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Papers by Patrick Were

Environmental Earth Sciences, 2018

Accurate assessment of deep geothermal resources remains a challenge from the practical point of ... more Accurate assessment of deep geothermal resources remains a challenge from the practical point of view. Parameter uncertainties and partial knowledge of initial conditions limit the prediction of subsurface temperatures using a variety of thermal models strongly unreliable, and the temperature is highly dependent on the radiogenic heat production in the geological layers mainly affected by a number of factors including the concentrations of uranium, thorium and potassium, and rock density. In this paper, geostatistical methods were applied to investigate the spatial distribution of radiogenic elements (e.g., uranium, thorium, potassium) and their corresponding concentrations and radiogenic heat production. A representative region measuring 35 km × 80 km in the southwestern Québec, and covering the domains of Portneuf-Mauricie, Morin Terrane and Parc des Laurentides in the Grenville Province was selected for this study because of its easy accessibility. Analysis results show that the concentrations of uranium, thorium and potassium for most rocks of the Grenville basement in the research region are in the range of 1-2 ppm, 3-10 ppm and 1-4%, respectively. Furthermore, 90% of the total samples analysed in this study show a uranium concentration of less than 3 ppm, 64% of the samples show a thorium concentration of less than 5 ppm, and 56% of the samples show a potassium concentration of less than 3%. This paper engaged both the ordinary kriging interpolation and sequential Gaussian simulation (SGS) methods to study the spatial distribution of radiogenic elements. Using density data for specific rocks, the distribution of radiogenic heat production in the study area of the southwestern Grenville Province was also simulated using the SGS method. Conclusively, results show that the difference between the minimum and the maximum value of radiogenic heat production is 30%, considering a significant proportion of heterogeneity in rock density.

Journal of Petroleum Science and Engineering, 2016

Theoretical and numerical studies of coupled hydro-mechanical processes during geothermal and hyd... more Theoretical and numerical studies of coupled hydro-mechanical processes during geothermal and hydrocarbon production, nuclear waste disposal, and CO 2 sequestration in geological media have become a research focus worldwide. However, their application in China, especially CO 2 sequestration, is still in infancy and may need several years to develop. In this paper coupled hydro-mechanical processes in CO 2 migration, formation uplift and the state of caprock integrity as a result of CO 2 sequestration have been investigated by simulation at field scale for an integrated CCS (CO 2 Capture and Sequestration) project launched in the Ordos Basin, China. The paper aims at attaining long-term safety and integrity of caprock to ensure success for subsequent project operations at field scale. The linked TOUGH2MP-FLAC3D simulator has been used to study the coupled hydraulic-mechanical responses to primary stresses, injection rate and reservoir permeability during CO 2 injection and post-injection periods by means of 4 case studies. In the short-term operation of a CO 2 sequestration project launched in the Ordos Basin 0.1 Mt CO 2 has been injected in four saline aquifers within a period of 1 year and 9 years of relaxation time allotted for simulation runs. Contrary to using hydraulic field conditions alone, a combination of hydraulic-mechanical effects accounts for lateral migration of CO 2 in aquifer layers that leads to partial volumetric expansion of the reservoir. Amongst the main factors affecting hydro-mechanical processes, injection rate has the most significant impact. This is followed by reservoir permeability that is responsible for gauging the uplift trend rate in the post-injection period through hindering the dissipation of pore pressure. Results indicate that the scope of pore pressure attained will be larger in the isotropic stress state than in the compressional stress condition, pointing to the significance of primary stress. Calculation of the maximum storage pressure within safety limits using the integrity and shear criteria reveals that failure of caprock integrity increases with the amount of CO 2 injected. During injection however, the risk of caprock failure, especially in thin sandstone-caprock interlayered systems, increases with time. This paper estimates the pressure buildup in the four injection aquifers in the Ordos Basin, as a basis to ensure safety in the long-term operation of the CO 2 sequestration project, hence providing important reference for future commercial operations at field scale.

Environmental Earth Sciences, 2016

Wellbore instability in shale results in annual expenditure for petroleum industry especially for... more Wellbore instability in shale results in annual expenditure for petroleum industry especially for shale gas/ oil development. Osmotic pressure is one of the most significant effects, which affect wellbore stability during drilling in shale formation using water-based mud. Until now, methods to calculate osmotic were only undertaken in the pre-drilling or post-drilling phase. This paper presents a new developed method to estimate shale osmotic pressure using an spontaneous potential (SP) log, which makes it possible to calculate osmotic pressure during drilling. The relationship between shale osmotic pressure and SP log was investigated. It was found that osmotic pressure and SP value both depend on the shale cation exchange capacity, which was used to bridge the SP log and osmotic pressure. An empirical equation relating the SP value and osmotic pressure was developed, which can compute osmotic pressure using the SP value. Moreover, the depth-dependent earth temperature was considered in the osmotic pressure calculation. A case study was undertaken using the SP log of a wellbore section located in Shengli Oil Field, China.

Journal of Petroleum Science and Engineering, 2016

Hydraulic fracturing is an engineering multi-process method used for stimulating the economic pro... more Hydraulic fracturing is an engineering multi-process method used for stimulating the economic production of energy carriers from unconventional resources. This technique has become indispensable to the exploitation of natural gas from tight gas reservoirs. In its appropriate operation, both pressurized fluid and Contents lists available at ScienceDirect

Springer Series in Geomechanics and Geoengineering, 2013

Many problems may occur in the process of CO 2 injection which usually lasts for many years. Inje... more Many problems may occur in the process of CO 2 injection which usually lasts for many years. Injection efficiency depends on many factors, such as temperature (T), well bottomhole pressure (P), well heading pressure, injection rate, reservoir properties (porosity, permeability, wettability) and scales of some minerals. The scaling problem in the wellbore and near-well region in reservoir (usually a few meters away from the borehole), may have a large negative effect on the reservoir properties such as a decrease in porosity and permeability near the well borehole region. As a result, the amount of CO 2 injected will be restricted. The purpose of this study is to predict mineral scales formation and distribution that happen in near-well reservoir using the simulation method. In this paper, TOUGH2 and TOUGHREACT software are used, and a 1D model has been set up. In this benchmark simulation of scaling problems, T and P are chosen to be at 100°C and 4MPa respectively. Simulation results show that pressure and gas saturation of the reservoir had been changed greatly after CO 2 injection. Illite and calcite may be the main mineral scales in the near-well region. At different time after the injection of CO 2 , there are different changes of mineral types and mineral volume, illustrating that at the beginning of the injection period, the precipitated minerals are illite, oligoclase and calcite, with time, ankerite, smectite-Ca and dawsonite will precipitate. In order to control the scale problems and alleviate destruction of the reservoir and petroleum equipment, chelants (such as EDTA, DTPA) should be used.

