Effect of Fracturing Fluid/Shale Rock Interaction on the Rock Physical and Mechanical Properties, the Proppant Embedment Depth and the Fracture Conductivity (original) (raw)
Related papers
Hydraulic Fracture Conductivity in Shale Reservoirs
IntechOpen eBooks, 2022
Optimum conductivity is essential for hydraulic fracturing due to its significant role in maintaining productivity. Hydraulic fracture networks with required fracture conductivities are decisive for the cost-effective production from unconventional shale reservoirs. Fracture conductivity reduces significantly in shale formations due to the high embedment of proppants. In this research, the mechanical properties of shale samples from Sungai Perlis beds, Terengganu, Malaysia, have been used for computational contact analysis of proppant between fracture surfaces. The finite element code in ANSYS is used to simulate the formation/ proppant contact-impact behavior in the fracture surface. In the numerical analysis, a material property of proppant and formation characteristics is introduced based on experimental investigation. The influences of formation load and resulted deformation of formation are calculated by total penetration of proppant. It has been found that the formation stresses on both sides of fractured result in high penetration of proppant in the fracture surfaces, although proppant remains un-deformed.
Interaction of Fracturing Fluids with Shales: Proppant Embedment Mechanisms
The University of Texas at Austin, MS Thesis, 2020
In petroleum engineering, hydraulic fracturing has been developed to mitigate the crucial problem of the world's dwindling oil supplies. Thanks to hydraulic fracturing, engineers can create new artificial apertures with pressurized fluids. The process includes the high-pressure injection of a fracking fluid, which is basically water and proppants. Hydrocarbons will flow more freely after the flow back of water. Once the pumping of fracturing fluid is stopped, created fractures begin to close, as the stress increases. This has become a critical issue because closing these fractures results in a rapid decline in productivity of the well. The primary reason for proppant usage is to settle between fracture apertures and prop them open in order to increase oil and gas productivity. Proppant embedment is a crucial problem that causes many fractures to fail over time. Fractured well productivity can be dramatically reduced by severe proppant embedment due to a reduction in fracture aperture. Accordingly, understanding the proppant embedment phenomena is essential for hydraulic fracturing treatments. In this thesis, the mechanisms of proppant embedment have been investigated by quantifying the stress-dependent deformations (elastic and plastic) as well as the time-dependent deformation (creep). A set of constitutive equations were developed to account for elastic, plastic, and creep deformation during proppant embedment. Two new experimental apparatuses have been built and used to quantify the shale rock proppant deformation behavior (elastic, plastic, and creep) after exposure to various fracture fluid additives such as surfactants and clay stabilizers. Results show that proppant embedment primarily occurs due to plastic deformation followed by time-dependent creep deformation, while elastic deformation is small. The impact of different fracturing fluids and rock mineralogy on proppant embedment were also studied. Our results show that fluid chemistry substantially affects the amount of plastic deformation and creep. For example, KCI with a Clay Inhibitor was quite successful in reducing the proppant embedment. Shales with high clay-content embedded proppant at lower stresses and showed more plastic deformation. The test results show that 15% more clay-content shale samples experienced almost 50% more deformation. Chemical treatments fostered the best improvements or degradations in high clay-content shales.
Laboratory study of proppant on shale fracture permeability and compressibility
Fuel, 2018
Hydraulic fracturing is key for shale gas production and fracture permeability or conductivity is one of the most important parameters for gas production rate. Investigating the proppant distribution and fracture permeability in the field is difficult, therefore, laboratory study is a good alternative. In this work, the effect of the layer number and type of proppant on fracture permeability and compressibility were investigated. A cubic shale sample from the Cambrian Niutitang Formation at Sangzhi, Hunan Province, China, was used in this work. Sands and glass beads of different number of layers were added into an artificial fracture and seven cases, including original sample, non-propped fracture, and four kinds of propped fractures were considered. Permeability at three gas pressure steps and five confining pressure steps were measured in each case at two flow directions. Microscopic X-ray computed tomography was used to detect the distributions of proppant, and the relationship with permeability and its anisotropy was studied. A permeability model combining the stress and Klinkenberg effects was used to match experimental data and a new fracture compressibility model was proposed to predict the change of fracture compressibility with the layer number of proppant. It was found that permeability and compressibility of proppant supported fracture are closely related to proppant packing pattern and layer number, as well as the permeability anisotropy. These results improve our understanding on permeability behaviour for the proppant supported fracture and can assist in the model of fracture permeability and simulation of shale gas production.
