Davide Gei | Istituto Nazionale di Oceanografia e Geofisica Sperimentale (original) (raw)

Papers by Davide Gei

Oil & Gas Science and Technology – Revue d’IFP Energies nouvelles, 2015

Risk Assessment-Led Characterisation of the SiteChar UK North Sea Site for the Geological Storage... more Risk Assessment-Led Characterisation of the SiteChar UK North Sea Site for the Geological Storage of CO 2 Caractérisation d'un site de stockage géologique de CO 2 situé en Mer du Nord (Royaume-Uni) sur la base d'une analyse de risques

Frontiers in Earth Science

Permafrost in the NE European Russian Arctic is suffering from some of the highest degradation ra... more Permafrost in the NE European Russian Arctic is suffering from some of the highest degradation rates in the world. The rising mean annual air temperature causes warming permafrost, the increase in the active layer thickness (ALT), and the reduction of the permafrost extent. These phenomena represent a serious risk for infrastructures and human activities. ALT characterization is important to estimate the degree of permafrost degradation. We used a multidisciplinary approach to investigate the ALT distribution in the Khanovey railway station area (close to Vorkuta, Arctic Russia), where thaw subsidence leads to railroad vertical deformations up to 2.5 cm/year. Geocryological surveys, including vegetation analysis and underground temperature measurements, together with the faster and less invasive electrical resistivity tomography (ERT) geophysical method, were used to investigate the frozen/unfrozen ground settings between the railroad and the Vorkuta River. Borehole stratigraphy and...

Carbon dioxide (CO2) sequestration consists in injecting and storage the gas into a geologic form... more Carbon dioxide (CO2) sequestration consists in injecting and storage the gas into a geologic formation as a means of mitigating the greenhouse effect. Among the storage sites, saline aquifers are very promising because of their large capacity and wide availability. In this work, we perform a sensitivity analysis on the response of a flow simulator to study the impact of capillary pressure in CO2 injection and storage. Capillary pressure is represented as a exponential function of CO2 saturation. The simultaneous flow of CO2 and brine in porous media is based on the well known Black-Oil formulation, applied to two-phase fluid flow. It considers that CO2 may dissolve in the brine but the brine is not allowed to vaporize into the CO2 phase. This formulation uses the PVT data as a simplified thermodynamic model. The numerical solution is obtained applying the public-domain software BOAST, which solves the differential equations by finite differences with the IMPES algorithm. Besides, we...

Netherlands Journal of Geosciences, 2016

The Boom Clay is being investigated as a host rock for disposal purposes of radioactive wastes. A... more The Boom Clay is being investigated as a host rock for disposal purposes of radioactive wastes. Although the formation is relatively uniform and homogeneous, there are embedded septaria bodies (carbonates) or layers of septaria that may constitute a problem regarding the integrity of the clay. It is therefore essential to locate these geobodies, particularly with seismic experiments. Since the medium shows strong attenuation it is necessary to correct for this amplitude loss if true amplitudes of the reflections are required when imaging these bodies after the stack. To achieve this task, we implement a reverse-time migration algorithm based on a dispersionless anelastic rheology, that is, the phase velocity and attenuation factor are frequency independent, and back-propagation is performed with a negative quality factor,Q. The algorithm is tested on synthetic data. For this we assume that the septaria are diffractors generating waves synchronously to simulate a stacked seismic sect...