Environmental Earth Sciences, 2014

ABSTRACT Subsurface immobilization and conversion of CO2 into solid mineral phases in deep silici... more ABSTRACT Subsurface immobilization and conversion of CO2 into solid mineral phases in deep siliciclastic saline formations containing silicate minerals, commonly known as “mineral trapping”, is gaining research attention as a significant option to reduce CO2 emissions in the atmosphere. Although mineral trapping of CO2 is a long-term process, a combination of short-term results from both laboratory experiments and numerical simulations can lead to some general understanding of the required long-term CO2 sequestration mechanisms. This is a 100 year preliminary batch simulation study of four sandstone samples, under CO2 saturated water at 75 °C from the Upper Permian formations in the Ordos Basin, using the TOUGHREACT/ECO2N module to simulate the CO2-brine-rock interaction processes in deep siliciclastic multilayered saline aquifers. The samples approximately correspond to the four target saline formations selected by the Shenhua Group for a CO2 sequestration field demonstration project in the Ordos Basin, PR China. Preliminary simulation results show that the initial salinity of formation brine plays a significant role in determining the amount of CO2 that will be sequestered by solubility or mineral trapping in a deep saline aquifer. Minimal differences between experimental results and numerical calculation occur in low salinity waters, and significantly larger differences in high salinity waters, which is still under the maximum acceptable difference between experimental and computed data (10 %). The upper Liujiagou formation, with the highest level of salinity (ca. 88.7 g/L TDS) and lowest level of CO2 solubility, offers the highest mineral trapping capacity, with a maximum carbonate mineral storage of ca. 0.7 kg/m3 of bulk rock over a 100 year period. Regardless of the initial acidity or alkalinity of the aquifer brine, injection of CO2 will inflict a sudden drop in pH of the brine to acidity levels in a range of 3.0-4.6. The subsequent amount of dissolved and precipitated minerals, arising from the CO2-brine-rock interaction, is site specific and mainly dependent on initial aquifer mineralogy and brine composition.

Environmental Earth Sciences, 2013

ABSTRACT Deep saline aquifers still remain a significant option for the disposal of large amounts... more ABSTRACT Deep saline aquifers still remain a significant option for the disposal of large amounts of CO2 from the atmosphere as a means of mitigating global climate change. The small scale Carbon Capture and Sequestration demonstration project in Ordos Basin, China, operated by the Shenhua Group, is the only one of its kind in Asia, to put the multilayer injection technology into practice. This paper aims at studying the influence of temperature, injection rate and horizontal boundary effects on CO2 plume transport in saline formation layers at different depths and thicknesses, focusing on the variations in CO2 gas saturation and mass fraction of dissolved CO2 in the formation of brine in the plume’s radial three-dimensional field around the injection point, and interlayer communication between the aquifer and its confining beds of relatively lower permeability. The study uses the ECO2N module of TOUGH2 to simulate flow and pressure configurations in response to small-scale CO2 injection into multilayer saline aquifers. The modelling domain involves a complex multilayer reservoir–caprock system, comprising of a sequence of sandstone aquifers and sealing units of mudstone and siltstone layers extending from the Permian Shanxi to the Upper Triassic Liujiagou formation systems in the Ordos Basin. Simulation results indicate that CO2 injected for storage into deep saline aquifers cause a significant pressure perturbation in the geological system that may require a long duration in the post-injection period to establish new pressure equilibrium. The multilayer simultaneous injection scheme exhibits mutual interference with the intervening sealing layers, especially when the injection layers are very close to each other and the corresponding sealing layers are thin. The study further reveals that injection rate and temperature are the most significant factors for determining the lateral and vertical extent that the CO2 plume reaches and which phase and amount will exist at a particular time during and after the injection. In general, a large number of factors may influence the CO2–water fluid flow system considering the complexity in the real geologic sequence and structural configurations. Therefore, optimization of a CO2 injection scheme still requires pursuance of further studies.

Environmental Earth Sciences, 2015

CO 2 sequestration in deep saline formations has been proved to be an effective method for reduci... more CO 2 sequestration in deep saline formations has been proved to be an effective method for reducing greenhouse emissions into the atmosphere. However, pure sequestration of CO 2 will add to the costs incurred by both industries and governments. A win-win method of CO 2 injection and hot brine (water) extraction can become attractive to the investors, as it will not only increase the storage capacity of the injected CO 2 , but also offset the costs by selling and using the produced hot water for industrial, agricultural or household purposes. For instance, water from very hot geothermal reservoirs (T C 150°C) can be used for electricity generation in power plants and water from low-medium temperature reservoirs, the most predominant in natural systems, are more popular for direct use, e.g., in heating systems, household hot water, baths, aquaculture, etc. In this paper, low-medium geothermal reservoirs widely distributed in China, especially those in the Ordos Basin, were selected for the numerical case studies using TOUGH2MP with the ECO2N module for the simulations. Generally, simulation parameters were taken from the Ordos Basin, where the first full-integration CO 2 sequestration project had been operated since 2010. The simulations in the base case study lasted 35 years, based on the lifespan of a normal geothermal project. Shallow re-injection systems were also considered to investigate the influence of thermal breakthrough, pressure perturbation, etc. Results show that injection of cold CO 2 causes sharp decrease in temperature in the reservoir region near the injection well, which is enlarged with continuous injection. The region near the production well is dominated by different fluid phases during the CO 2 driven process, including a single water phase, a two-phase fluid (water and CO 2) and a phase of almost pure CO 2. Results also show that the CO 2 breakthrough lags far behind the pressure response in the geothermal production system. Before breakthrough, the injected CO 2 pressurizes the reservoir, improving the overall performance of the geothermal reservoir. Furthermore, the heat extraction efficiency of CO 2based system is obviously higher than H 2 O-based system.

Environmental Earth Sciences, 2015

Because of the high costs involved in CO 2 sequestration in deep saline formations, it has been d... more Because of the high costs involved in CO 2 sequestration in deep saline formations, it has been difficult for its widespread application in the short term. Recently, however, a new technology called ''combined geothermal production-CO 2 sequestration technology'' has made the technique more attractive not only by increasing the CO 2 storage capacity, but also by decreasing the operational costs, through the utilization of geothermal energy, either directly or for the purpose of producing electricity. A CO 2aided (or CO 2-involved) geothermal extraction system (CO 2-AGES) is presented, based on existing technologies and the new ideas of ''combined geothermal production-CO 2 sequestration'' introduced in this paper. This system can be used to extract geothermal energy from sedimentary aquifer formations (low-medium temperature) at different stages. The purpose of this paper is to set up an evaluation system for selecting a suitable site for this CO 2-AGES system. There are both similarities and differences in the procedure needed for selecting a site for a conventional pure CO 2 sequestration and for this CO 2-AGES system. Both are carried out at different scales, including basin, region and target formation scales. The biggest difference is the temperature gradient, which plays an opposite role in the two systems. By using the preliminary ranking and screening method presented in this paper, it can be shown that the Bohaiwan, Songliao and Qiangtang basins have the highest potential for the application of this CO 2-AGES technology. However, if CO 2 sequestration is considered alone, the Ordos, Tarim and Bohaiwan basins should be selected as the best three operational sites in China. While preliminary evaluation methods can provide some useful information on the selection of the best sites for the ''combined geothermal production-CO 2 sequestration technology'', more detailed work is still required because of the strong uncertainties that exist in the determination of the upper and lower boundaries for each indicator.

