Pore Pressure Field Research Articles

Tomographic evidences for hydraulic fracturing induced seismicity in the Changning shale gas field, southern Sichuan Basin, China

Seismic activity near the shale gas fields in the Sichuan Basin of China has risen significantly in the past few years. Due to the lack of industrial stimulation data to the public, it is difficult to directly link the surge in seismicity near the shale gas fields with hydraulic fracturing stimulation. Meanwhile, it is also challenging to better understand the mechanisms inducing/triggering seismicity in these areas. In the absence of stimulation data, understanding subsurface fluid distribution can go a long way toward addressing these problems. Therefore, in this study, we have applied the double-difference seismic tomography method and the focal mechanism tomography method to the monitoring data of a local seismic array in the Changning shale gas field to build high-resolution three-dimensional models of seismic velocity structure and pore pressure field, then used these models to characterize fluid distributions beneath the shale gas field. From the inverted models, the correlation of overpressure regions (>12 MPa) with moderate-to-high Vp/Vs (1.75-1.90) is clearly observed. With the aid of the exploration data (including well log, active and passive seismic data), the overpressure regions are interpreted to be created by pore pressure or fluid diffusion along pre-existing faults from the shale gas reservoirs, while variations of the Vp/Vs model partly indicate water- and gas-bearing rocks. By jointly analyzing the geophysical model features and earthquake spatio-temporal characteristics, we infer that the earthquakes are primarily caused by increased pore pressures on the seismogenic faults due to the presence of water and/or gas. Besides, the poroelastic stress perturbation and aseismic slip produced by hydraulic fracturing also contribute to the generation of certain earthquake swarms.

Centrifuge experiment and numerical simulation of suction caisson installation in sand

<p indent="0mm">Suction caissons are widely used foundations in offshore engineering. Compared with traditional forms of foundations, a significant advantage of the suction caisson is its cost-effectiveness owing to its simple installation equipment requirement, short installation time, convenience in transportation, and recycle and reuse potential. Changes in excess pore pressure and seepage field caused by penetration and suction have a significant impact on soil resistance around the caisson wall and tip. The centrifuge test and Arbitrary Lagrangian-Eulerian (ALE) large deformation solid-fluid coupled FEM are used in this study to investigate the changes in suction pressure and the seepage field during the suction caisson installation in the sand. The ALE solid-fluid coupled FEM successfully simulates suction caisson installation processes in sand observed in centrifuge tests. The difference in suction penetration resistance is found to be caused by seepage and excess pore pressure generated during suction caisson installation. Therefore, the solid-fluid effective stress analysis is needed in the evaluation of suction caisson penetration to consider the importance of seepage.

Numerical simulation of fluid-driven fracturing in orthotropic poroelastic media based on a peridynamics-finite element coupling approach

In the present manuscript a peridynamics-finite element coupling approach proposed by Sun and Fish (2021) is generalized to simulate fluid-driven fracturing in orthotropic poroelastic media. For this purpose, an orthotropic non-ordinary state-based peridynamics (NOSBPD) model with a bond micromodulus and a critical energy density continuously varying with the bond orientation is proposed. The governing equations of the coupled system involving an orthotropic NOSBPD model aimed at capturing the solid fracture and finite element model (FEM) aimed at modeling fluid flow with an anisotropic permeability tensor is derived. Both the m -convergence and δ -convergence studies are performed to determine appropriate discretization parameters for the proposed orthotropic NOSBPD model. The fracture propagation in the compact tension test with different material orientations is simulated for model verification. The effects of the elasticity and permeability anisotropy, injection rates as well as the existences of multiple fractures and natural fractures have been found to have a significant effect on the fluid-driven fracture propagation in orthotropic poroelastic media. The fracture seems to deviate from the initial notch direction under different material angles, and this trend becomes more prominent with reduction of the elastic modulus and the fracture release rate perpendicular to the bending plane. The permeability anisotropy affects the fluid pattern and the fracture propagation length. The rapid injection rate gives rise to a wider damage zone and a more diffuse pore pressure field .

Numerical simulation of hydraulic fracturing with consideration of the pore pressure distribution based on the unified pipe-interface element model

Understanding hydraulic fracture propagation behaviours is vital for optimizing fracturing design in oil and gas production. It has been observed that the stress and pore pressure distribution are much more complex during fracturing treatment as a result of long-term fluid injection and production in the reservoir. When rock is subjected to internal hydraulic pressure and external mechanical loading, the closing, opening, or other interactions of pre-existing weaknesses or induced new fractures will be altered. In this paper, a number of hydraulic simulation scenarios are performed to investigate the effect of pore pressure distribution on fracture initiation and propagation using the unified pipe-interface element method (UP-IEM). The proposed method for predicting the hydraulic fracture path in the permeable porous medium with non-uniform pore pressure field is validated with experimental results. Considering the arrangement of well patterns and natural fractures in the rock, the problems of hydraulic fracture propagation behaviours with wellbore interaction and pore pressure induced natural fracture activation are simulated. The influence of pore pressure saturation and the injection sequence of wellbores on hydraulic fracturing are researched. A sensitivity analysis is carried out to investigate the parameter effects on pre-existing fracture tortuosity, opening width and breakdown pressure under pore pressure. These numerical examples provide guidance to engineers to estimate the probability of danger or possibilities to improve the injection and production of wells.