Energy Procedia, 2017

CO 2 geological storage (CGS) is an efficient tool to mitigate climate change and to continue use... more CO 2 geological storage (CGS) is an efficient tool to mitigate climate change and to continue use of fossil fuels for energy production. Renewable energy is becoming more popular to reduce impact of human activity on the Earth. However, oil is still the most profitable energy source and will remain the same in the future. Taking into account the development of CGS and profit of oil production, the combining Enhanced Oil Recovery (EOR) technology in the depleted oil reservoirs with CGS (CO 2-EOR) is well-known commercial practice and has been already successfully implemented during last decades. Use of CO 2 offers an attractive opportunity to increase effectively oil production in the depleted oil reservoirs. CO 2-EOR ensures permanent storage of large quantities of CO 2 underground (EOR+) reaching the goals to reduce the greenhouse gas effect on the Earth's atmosphere, achieving win-win situation [1]. Nevertheless, when using underground oil recovery usually has the highest priority and can cause a conflict of interest with other underground uses. Such a case in the Baltic Sea Region was studied for the first time. The E6 offshore structure was found by seismic exploration and explored in 1984 by one well E6-1 (depth 1068 m), located 37 km from coast of Latvia [2, 3]. The structure was estimated as prospective for oil in 10.5 m thick oil-bearing reservoir layer of the Saldus Formation in the Upper Ordovician Porkuni Stage. The fractured-porous reservoir is represented by carbonate rocks with average porosity 18% and gas permeability up to 39 mD. Oil flow was 2.7 m 3 per 24 hours [4]. Oil shows were found in the sandstones of the Cambrian Series 3 Deimena Formation and in the Devonian rocks. The E6 structure was licensed to the Odin oil company in 2008 for oil exploration and production. The Deimena Formation (53 m thick reservoir sandstones) in the E6 structure, located 135.5 m lower under the oil reservoir of Saldus Formation. It is covered by Lower Ordovician clayey primary cap rock and additionally covered by Ordovician and Silurian (in total 266 m thick) clayey carbonate rocks and Devonian siliciclastic and carbonate rocks [2, 3]. According to [5], fault system within the structure has led to the migration of hydrocarbons from the Cambrian reservoir to the upper Ordovician reservoir. The E6 structure was estimated as the most prospective for CO 2 storage in the Deimena Formation sandstones offshore Latvia [2, 3]. In our study we are discussing possibility of CGS in the Cambrian Deimena Formation and CO 2-EOR from the Upper Ordovician Saldus Formation. During exploration, the inflow of light oil was insignificant due to low pressure in the reservoir. Injection of CO 2 into the oil reservoir will significantly increase pressure and consequently oil extraction. CO 2-EOR method has been already tested for the Cambrian Deimena Formation ROZ (residual oil zone) in the Baltic Sea Region by the oil company. Two pilot injections have been made into Lithuanian onshore oil fields (about 2 km depth) for EOR-CCS in 2013, investigating potential of CO 2 to be used for EOR. The results

Petroleum Geoscience, 2016

Time-lapse numerical seismic modelling based on rock physics studies was for the first time appli... more Time-lapse numerical seismic modelling based on rock physics studies was for the first time applied to analyse the feasibility of CO 2 storage monitoring in the largest Latvian offshore geological structure E6 in the Baltic Sea. The novelty of this approach was the coupling of the chemically induced petrophysical alteration effect of CO 2-hosting rocks measured in laboratory with time-lapse numerical seismic modelling. Synthetic seismograms were computed for the E6 structure, where the sandstone reservoir of the Deimena Formation of Cambrian Series 3 (earlier Middle Cambrian) was saturated with different concentrations of CO 2. The synthetic seismograms obtained after CO 2 injection were compared with the baseline. The following four scenarios were considered: (1) a uniform model without the alteration effect; (2) a uniform model with the alteration effect; (3) a plume model without the alteration effect; and (4) a plume model with the alteration effect. The presence of CO 2 in the reservoir layers can be detected by direct comparison and interpretation of plane-wave synthetic seismic sections, and is clearly observed when one displays the difference between the baseline and post-CO 2 injection synthetics. The normalized root-mean-square imaging techniques also clearly highlight the time-lapse differences between the baseline and post-injection seismic data. The laboratory-conducted alteration of the petrophysical properties of the reservoir had a strong influence on the reflected signals in the seismic sections. The greatest difference was revealed on seismic sections with 1% CO 2 saturation, increasing the detectability of the stored CO 2. The difference decreased with an increase in CO 2 content. The saturation of CO 2 could be qualitatively estimated up to a value of 5%. Higher saturation produced a strong signal in the repeatability metrics but the seismic velocity varied so slightly with the increasing gas content that the estimation was challenging. A time shift or push-down of the reflectors below the CO 2 storage area was observed for all scenarios. According to changes in the amplitude and two-way travel times in the presence of CO 2 , reflection seismics could detect CO 2 injected into the deep aquifer formations even with low CO 2 saturation values. Our data showed the effectiveness of the implemented time-lapse rock physics and seismic methods in the monitoring of the CO 2 plume evolution and migration in the E6 offshore oil-bearing structure. The new results obtained could be applied to other prospective structures in the Baltic region.

London 2013, 75th eage conference en exhibition incorporating SPE Europec, 2013

ABSTRACT This study is a part of CO2 geological storage research in the Baltic Region. Our resear... more ABSTRACT This study is a part of CO2 geological storage research in the Baltic Region. Our research shows the effectiveness of seismic numerical modelling methodology, in particular, synthetic plane-wave and difference sections, to detect the presence of CO2 in deep saline aquifer of Cambrian Deimena Sandstone Reservoir Formation in the E6 Baltic offshore structure for various saturation levels. Our results clearly show the applicability of seismic methods to monitor CO2 plume within the studied Baltic Sea offshore storage site E6. This study plays a crucial role in developing an optimal seismic monitoring plan in the studied area.