Acta Geotechnica, 2013

This paper studied the CO 2-EGR in Altmark natural gas field with numerical simulations. The hydr... more This paper studied the CO 2-EGR in Altmark natural gas field with numerical simulations. The hydromechanical coupled simulations were run using a linked simulator TOUGH2MP-FLAC3D. In order to consider the gas mixing process, EOS7C was implemented in TOUGH2MP. A multi-layered 3D model (4.4 km 9 2 km 9 1 km) which consists of the whole reservoir, caprock and base rock was generated based on a history-matched PETREL model, originally built by GDF SUEZ E&P Deutschland GmbH for Altmark natural gas field. The model is heterogeneous and discretized into 26,015 grid blocks. In the simulation, 100,000 t CO 2 was injected in the reservoir through well S13 within 2 years, while gas was produced from the well S14. Some sensitivity analyses were also carried out. Simulation results show that CO 2 tends to migrate toward the production well S14 along a NW-SE fault. It reached the observation wells S1 and S16 after 2 years, but no breakthrough occurred in the production well. After 2 years of CO 2 injection, the reservoir pressure increased by 2.5 bar, which is beneficial for gas recovery. The largest uplift (1 mm) occurred at the bottom of the caprock. The deformation was small (elastic) and caprock integrity was not affected. With the injection rate doubled the average pressure increased by 5.3 bar. Even then the CO 2 did not reach the production well S14 after 2 years of injection. It could be concluded that the previous flow field was established during the primary gas production history. This former flow field, including CO 2 injection/CH 4 production rate during CO 2-EGR and fault directions and intensity are the most important factors affecting the CO 2 transport.

I would like to thank my in-laws, Mr. and Mrs. Higwira for keeping my family during the period I ... more I would like to thank my in-laws, Mr. and Mrs. Higwira for keeping my family during the period I have been abroad. I am indeed grateful for their support, patience and understanding. Funding of this research was provided by DFG grant Gottfried Wilhelm Leibniz-Preis 2001 to Professor Dr. Hans Keppler. I am very grateful to him for having given me the chance to do my Ph.D research work, benefiting from his grant. As my supervisor, he carefully read through the entire dissertation and made many suggestions for its improvement, in matters of substance as well as style. I am grateful for his generous effort on his part, but any remaining errors are of course mine. Professor Dr. Muharrem Satir opened my eyes as regards to the use of stable and radioactive isotopes in geochemistry. I would also like to thank him for the moral support and advice he often offered me whenever I had difficulties of any kind. I should like to extend my thanks to the entire academic staff of the Institute of Mineralogy at the University of Tübingen for their endeavours in teaching me the theory and practical aspects necessary for safe use of experimental and analytical equipment. Dr. Thomas Wenzel, the chief of the Microprobe laboratory, helped me a great deal with the chemical analysis of samples using the Electron microprobe. Andreas Audetat, then a postgraduate and head of our research team, helped me a great deal with the calculations necessary for the preparation of the starting glasses and mixtures for my experiments. The technical team at the Bayerisches Geoinstitut, particularly Mr. Detlef and Anke, also deserve many thanks. They helped me to get some analytical results of my problematic samples using Electron microprobe. I would also like to thank the workshop staff, particularly the Meister, Mr. Walker, and Barbara, for the care and maintenance of the experimental and analytical equipment, and Mrs. Gill-Kopp, for fine polishing of my samples. Finally I would like to thank my family for all their love and prayers.

Geofluids, 2017

Carbon capture, utilization, and storage (CCUS) is a gas injection technology that enables the st... more Carbon capture, utilization, and storage (CCUS) is a gas injection technology that enables the storage of CO2 underground. The aims are twofold, on one hand to reduce the emissions of CO2 into the atmosphere and on the other hand to increase oil/gas/heat recovery. Different types of CCUS technologies and related engineering projects have a long history of research and operation in the USA. However, in China they have a short development period ca. 10 years. Unlike CO2 capture and CO2-EOR technologies that are already operating on a commercial scale in China, research into other CCUS technologies is still in its infancy or at the pilot-scale. This paper first reviews the status and development of the different types of CCUS technologies and related engineering projects worldwide. Then it focuses on their developments in China in the last decade. The main research projects, international cooperation, and pilot-scale engineering projects in China are summarized and compared. Finally, t...

Journal of Petroleum Science and Engineering, 2016

Abstract Through integral studies on cores, seismics, well-logs and laboratory sediment analysis ... more Abstract Through integral studies on cores, seismics, well-logs and laboratory sediment analysis a new sequence stratigraphic framework of the Late Cretaceous first member of the Yaojia Formation in the Songliao Basin, in China's Zh ao y uan- T ai p ing ch uan (Zhy-Tpch) region, has been reconstructed. In this study, the first member of Yaojia Formation is divided into 3 three-order depositional sequences, each composed of three systems tracts, i.e. the lowstand systems tract (LST), the transgressive systems tract (TST) and the highstand systems tract (HST). Using a dense network of cored boreholes data and data acquired from a combination of spontaneous potential and electric resistivity logging methods, the paper studies the characteristics and distribution of sedimentary environments within the cyclic patterns (systems tracts) of the major depositional sequences, and ascertains that deposition of sediments in a predominantly shallow lake delta environment which prevailed in the region during the Late Cretaceous period was responsible for the creation of the first member of the Yaojia Formation. Results show that this formation is mainly composed of three sub-facies namely, the delta plain, delta front and prodelta (the shore shallow lake). Each sub-facies is further subdivided into eleven sedimentary environments including distributary channels, natural levees, crevasse splays, inter-distributary channels, flood plains, subaqueous distributary channels, inter-distributary bays, subaqueous crevasse splays, mouth bars, distal bars and sheet sands. The delta plains and delta fronts are widely developed. Deposited with frequent bifurcations, the distributary channels gradually moved towards the center of the lacustrine basin where prodelta sedimentation had hardly occurred. Meanwhile, subaqueous channels were frequently developed within the shallow water areas of the delta fronts. Sandstone facies in the distributary and subaqueous distributary channels mutually interacted and stacked up to form dendritic-like sands. Distributary channel sands formed the most favorable reservoirs available today and are considered to be the most important exploration targets for the oil industry. Overall, this study is crucial for improving the understanding of the geological and paleo-geographic record in the Zhy-Tpch region including the facies types, relationships and cyclicity in response to changes in depositional environments (i.e. sedimentation and base-level changes) that prevailed in the region during the Late Cretaceous period and hence important for the predictive aspects of the economic petroleum exploration and development in the Songliao Basin.