Applied Sciences, 2017
The aim of this study was to identify the influence of reservoir depth on reservoir rock mass breakdown pressure and the influence of reservoir depth and injecting fluid pressure on the flow ability of reservoirs before and after the hydraulic fracturing process. A series of fracturing tests was conducted under a range of confining pressures (1, 3, 5 and 7 MPa) to simulate various depths. In addition, permeability tests were conducted on intact and fractured samples under 1 and 7 MPa confining pressures to determine the flow characteristic variations upon fracturing of the reservoir, depending on the reservoir depth and injecting fluid pressure. N 2 permeability was tested under a series of confining pressures (5, 10, 15, 20 and 25 MPa) and injection pressures (1-10 MPa). According to the results, shale reservoir flow ability for gas movement may reduce with increasing injection pressure and reservoir depth, due to the Klinkenberg phenomenon and pore structure shrinkage, respectively. The breakdown pressure of the reservoir rock linearly increases with increasing reservoir depth (confining pressure). Interestingly, 81% permeability reduction was observed in the fractured rock mass due to high (25 MPa) confinement, which shows the importance of proppants in the fracturing process.
Experimental study of permeability and its anisotropy for shale fracture supported with proppant
Journal of Natural Gas Science and Engineering, 2017
Shale gas is an important unconventional natural gas resource, but shale has extremely low permeability. Production of shale gas can be improved by using proppants for hydraulic fracturing and maintaining fracture conductivity, and a better understanding of the permeability and its anisotropy of proppant-supported fractures would be useful in optimising gas production. This paper described experiments on shale permeability and its anisotropy with respect to gas pressure, effective stress and gas type for a natural fracture supported with two sizes of proppant. A cubic sample from the Silurian Longmaxi formation in the Sichuan Basin, China, was used in the study; the testing direction of the sample was altered, and both helium (non-sorbing) and methane (sorbing) were tested. Microscopic X-ray computerised tomography (CT) scanning was used to reveal the proppant distribution and fracture shape. Finally, an analytical model was applied to describe the permeability with respect to pore pressure and effective stress and the results were used to determine the relationships between initial fracture compressibility, Klinkenberg coefficient and absolute permeability. The permeabilities of propped fractures were found to be a few hundred or even a few thousand times higher than those of the natural fracture under the same experimental conditions, with both the proppant size and the amount of proppants added affecting this increase. The permeability was anisotropic in two horizontal directions. The direction and ratio of permeability anisotropy of the proppant-supported fracture differed from those of the natural
Fracturing Fluids Effect on Mechanical Properties in Shales
Proceedings of the 8th Unconventional Resources Technology Conference
The URTeC Technical Program Committee accepted this presentation on the basis of information contained in an abstract submitted by the author(s). The contents of this paper have not been reviewed by URTeC and URTeC does not warrant the accuracy, reliability, or timeliness of any information herein. All information is the responsibility of, and, is subject to corrections by the author(s). Any person or entity that relies on any information obtained from this paper does so at their own risk. The information herein does not necessarily reflect any position of URTeC. Any reproduction, distribution, or storage of any part of this paper by anyone other than the author without the written consent of URTeC is prohibited.
OTC 2022, 2022
Shale gas reservoirs in the LongMaxi Formation, Sichuan Basin, are the biggest proven shale gas reservoirs in China. The natural fracture system plays an important role in the shale gas exploration and development. Both high-angle calcite-cemented fractures and low-angle bedding fractures were observed in the core and resistivity image logs but the relationships between these two kinds of fractures is still not clear. Identifying and characterizing the natural fracture system in the shale gas reservoir were the objective of this case study. The borehole resistivity image logs were acquired in several shale gas wells in southeast Sichuan Basin. We observed a spiky resistivity log response in some intervals of Longmaxi Formation. Image logs reveal that all these intervals have low-angle conductive features. Based on the crosscutting relationship of bedding and fractures, these features are conductive bedding fractures, which cut the high-angle resistive fractures in the formation. Both conductive and resistive bedding fractures are identified in the image logs and core data. The geochemical spectroscopy logs in this interval show low pyrite volume, which indicates the conductive bedding fractures are not filled by conductive minerals. It could be an effective pathway of fluid flow. The natural fractures can be divided into three types: low-angle conductive bedding fracture, low-angle resistive bedding fracture, and high-angle resistive fracture. The bedding fractures and high-angle resistive fractures are mainly in the brittle zone of the LongMaxi Formation. We observed that the high density of the conductive bedding fractures will appear spiky on the resistivity logs, which will cause high horizontal permeability. Multiwell correlation shows that there will be more conductive bedding fractures in areas where the shale gas reservoir was undergoing tectonic stresses, which could have a negative impact on the shale gas enrichment. The bedding fractures were formed after the high-angle resistive fractures. Some bedding fractures were cemented by calcite, while some are still open. The identification of bedding fractures will help to understand the complex natural fracture system in the tight shale gas reservoir. The conductive bedding fractures will greatly enhance the horizontal permeability and will influence the fluid flow dramatically.