The Leading Edge, 2011

The monitoring of induced or triggered microseismic events increasingly is being used to inform t... more The monitoring of induced or triggered microseismic events increasingly is being used to inform the efficient production of unconventional reservoirs. A key aspect of economic production in these low-permeability rocks is hydraulic fracture stimulation, usually in horizontal wells. To evaluate the success of the stimulation, engineers rely on monitoring the induced (or triggered) microseismic events that are then interpreted to map the stimulated reservoir volume and likely drainage area of the well. These microseismic events can be mapped either from downhole or surface monitoring arrays. In this study, we discuss a newly developed methodology that allows economic and consistent mapping of microseismic events from multiple stimulated wells across an entire field. This approach allows better comparison of stimulation techniques between wells in order to optimize long-term development of the reservoir. As well, the method enables a relatively robust observation of velocity anisotropy...

The Leading Edge, 2013

CO2 plume imaging is a required step in CO2 geological storage for both performance assessment an... more CO2 plume imaging is a required step in CO2 geological storage for both performance assessment and risk management purposes. This work has been performed in the frame of the CO2CARE project, and its aim is to develop tools and methodologies to monitor CO2 migration and verify the long-term well integrity after site abandonment. The timely detection of an anomaly is essential to perform a suitable remediation. For this purpose, downhole tools are permanently installed, but it is important to check the resolution and efficiency of the adopted techniques. In particular, this study investigates the possibility of using electrical resistivity tomography (ERT) to image CO2 migrations around observation boreholes through a sensitivity study.

GEOPHYSICS, 2013

Direct seismic waves (P- or S-waves) are used to locate and further characterize microseismic eve... more Direct seismic waves (P- or S-waves) are used to locate and further characterize microseismic events. The resolution of information obtained from direct waves depends on the peak frequencies of the waveforms. The peak frequency results from combination of the source, propagation, and the receiver effects. For frequencies below the corner frequency, propagation effects control the peak frequency in observed seismograms of microseismic events. The frequency dependence of direct body waves can be modeled by attenuation, specifically the global attenuation factor. This model is consistent with observed data along surface profiles explaining the difference between the peak frequencies of P- and S-waves. In addition, the model is consistent with the peak frequencies observed on downhole monitoring arrays. This can be used to invert effective attenuation providing additional unique measurement from microseismic events. The corner frequency can be estimated from the average stress drop and ...

Journal of Physics: Conference Series, 2013

Carbon Dioxide (CO2) sequestration into geologic formations is a means of mitigating greenhouse e... more Carbon Dioxide (CO2) sequestration into geologic formations is a means of mitigating greenhouse effect. In this work we present a new numerical simulation technique to model and monitor CO2 sequestration in aquifers. For that purpose we integrate numerical simulators of CO2-brine flow and seismic wave propagation (time-lapse seismics). The simultaneous flow of brine and CO2 is modeled applying the Black-Oil formulation for two phase flow in porous media, which uses the Pressure-Volume-Temperature (PVT) behavior as a simplified thermodynamic model. Seismic wave propagation uses a simulator based on a space-frequency domain formulation of the viscoelastic wave equation. In this formulation, the complex and frequency dependent coefficients represent the attenuation and dispersion effect suffered by seismic waves travelling in fluid-saturated heterogeneous porous formations. The spatial discretization is achieved employing a nonconforming finite element space to represent the displacement vector. Numerical examples of CO2 injection and time-lapse seismics in the Utsira formation at the Sleipner field are analyzed. The Utsira formation is represented using a new petrophysical model that allows a realistic inclusion of shale seals and fractures. The results of the simulations show the capability of the proposed methodology to monitor the spatial distribution of CO2 after injection.

GEOPHYSICS, 2003

We use seismic modeling methods to validate the interpretation of deep‐crust seismic exploration.... more We use seismic modeling methods to validate the interpretation of deep‐crust seismic exploration. An approximation of the stacked section is obtained with the nonreflecting wave equation and the exploding‐reflector approach. Using this technique and ray‐tracing algorithms, we obtain a geological model by comparing the synthetic section with the real stacked section. An isotropic constitutive equation is assumed in this phase. The exact synthetic stacked section is then obtained by applying the standard processing sequence to a set of synthetic common‐shot profiles computed with the variable‐density acoustic wave equation. We introduce elliptical P‐wave anisotropy and the effects of small‐scale inhomogeneities by using a von Kármán autocovariance probability function that simulates scattering Q effects. Verification of the geological model by poststack migration constitutes an additional test. The methodology, which is suitable for areas of complex geology, is applied to a seismic li...