Contributions to Mineralogy and Petrology

The partitioning of a large suite of trace elements between biotite and water-saturated granitic ... more The partitioning of a large suite of trace elements between biotite and water-saturated granitic melt was measured at 2 kbar and 700—800 ˚C. To reach equilibrium and to grow biotite crystals large enough for analysis, runs usually lasted from 30 to 45 days. In every charge, a few trace elements were initially doped at the 0.1—0.5 wt. % level and analyzed by electron microprobe after the run. First-row transition metal ions are highly compatible in biotite with Dbiotite/melt of 17 for Ti, 35 for V, 47 for Co, 174 for Ni, and 5.8 for Zn. A very notable exception is Cu with Dbiotite/melt

Environmental Earth Sciences, 2015

Natural gas storage caverns are usually built either in a rock salt dome or in a bedded rock salt... more Natural gas storage caverns are usually built either in a rock salt dome or in a bedded rock salt formation. In China, the rock salt stratum has the following characteristics: thin, inter-layered, bedded and highly impure. The height of a cavern in bedded rock salt deposit in China depends largely on the thickness of the salt formation (80-300 m) and is much smaller than in thick domes and thick layers of rock salt deposits in Europe and the USA, where a storage cavern is typically, cylindrically shaped. This renders the diameter of a bedded salt cavern in China to be greater than its height to obtain sufficient storage volume. The stability of horizontal salt caverns with different diameters and varied minimal internal pressures during its long-term operation was numerically analyzed in this paper regarding convergence, damage, stress to peak or dilatancy strength ratio and effective strain. The Hou/Lux constitutive model including some relevant parameters from previous laboratory investigations were used for the numerical simulations. The simulation results show that a horizontal cavern (height ( length) for underground gas storage is more efficient, profitable and suitable because of its few times larger storage capacity than the most used vertical cavern (height ) diameter) or a prolate cavern (height & or \ diameter) under the Chinese special geological condition of thin-bedded rock salt deposits, provided with appropriate construction and operation parameters.

Environmental Earth Sciences, 2015

In this paper, numerical and semi-analytical investigations were conducted to understand the hydr... more In this paper, numerical and semi-analytical investigations were conducted to understand the hydraulic fracturing operation in the tight gas reservoir only identified by the code A7 in the North German Basin. Two simulators, FLAC3D plus (numerical, full 3D model) and MFrac (semi-analytical, modified model based on the conventional pseudo-3D model), were used to model the fracturing operations including fracture propagation, proppant transport and settling. A comparison of the two simulators was carried out through simulations. Meanwhile, the function of the geological barrier integrity in A7 was also studied and confirmed. The simulations were based on the history matching of the in situ measured well head pressure. At the end of the simulation, a long fracture (length ) height) was modeled by both simulators. Although the results express some differences in the modeled fracture length and width (average), their results for fracture pressure, fluid leak off and proppant distribution are comparable. That means they would provide similar productivity in the later production. Investigation of the geological barrier integrity confirms that the cap rock, formed from rock salt and anhydrite, normally has higher minimum horizontal stress and lower permeability providing enough resistance to prevent the fracture from propagating in the vertical direction of the cap rocks. The case study reveals that, even when the injection volume was increased 10 times the initial volume, the integrity of the cap rocks could not be broken. Despite the presence of interbedded shale formations, in the reservoir as those of the cap rock an impediment function, if their thicknesses are too small to prevent their breakage and the injection volume too large for them to resist fracturing.

Environmental Earth Sciences, 2015

Hydraulic fracturing is widely used in the petroleum engineering to enhance the reservoir conduct... more Hydraulic fracturing is widely used in the petroleum engineering to enhance the reservoir conductivity. The most 3D simulators for modeling hydraulic fracturing assume that the created fracture propagates perpendicular to the minimum horizontal stress. In the reality, the fracture orientation is more or less affected by many factors, e.g. the reservoir heterogeneity. If the fracture is strongly reoriented, then such 3D simulators could not be used. Therefore, it is essential to investigate the orientation problem. In this paper, a 2D numerical approach to model hydraulic-driven fractures with arbitrary orientation in tight gas reservoirs is presented. The approach is based on the elasticity and lubricant theory with consideration of fully hydro-mechanical coupling effects. It was solved using a combination of the extended finite element and the finite volume methods. The approach was verified by modeling the dynamical growth of a KGD fracture (a model developed by Kristnovitch-Geertsma-Daneshy). Three numerical examples are illustrated. The first example is hydraulic fracturing in a heterogeneous reservoir. The hydraulic fracture propagated asymmetrically with approx. 7 m or 1.5°deviation in the direction of maximum principal stress. In the second example, a two stage multiple hydraulic fracturing was modeled. The influences of the perforation and the first fracture on the second fracture were observed from the modeling results. In the third example, a re-fracturing operation after 1 year of production was modeled. Due to the irregular stress change, an almost 90°fracture reorientation was observed.

Environmental Earth Sciences, 2015

Environmental Earth Sciences, 2015

Geothermal energy is renewable, sustainable and available in large amounts. The hot dry rock (HDR... more Geothermal energy is renewable, sustainable and available in large amounts. The hot dry rock (HDR) systems, in particular, have the largest potential for longterm sustainability and therefore draw a lot of attention. The combination of horizontal wells and the technology of multiple transverse fractures technology is an attractive approach of such HDRs, which is called in this paper as the specific EGS (enhanced geothermal system). The main objective of this paper is to study the heat extraction over a period of 20 years by water circulating in a deep geothermal reservoir using this specific EGS. The wellbore flow module, T2WELL/ECO2N, is implemented in the parallelized simulator TOUGH2MP as a new code TOUGH2MP-WELL/EOS3, which enables coupled wellbore-reservoir simulations. Using this newly developed code, the sensitivity of heat extraction against various parameters of the formation and fractures is assessed. The influences of the fracture geometry and the space of two neighborhood fractures on the geothermal energy performance are analyzed. According to the results in this paper, the injection rate, the wellbore radius and the fracture permeability are three main influence factors for the distribution of total flow through individual fractures. A large injection rate, a small wellbore radius and a large fracture width will cause the short-circuit effect, which reduces the performance of this specific EGS. Compared with the classic HDR system (doublet or triplet vertical wells ? single fracture), this specific EGS has a much higher performance and a longer duration of the economic production. Furthermore, the horizontal wells can also be drilled in both directions of the minimum horizontal stress instead of in just one direction, so that the energy performance is doubled and the investment cost per kW energy production is decreased and the advantage of this specific EGS is significantly enlarged.