Rock Mechanics and Rock Engineering, 2018
Shale gas has recently gained significant attention as one of the most important unconventional gas resources. Shales are fine-grained rocks formed from the compaction of silt-and clay-sized particles and are characterised by their fissured texture and very low permeability. Gas exists in an adsorbed state on the surface of the organic content of the rock and is freely available within the primary and secondary porosity. Geomechanical studies have indicated that, depending on the clay content of the rock, shales can exhibit a brittle failure mechanism. Brittle failure leads to the reduced strength of the plastic zone around a wellbore, which can potentially result in wellbore instability problems. Desorption of gas during production can cause shrinkage of the organic content of the rock. This becomes more important when considering the use of shales for CO 2 sequestration purposes, where CO 2 adsorption-induced swelling can play an important role. These phenomena lead to changes in the stress state within the rock mass, which then influence the permeability of the reservoir. Thus, rigorous simulation of material failure within coupled hydro-mechanical analyses is needed to achieve a more systematic and accurate representation of the wellbore. Despite numerous modelling efforts related to permeability, an adequate representation of the geomechanical behaviour of shale and its impact on permeability and gas production has not been achieved. In order to achieve this aim, novel coupled poro-elastoplastic analytical solutions are developed in this paper which take into account the sorption-induced swelling and the brittle failure mechanism. These models employ linear elasticity and a Mohr-Coulomb failure criterion in a plane-strain condition with boundary conditions corresponding to both open-hole and cased-hole completions. The post-failure brittle behaviour of the rock is defined using residual strength parameters and a non-associated flow rule. Swelling and shrinkage are considered to be elastic and are defined using a Langmuir-like curve, which is directly related to the reservoir pressure. The models are used to evaluate the stress distribution and the induced change in permeability within a reservoir. Results show that development of a plastic zone near the wellbore can significantly impact fracture permeability and gas production. The capabilities and limitations of the models are discussed and potential future developments related to modelling of permeability in brittle shales under elastoplastic deformations are identified.
International Journal of Coal Geology, 2019
A series of pressure-dependent permeability experiments were conducted on the Lower Silurian Longmaxi organic-rich shales in northeast Yunnan area, southern China, to investigate the effects of sedimentary bedding, fracture surface roughness, frac ture offset, fracture aperture and effective pressure on shale permeability. The results demonstrate that the sedimentary bedding of shale only has a slight influence on matrix permeability, while the fractures can enhance shale permeability dramatically. Even at a maximum effective pressure of 48 MPa, the permeabilities of aligned fractures (without fracture offset) can increase by about one order of magnitude over the permeabilities of the shale matrix. A power-law relation appears to better describe the pressure-dependency of permeability for the tested shale samples than the exponential relationship.
Experimental Investigation on the Crack Evolution of Marine Shale with Different Soaking Fluids
Frontiers in Earth Science, 2021
Hydration induced cracks could promote the complexity of hydraulic fractures in marine shale gas reservoir. But the evolution process and forming mechanism has not been fully investigated. In this paper, Longmaxi marine shale were collected and immersed in three types of fluids (distilled water, fracturing fluid, and mineral oil) for more than 10 days. The spatial-temporal evolution of soaking fractures was recorded and analyzed. A fracture mechanical model was established, considering the effects of in-situ stress, fluid pressure, hydration stress, and capillary force. The promotion mechanism of hydration cracks in forming complex fracking network was discussed. Results showed that hydration fractures were extremely developed and evenly distributed in a state of network for specimens immersed in distilled water. For specimens soaked in fracturing fluid, the hydration cracks were moderately developed for the addition of anti-swelling agent. Fractures were rarely developed for specim...