Pure and Applied Geophysics, 2019

Absolute rock permeability depends on several factors, mainly porosity, φ, the geometry of the po... more Absolute rock permeability depends on several factors, mainly porosity, φ, the geometry of the pore network (tortuosity), and the grain geometry, dimension and composition. The mineralogy composition plays an important role, mostly clay, which involves several components such as illite, smectite, kaolinite and chlorite. The presence of quartz and feldspar increases permeability, while clay minerals and calcite tend in the opposite direction. Basically, permeability decreases with a smaller grain radius, increasing tortuosity of the pore space and decreasing porosity. As the specific surface area of the pores increases, permeability decreases. Here, we compare four expressions for permeability based on clay content, grain dimension, tortuosity and mineral composition. All the expressions contain somehow the Kozeny-Carman (KC) factor φ 3 /(1−φ) 2 , which is obtained on physical grounds, and rely on fitting parameters related to the geometrical characteristics of the rock and its composition. Herron model is based on geochemical mineralogy composition. Despite the highly idealizations on which these models are based, the results indicate the prediction power of the Kozeny-Carman equation, provided that proper calibration is performed.

GEOPHYSICS, 2019

The generation of microseismic events is often associated with induced fractures/faults during th... more The generation of microseismic events is often associated with induced fractures/faults during the extraction/injection of fluids. A full characterization of the spatiotemporal distribution of microseismic events provides constraints on fluid migration paths in the formations. We have developed a high-resolution source imaging method — a hybrid multiplicative time-reversal imaging (HyM-TRI) algorithm, for automatically tracking the spatiotemporal distribution of microseismic events. HyM-TRI back propagates the data traces from groups of receivers (in space and time) as receiver wavefields, multiplies receiver wavefields between all groups, and applies a causal integration over time to obtain a source evolution image. Using synthetic and field-data examples, we revealed the capability of the HyM-TRI technique to image the spatiotemporal sequence of asynchronous microseismic events, which poses a challenge to standard TRI methods. Moreover, the HyM-TRI technique is robust enough to pr...

Geophysical Journal International, 2017

In a lossy medium with complex frequency-dependent wave speed both rays and plane waves at an int... more In a lossy medium with complex frequency-dependent wave speed both rays and plane waves at an interface should satisfy the dispersion relation (that is, the wave equation), the radiation condition (the amplitude should go to zero at infinity) and the horizontal complex slowness should be continuous (Snell's law). It is known that this may lead to a transmitted wave which violates the radiation condition and which also causes problems with the phase of the reflection coefficient. In fact, ray-tracing algorithms and analytical evaluations of the reflection and transmission coefficients in anelastic media may lead to non-physical solutions related to the complex square roots of the vertical slowness and polarizations. The steepest-descent approximation with complex horizontal slowness involves non-physical complex horizontal distances, and in some cases also a non-physical vertical slowness that violates the radiation condition. Similarly, the reflection and transmission coefficients and ray-tracing codes obtained with this approach yields wrong results. In order to tackle this problem, we choose the stationary-phase approximation with real horizontal slowness. This gives real horizontal distances, the radiation condition is always satisfied and the reflection and transmission coefficients are correct. This is shown by comparison to full-wave space-time modelling results by computing the reflection and transmission coefficients and respective phase angles from synthetic seismograms. This numerical evaluation is based on a 2-D wavenumber-frequency Fourier transform. The results indicate that the stationary-phase method with a real horizontal slowness provides the correct physical solution.

Journal of Earthquake Engineering, 2014

We present a detailed approach to implement a moment-tensor point source to compute displacements... more We present a detailed approach to implement a moment-tensor point source to compute displacements, particle velocities and accelerations using direct grid methods. Here, the wave modeling algorithm is based on pseudospectral methods to compute the partial derivatives. A comparison to the analytical solution in the 3D acoustic case verifies the discrete implementation of the source in the mesh. Then, the more general 3D elastic case is illustrated and simulations, with and without free surface, are performed that can be used as a reference solution for other grid methods.

Environmental Semeiotics, 2009

We design a numerical algorithm for wave simulation in anelastic media in the presence of free su... more We design a numerical algorithm for wave simulation in anelastic media in the presence of free surface, which can be used to model seismic waves at the Earth's surface. The modeling simulates 3-D waves by using the Fourier and Chebyshev methods to compute the spatial derivatives along the horizontal and vertical directions, respectively. The stress-strain relation is based on the Kelvin-Voigt mechanical model, which has the advantage of not requiring additional field variables. The model is based on two anelastic parameters and twice the spatial derivatives of the lossless case. The methodology is illustrated by simulating the Messina 1908 earthquake.

GEOPHYSICS, 2004

We propose the exploding‐reflector method to simulate a monostatic survey with a single simulatio... more We propose the exploding‐reflector method to simulate a monostatic survey with a single simulation. The exploding reflector, used in seismic modeling, is adapted for ground‐penetrating radar (GPR) modeling by using the analogy between acoustic and electromagnetic waves. The method can be used with ray tracing to obtain the location of the interfaces and estimate the properties of the medium on the basis of the traveltimes and reflection amplitudes. In particular, these can provide a better estimation of the conductivity and geometrical details. The modeling methodology is complemented with the use of the plane‐wave method. The technique is illustrated with GPR data from an excavated tomb of the nineteenth century.