Environmental Earth Sciences, 2018

Accurate assessment of deep geothermal resources remains a challenge from the practical point of ... more Accurate assessment of deep geothermal resources remains a challenge from the practical point of view. Parameter uncertainties and partial knowledge of initial conditions limit the prediction of subsurface temperatures using a variety of thermal models strongly unreliable, and the temperature is highly dependent on the radiogenic heat production in the geological layers mainly affected by a number of factors including the concentrations of uranium, thorium and potassium, and rock density. In this paper, geostatistical methods were applied to investigate the spatial distribution of radiogenic elements (e.g., uranium, thorium, potassium) and their corresponding concentrations and radiogenic heat production. A representative region measuring 35 km × 80 km in the southwestern Québec, and covering the domains of Portneuf-Mauricie, Morin Terrane and Parc des Laurentides in the Grenville Province was selected for this study because of its easy accessibility. Analysis results show that the concentrations of uranium, thorium and potassium for most rocks of the Grenville basement in the research region are in the range of 1-2 ppm, 3-10 ppm and 1-4%, respectively. Furthermore, 90% of the total samples analysed in this study show a uranium concentration of less than 3 ppm, 64% of the samples show a thorium concentration of less than 5 ppm, and 56% of the samples show a potassium concentration of less than 3%. This paper engaged both the ordinary kriging interpolation and sequential Gaussian simulation (SGS) methods to study the spatial distribution of radiogenic elements. Using density data for specific rocks, the distribution of radiogenic heat production in the study area of the southwestern Grenville Province was also simulated using the SGS method. Conclusively, results show that the difference between the minimum and the maximum value of radiogenic heat production is 30%, considering a significant proportion of heterogeneity in rock density.

Journal of Petroleum Science and Engineering, 2016

Theoretical and numerical studies of coupled hydro-mechanical processes during geothermal and hyd... more Theoretical and numerical studies of coupled hydro-mechanical processes during geothermal and hydrocarbon production, nuclear waste disposal, and CO 2 sequestration in geological media have become a research focus worldwide. However, their application in China, especially CO 2 sequestration, is still in infancy and may need several years to develop. In this paper coupled hydro-mechanical processes in CO 2 migration, formation uplift and the state of caprock integrity as a result of CO 2 sequestration have been investigated by simulation at field scale for an integrated CCS (CO 2 Capture and Sequestration) project launched in the Ordos Basin, China. The paper aims at attaining long-term safety and integrity of caprock to ensure success for subsequent project operations at field scale. The linked TOUGH2MP-FLAC3D simulator has been used to study the coupled hydraulic-mechanical responses to primary stresses, injection rate and reservoir permeability during CO 2 injection and post-injection periods by means of 4 case studies. In the short-term operation of a CO 2 sequestration project launched in the Ordos Basin 0.1 Mt CO 2 has been injected in four saline aquifers within a period of 1 year and 9 years of relaxation time allotted for simulation runs. Contrary to using hydraulic field conditions alone, a combination of hydraulic-mechanical effects accounts for lateral migration of CO 2 in aquifer layers that leads to partial volumetric expansion of the reservoir. Amongst the main factors affecting hydro-mechanical processes, injection rate has the most significant impact. This is followed by reservoir permeability that is responsible for gauging the uplift trend rate in the post-injection period through hindering the dissipation of pore pressure. Results indicate that the scope of pore pressure attained will be larger in the isotropic stress state than in the compressional stress condition, pointing to the significance of primary stress. Calculation of the maximum storage pressure within safety limits using the integrity and shear criteria reveals that failure of caprock integrity increases with the amount of CO 2 injected. During injection however, the risk of caprock failure, especially in thin sandstone-caprock interlayered systems, increases with time. This paper estimates the pressure buildup in the four injection aquifers in the Ordos Basin, as a basis to ensure safety in the long-term operation of the CO 2 sequestration project, hence providing important reference for future commercial operations at field scale.

Environmental Earth Sciences, 2016

Wellbore instability in shale results in annual expenditure for petroleum industry especially for... more Wellbore instability in shale results in annual expenditure for petroleum industry especially for shale gas/ oil development. Osmotic pressure is one of the most significant effects, which affect wellbore stability during drilling in shale formation using water-based mud. Until now, methods to calculate osmotic were only undertaken in the pre-drilling or post-drilling phase. This paper presents a new developed method to estimate shale osmotic pressure using an spontaneous potential (SP) log, which makes it possible to calculate osmotic pressure during drilling. The relationship between shale osmotic pressure and SP log was investigated. It was found that osmotic pressure and SP value both depend on the shale cation exchange capacity, which was used to bridge the SP log and osmotic pressure. An empirical equation relating the SP value and osmotic pressure was developed, which can compute osmotic pressure using the SP value. Moreover, the depth-dependent earth temperature was considered in the osmotic pressure calculation. A case study was undertaken using the SP log of a wellbore section located in Shengli Oil Field, China.

Journal of Petroleum Science and Engineering, 2016

Hydraulic fracturing is an engineering multi-process method used for stimulating the economic pro... more Hydraulic fracturing is an engineering multi-process method used for stimulating the economic production of energy carriers from unconventional resources. This technique has become indispensable to the exploitation of natural gas from tight gas reservoirs. In its appropriate operation, both pressurized fluid and Contents lists available at ScienceDirect

Springer Series in Geomechanics and Geoengineering, 2013

Many problems may occur in the process of CO 2 injection which usually lasts for many years. Inje... more Many problems may occur in the process of CO 2 injection which usually lasts for many years. Injection efficiency depends on many factors, such as temperature (T), well bottomhole pressure (P), well heading pressure, injection rate, reservoir properties (porosity, permeability, wettability) and scales of some minerals. The scaling problem in the wellbore and near-well region in reservoir (usually a few meters away from the borehole), may have a large negative effect on the reservoir properties such as a decrease in porosity and permeability near the well borehole region. As a result, the amount of CO 2 injected will be restricted. The purpose of this study is to predict mineral scales formation and distribution that happen in near-well reservoir using the simulation method. In this paper, TOUGH2 and TOUGHREACT software are used, and a 1D model has been set up. In this benchmark simulation of scaling problems, T and P are chosen to be at 100°C and 4MPa respectively. Simulation results show that pressure and gas saturation of the reservoir had been changed greatly after CO 2 injection. Illite and calcite may be the main mineral scales in the near-well region. At different time after the injection of CO 2 , there are different changes of mineral types and mineral volume, illustrating that at the beginning of the injection period, the precipitated minerals are illite, oligoclase and calcite, with time, ankerite, smectite-Ca and dawsonite will precipitate. In order to control the scale problems and alleviate destruction of the reservoir and petroleum equipment, chelants (such as EDTA, DTPA) should be used.

Environmental Earth Sciences, 2014

ABSTRACT Subsurface immobilization and conversion of CO2 into solid mineral phases in deep silici... more ABSTRACT Subsurface immobilization and conversion of CO2 into solid mineral phases in deep siliciclastic saline formations containing silicate minerals, commonly known as “mineral trapping”, is gaining research attention as a significant option to reduce CO2 emissions in the atmosphere. Although mineral trapping of CO2 is a long-term process, a combination of short-term results from both laboratory experiments and numerical simulations can lead to some general understanding of the required long-term CO2 sequestration mechanisms. This is a 100 year preliminary batch simulation study of four sandstone samples, under CO2 saturated water at 75 °C from the Upper Permian formations in the Ordos Basin, using the TOUGHREACT/ECO2N module to simulate the CO2-brine-rock interaction processes in deep siliciclastic multilayered saline aquifers. The samples approximately correspond to the four target saline formations selected by the Shenhua Group for a CO2 sequestration field demonstration project in the Ordos Basin, PR China. Preliminary simulation results show that the initial salinity of formation brine plays a significant role in determining the amount of CO2 that will be sequestered by solubility or mineral trapping in a deep saline aquifer. Minimal differences between experimental results and numerical calculation occur in low salinity waters, and significantly larger differences in high salinity waters, which is still under the maximum acceptable difference between experimental and computed data (10 %). The upper Liujiagou formation, with the highest level of salinity (ca. 88.7 g/L TDS) and lowest level of CO2 solubility, offers the highest mineral trapping capacity, with a maximum carbonate mineral storage of ca. 0.7 kg/m3 of bulk rock over a 100 year period. Regardless of the initial acidity or alkalinity of the aquifer brine, injection of CO2 will inflict a sudden drop in pH of the brine to acidity levels in a range of 3.0-4.6. The subsequent amount of dissolved and precipitated minerals, arising from the CO2-brine-rock interaction, is site specific and mainly dependent on initial aquifer mineralogy and brine composition.

Environmental Earth Sciences, 2013

ABSTRACT Deep saline aquifers still remain a significant option for the disposal of large amounts... more ABSTRACT Deep saline aquifers still remain a significant option for the disposal of large amounts of CO2 from the atmosphere as a means of mitigating global climate change. The small scale Carbon Capture and Sequestration demonstration project in Ordos Basin, China, operated by the Shenhua Group, is the only one of its kind in Asia, to put the multilayer injection technology into practice. This paper aims at studying the influence of temperature, injection rate and horizontal boundary effects on CO2 plume transport in saline formation layers at different depths and thicknesses, focusing on the variations in CO2 gas saturation and mass fraction of dissolved CO2 in the formation of brine in the plume’s radial three-dimensional field around the injection point, and interlayer communication between the aquifer and its confining beds of relatively lower permeability. The study uses the ECO2N module of TOUGH2 to simulate flow and pressure configurations in response to small-scale CO2 injection into multilayer saline aquifers. The modelling domain involves a complex multilayer reservoir–caprock system, comprising of a sequence of sandstone aquifers and sealing units of mudstone and siltstone layers extending from the Permian Shanxi to the Upper Triassic Liujiagou formation systems in the Ordos Basin. Simulation results indicate that CO2 injected for storage into deep saline aquifers cause a significant pressure perturbation in the geological system that may require a long duration in the post-injection period to establish new pressure equilibrium. The multilayer simultaneous injection scheme exhibits mutual interference with the intervening sealing layers, especially when the injection layers are very close to each other and the corresponding sealing layers are thin. The study further reveals that injection rate and temperature are the most significant factors for determining the lateral and vertical extent that the CO2 plume reaches and which phase and amount will exist at a particular time during and after the injection. In general, a large number of factors may influence the CO2–water fluid flow system considering the complexity in the real geologic sequence and structural configurations. Therefore, optimization of a CO2 injection scheme still requires pursuance of further studies.

Environmental Earth Sciences, 2015

CO 2 sequestration in deep saline formations has been proved to be an effective method for reduci... more CO 2 sequestration in deep saline formations has been proved to be an effective method for reducing greenhouse emissions into the atmosphere. However, pure sequestration of CO 2 will add to the costs incurred by both industries and governments. A win-win method of CO 2 injection and hot brine (water) extraction can become attractive to the investors, as it will not only increase the storage capacity of the injected CO 2 , but also offset the costs by selling and using the produced hot water for industrial, agricultural or household purposes. For instance, water from very hot geothermal reservoirs (T C 150°C) can be used for electricity generation in power plants and water from low-medium temperature reservoirs, the most predominant in natural systems, are more popular for direct use, e.g., in heating systems, household hot water, baths, aquaculture, etc. In this paper, low-medium geothermal reservoirs widely distributed in China, especially those in the Ordos Basin, were selected for the numerical case studies using TOUGH2MP with the ECO2N module for the simulations. Generally, simulation parameters were taken from the Ordos Basin, where the first full-integration CO 2 sequestration project had been operated since 2010. The simulations in the base case study lasted 35 years, based on the lifespan of a normal geothermal project. Shallow re-injection systems were also considered to investigate the influence of thermal breakthrough, pressure perturbation, etc. Results show that injection of cold CO 2 causes sharp decrease in temperature in the reservoir region near the injection well, which is enlarged with continuous injection. The region near the production well is dominated by different fluid phases during the CO 2 driven process, including a single water phase, a two-phase fluid (water and CO 2) and a phase of almost pure CO 2. Results also show that the CO 2 breakthrough lags far behind the pressure response in the geothermal production system. Before breakthrough, the injected CO 2 pressurizes the reservoir, improving the overall performance of the geothermal reservoir. Furthermore, the heat extraction efficiency of CO 2based system is obviously higher than H 2 O-based system.

Environmental Earth Sciences, 2015

Because of the high costs involved in CO 2 sequestration in deep saline formations, it has been d... more Because of the high costs involved in CO 2 sequestration in deep saline formations, it has been difficult for its widespread application in the short term. Recently, however, a new technology called ''combined geothermal production-CO 2 sequestration technology'' has made the technique more attractive not only by increasing the CO 2 storage capacity, but also by decreasing the operational costs, through the utilization of geothermal energy, either directly or for the purpose of producing electricity. A CO 2aided (or CO 2-involved) geothermal extraction system (CO 2-AGES) is presented, based on existing technologies and the new ideas of ''combined geothermal production-CO 2 sequestration'' introduced in this paper. This system can be used to extract geothermal energy from sedimentary aquifer formations (low-medium temperature) at different stages. The purpose of this paper is to set up an evaluation system for selecting a suitable site for this CO 2-AGES system. There are both similarities and differences in the procedure needed for selecting a site for a conventional pure CO 2 sequestration and for this CO 2-AGES system. Both are carried out at different scales, including basin, region and target formation scales. The biggest difference is the temperature gradient, which plays an opposite role in the two systems. By using the preliminary ranking and screening method presented in this paper, it can be shown that the Bohaiwan, Songliao and Qiangtang basins have the highest potential for the application of this CO 2-AGES technology. However, if CO 2 sequestration is considered alone, the Ordos, Tarim and Bohaiwan basins should be selected as the best three operational sites in China. While preliminary evaluation methods can provide some useful information on the selection of the best sites for the ''combined geothermal production-CO 2 sequestration technology'', more detailed work is still required because of the strong uncertainties that exist in the determination of the upper and lower boundaries for each indicator.

Acta Geotechnica, 2013

This paper studied the CO 2-EGR in Altmark natural gas field with numerical simulations. The hydr... more This paper studied the CO 2-EGR in Altmark natural gas field with numerical simulations. The hydromechanical coupled simulations were run using a linked simulator TOUGH2MP-FLAC3D. In order to consider the gas mixing process, EOS7C was implemented in TOUGH2MP. A multi-layered 3D model (4.4 km 9 2 km 9 1 km) which consists of the whole reservoir, caprock and base rock was generated based on a history-matched PETREL model, originally built by GDF SUEZ E&P Deutschland GmbH for Altmark natural gas field. The model is heterogeneous and discretized into 26,015 grid blocks. In the simulation, 100,000 t CO 2 was injected in the reservoir through well S13 within 2 years, while gas was produced from the well S14. Some sensitivity analyses were also carried out. Simulation results show that CO 2 tends to migrate toward the production well S14 along a NW-SE fault. It reached the observation wells S1 and S16 after 2 years, but no breakthrough occurred in the production well. After 2 years of CO 2 injection, the reservoir pressure increased by 2.5 bar, which is beneficial for gas recovery. The largest uplift (1 mm) occurred at the bottom of the caprock. The deformation was small (elastic) and caprock integrity was not affected. With the injection rate doubled the average pressure increased by 5.3 bar. Even then the CO 2 did not reach the production well S14 after 2 years of injection. It could be concluded that the previous flow field was established during the primary gas production history. This former flow field, including CO 2 injection/CH 4 production rate during CO 2-EGR and fault directions and intensity are the most important factors affecting the CO 2 transport.

I would like to thank my in-laws, Mr. and Mrs. Higwira for keeping my family during the period I ... more I would like to thank my in-laws, Mr. and Mrs. Higwira for keeping my family during the period I have been abroad. I am indeed grateful for their support, patience and understanding. Funding of this research was provided by DFG grant Gottfried Wilhelm Leibniz-Preis 2001 to Professor Dr. Hans Keppler. I am very grateful to him for having given me the chance to do my Ph.D research work, benefiting from his grant. As my supervisor, he carefully read through the entire dissertation and made many suggestions for its improvement, in matters of substance as well as style. I am grateful for his generous effort on his part, but any remaining errors are of course mine. Professor Dr. Muharrem Satir opened my eyes as regards to the use of stable and radioactive isotopes in geochemistry. I would also like to thank him for the moral support and advice he often offered me whenever I had difficulties of any kind. I should like to extend my thanks to the entire academic staff of the Institute of Mineralogy at the University of Tübingen for their endeavours in teaching me the theory and practical aspects necessary for safe use of experimental and analytical equipment. Dr. Thomas Wenzel, the chief of the Microprobe laboratory, helped me a great deal with the chemical analysis of samples using the Electron microprobe. Andreas Audetat, then a postgraduate and head of our research team, helped me a great deal with the calculations necessary for the preparation of the starting glasses and mixtures for my experiments. The technical team at the Bayerisches Geoinstitut, particularly Mr. Detlef and Anke, also deserve many thanks. They helped me to get some analytical results of my problematic samples using Electron microprobe. I would also like to thank the workshop staff, particularly the Meister, Mr. Walker, and Barbara, for the care and maintenance of the experimental and analytical equipment, and Mrs. Gill-Kopp, for fine polishing of my samples. Finally I would like to thank my family for all their love and prayers.

Geofluids, 2017

Carbon capture, utilization, and storage (CCUS) is a gas injection technology that enables the st... more Carbon capture, utilization, and storage (CCUS) is a gas injection technology that enables the storage of CO2 underground. The aims are twofold, on one hand to reduce the emissions of CO2 into the atmosphere and on the other hand to increase oil/gas/heat recovery. Different types of CCUS technologies and related engineering projects have a long history of research and operation in the USA. However, in China they have a short development period ca. 10 years. Unlike CO2 capture and CO2-EOR technologies that are already operating on a commercial scale in China, research into other CCUS technologies is still in its infancy or at the pilot-scale. This paper first reviews the status and development of the different types of CCUS technologies and related engineering projects worldwide. Then it focuses on their developments in China in the last decade. The main research projects, international cooperation, and pilot-scale engineering projects in China are summarized and compared. Finally, t...

Journal of Petroleum Science and Engineering, 2016

Abstract Through integral studies on cores, seismics, well-logs and laboratory sediment analysis ... more Abstract Through integral studies on cores, seismics, well-logs and laboratory sediment analysis a new sequence stratigraphic framework of the Late Cretaceous first member of the Yaojia Formation in the Songliao Basin, in China's Zh ao y uan- T ai p ing ch uan (Zhy-Tpch) region, has been reconstructed. In this study, the first member of Yaojia Formation is divided into 3 three-order depositional sequences, each composed of three systems tracts, i.e. the lowstand systems tract (LST), the transgressive systems tract (TST) and the highstand systems tract (HST). Using a dense network of cored boreholes data and data acquired from a combination of spontaneous potential and electric resistivity logging methods, the paper studies the characteristics and distribution of sedimentary environments within the cyclic patterns (systems tracts) of the major depositional sequences, and ascertains that deposition of sediments in a predominantly shallow lake delta environment which prevailed in the region during the Late Cretaceous period was responsible for the creation of the first member of the Yaojia Formation. Results show that this formation is mainly composed of three sub-facies namely, the delta plain, delta front and prodelta (the shore shallow lake). Each sub-facies is further subdivided into eleven sedimentary environments including distributary channels, natural levees, crevasse splays, inter-distributary channels, flood plains, subaqueous distributary channels, inter-distributary bays, subaqueous crevasse splays, mouth bars, distal bars and sheet sands. The delta plains and delta fronts are widely developed. Deposited with frequent bifurcations, the distributary channels gradually moved towards the center of the lacustrine basin where prodelta sedimentation had hardly occurred. Meanwhile, subaqueous channels were frequently developed within the shallow water areas of the delta fronts. Sandstone facies in the distributary and subaqueous distributary channels mutually interacted and stacked up to form dendritic-like sands. Distributary channel sands formed the most favorable reservoirs available today and are considered to be the most important exploration targets for the oil industry. Overall, this study is crucial for improving the understanding of the geological and paleo-geographic record in the Zhy-Tpch region including the facies types, relationships and cyclicity in response to changes in depositional environments (i.e. sedimentation and base-level changes) that prevailed in the region during the Late Cretaceous period and hence important for the predictive aspects of the economic petroleum exploration and development in the Songliao Basin.

Contributions to Mineralogy and Petrology

The partitioning of a large suite of trace elements between biotite and water-saturated granitic ... more The partitioning of a large suite of trace elements between biotite and water-saturated granitic melt was measured at 2 kbar and 700—800 ˚C. To reach equilibrium and to grow biotite crystals large enough for analysis, runs usually lasted from 30 to 45 days. In every charge, a few trace elements were initially doped at the 0.1—0.5 wt. % level and analyzed by electron microprobe after the run. First-row transition metal ions are highly compatible in biotite with Dbiotite/melt of 17 for Ti, 35 for V, 47 for Co, 174 for Ni, and 5.8 for Zn. A very notable exception is Cu with Dbiotite/melt

Environmental Earth Sciences, 2015

Natural gas storage caverns are usually built either in a rock salt dome or in a bedded rock salt... more Natural gas storage caverns are usually built either in a rock salt dome or in a bedded rock salt formation. In China, the rock salt stratum has the following characteristics: thin, inter-layered, bedded and highly impure. The height of a cavern in bedded rock salt deposit in China depends largely on the thickness of the salt formation (80-300 m) and is much smaller than in thick domes and thick layers of rock salt deposits in Europe and the USA, where a storage cavern is typically, cylindrically shaped. This renders the diameter of a bedded salt cavern in China to be greater than its height to obtain sufficient storage volume. The stability of horizontal salt caverns with different diameters and varied minimal internal pressures during its long-term operation was numerically analyzed in this paper regarding convergence, damage, stress to peak or dilatancy strength ratio and effective strain. The Hou/Lux constitutive model including some relevant parameters from previous laboratory investigations were used for the numerical simulations. The simulation results show that a horizontal cavern (height ( length) for underground gas storage is more efficient, profitable and suitable because of its few times larger storage capacity than the most used vertical cavern (height ) diameter) or a prolate cavern (height & or \ diameter) under the Chinese special geological condition of thin-bedded rock salt deposits, provided with appropriate construction and operation parameters.

Environmental Earth Sciences, 2015

In this paper, numerical and semi-analytical investigations were conducted to understand the hydr... more In this paper, numerical and semi-analytical investigations were conducted to understand the hydraulic fracturing operation in the tight gas reservoir only identified by the code A7 in the North German Basin. Two simulators, FLAC3D plus (numerical, full 3D model) and MFrac (semi-analytical, modified model based on the conventional pseudo-3D model), were used to model the fracturing operations including fracture propagation, proppant transport and settling. A comparison of the two simulators was carried out through simulations. Meanwhile, the function of the geological barrier integrity in A7 was also studied and confirmed. The simulations were based on the history matching of the in situ measured well head pressure. At the end of the simulation, a long fracture (length ) height) was modeled by both simulators. Although the results express some differences in the modeled fracture length and width (average), their results for fracture pressure, fluid leak off and proppant distribution are comparable. That means they would provide similar productivity in the later production. Investigation of the geological barrier integrity confirms that the cap rock, formed from rock salt and anhydrite, normally has higher minimum horizontal stress and lower permeability providing enough resistance to prevent the fracture from propagating in the vertical direction of the cap rocks. The case study reveals that, even when the injection volume was increased 10 times the initial volume, the integrity of the cap rocks could not be broken. Despite the presence of interbedded shale formations, in the reservoir as those of the cap rock an impediment function, if their thicknesses are too small to prevent their breakage and the injection volume too large for them to resist fracturing.

Environmental Earth Sciences, 2015

Hydraulic fracturing is widely used in the petroleum engineering to enhance the reservoir conduct... more Hydraulic fracturing is widely used in the petroleum engineering to enhance the reservoir conductivity. The most 3D simulators for modeling hydraulic fracturing assume that the created fracture propagates perpendicular to the minimum horizontal stress. In the reality, the fracture orientation is more or less affected by many factors, e.g. the reservoir heterogeneity. If the fracture is strongly reoriented, then such 3D simulators could not be used. Therefore, it is essential to investigate the orientation problem. In this paper, a 2D numerical approach to model hydraulic-driven fractures with arbitrary orientation in tight gas reservoirs is presented. The approach is based on the elasticity and lubricant theory with consideration of fully hydro-mechanical coupling effects. It was solved using a combination of the extended finite element and the finite volume methods. The approach was verified by modeling the dynamical growth of a KGD fracture (a model developed by Kristnovitch-Geertsma-Daneshy). Three numerical examples are illustrated. The first example is hydraulic fracturing in a heterogeneous reservoir. The hydraulic fracture propagated asymmetrically with approx. 7 m or 1.5°deviation in the direction of maximum principal stress. In the second example, a two stage multiple hydraulic fracturing was modeled. The influences of the perforation and the first fracture on the second fracture were observed from the modeling results. In the third example, a re-fracturing operation after 1 year of production was modeled. Due to the irregular stress change, an almost 90°fracture reorientation was observed.

Environmental Earth Sciences, 2015

Environmental Earth Sciences, 2015

Geothermal energy is renewable, sustainable and available in large amounts. The hot dry rock (HDR... more Geothermal energy is renewable, sustainable and available in large amounts. The hot dry rock (HDR) systems, in particular, have the largest potential for longterm sustainability and therefore draw a lot of attention. The combination of horizontal wells and the technology of multiple transverse fractures technology is an attractive approach of such HDRs, which is called in this paper as the specific EGS (enhanced geothermal system). The main objective of this paper is to study the heat extraction over a period of 20 years by water circulating in a deep geothermal reservoir using this specific EGS. The wellbore flow module, T2WELL/ECO2N, is implemented in the parallelized simulator TOUGH2MP as a new code TOUGH2MP-WELL/EOS3, which enables coupled wellbore-reservoir simulations. Using this newly developed code, the sensitivity of heat extraction against various parameters of the formation and fractures is assessed. The influences of the fracture geometry and the space of two neighborhood fractures on the geothermal energy performance are analyzed. According to the results in this paper, the injection rate, the wellbore radius and the fracture permeability are three main influence factors for the distribution of total flow through individual fractures. A large injection rate, a small wellbore radius and a large fracture width will cause the short-circuit effect, which reduces the performance of this specific EGS. Compared with the classic HDR system (doublet or triplet vertical wells ? single fracture), this specific EGS has a much higher performance and a longer duration of the economic production. Furthermore, the horizontal wells can also be drilled in both directions of the minimum horizontal stress instead of in just one direction, so that the energy performance is doubled and the investment cost per kW energy production is decreased and the advantage of this specific EGS is significantly enlarged.