Closed-form solution for determination of pore pressure field around horizontal wellbore

Wellbores are usually located in saturated geological layers. The determination of pore water pressure field around the wellbore is necessary during the design calculation and drilling stages. This paper presents analytical approach to determine the pore water pressure field around a horizontal wellbore at deep geological layer that exhibits heterogeneous, isotropic or transversely isotropic behavior. Thus, the wellbore is considered to be in an infinite medium. The pore water pressure at the well wall, equal to the drilling mud pressure, together with the pore water pressure at infinity is assumed to be constant. The closed-form solutions are based on the theory of fluid transport in porous medium and the conformal mapping technique of the complex variable method. The closed-form solutions are established with the condition of transient fluid flow for the case of isotropic medium and with the condition of steady state fluid flow for the case of transversely isotropic medium. The accuracy of the closed-form solutions is validated by numerical solutions based on the finite element method. The obtained solutions can be used as tools to determine quickly and accurately the pore pressure field around the horizontal wellbore, which serves to evaluate the stability of the well wall in preliminary design of the wellbore, as well as the amount of water inflow into it. Furthermore, the closed-form solutions are also considered as reference solutions to evaluate the accuracy and reliability of numerical models.

Analysis of wellbore stability for overbalanced drilling in marine methane hydrate-bearing sediments under non-hydrostatic stresses

New challenges have emerged in drilling marine methane hydrate-bearing sediments (MHBSs); these challenges are closely related to methane hydrate (MH) dissociation and complex multi-physics coupling. This study investigates wellbore stability in overbalanced drilling by proposing exact solutions for multi-physics fields, taking into account the more general non-hydrostatic far-field stresses, partial multi-field coupling and the influences of hydrate dissociation, i.e., the water/heat production/absorption and the change in the physical properties of reservoirs due to hydrate dissociation.Wellbore drilling is simplified as a non-axisymmetric plane strain problem for the mechanical field and an axisymmetric problem for the pressure/temperature/salt concentration fields, in which the drilling domain includes four annular regions with different physical properties. The closed-form solutions for the temperature, pore pressure and salt concentration fields are obtained by considering the influence of water/heat production/absorption in the compatibility conditions of the steady-state governing equations. The solutions can explain the secondary formation of MHs around the dissociation front. The stress and drilling-induced displacement are then derived for the multi-region elastic problem subjected to non-hydrostatic far-field stresses considering seepage flow, with which the initial failure zone can be addressed by considering a failure criterion. The analytical solutions are verified by the good agreement between analytical and numerical results based on the same assumptions. Furthermore, the comparison between the analytical solutions and the results from complex numerical models suggests the applicability of the proposed solutions. Based on the conditions in marine hydrate field trials in the Shenhu area of the South China Sea, the influence of key parameters on wellbore stability in drilling in MHBSs is investigated in detail by the proposed solutions. Some useful dimensionless charts are provided.

A Porothermoelastic Model Considering the Dynamic Temperature-Perturbation Boundary Effect for Borehole Stability in Fractured Porous Rocks

Summary Conventional drilling design tends to inappropriately predict the mud density required for borehole stability of deep fractured porous rocks, such as shale, tight sandstone, and hot dry rock, because it is treated as a single-porosity case and even introduces the influence of weak plane to cover the effect on the fracture system. When the external loadings are applied, fractured porous rocks naturally display two different poromechanical responses of the matrix system and the fracture system considering respective hydraulic and mechanical properties. Besides, the constant temperature difference between the drilling mud and formation rock is often chosen as a boundary condition to solve the temperature balance equation, and thus the incorrect prediction of temperature variations of fractured rock further leads to inappropriate evaluation of the pore pressure and stress fields considering the thermo-hydromechanical (THM) coupling (porothermoelastic model), since a dynamic temperature-perturbation boundary condition related to the temperature at the borehole wall actually accounts for the circulating effect of the drilling mud. Therefore, this paper first uses the API RP 13D (1995) model in combination with the circulating temperature-fields model of Raymond (Raymond 1969) to obtain a set of fully transient analytical solutions to circulating temperature fields, including the four types of temperature inside the drilling pipe, borehole annulus, at the borehole wall, and formation. Furthermore, under local thermal equilibrium (LTE) condition, one considers the dynamic temperature-perturbation boundary condition and provides semianalytical porothermoelastic solutions to the field variables around a vertical borehole subjected to nonhydrostatic stresses in fractured porous rock with dual porosity and dual permeability. The solutions for field variables are obtained in line with the plane strain assumption. The variables include displacements, stresses, and two pore pressures of the matrix system and fracture system. The model is verified by the analytical solutions in the case of a porous medium with a single-porosity one under LTE condition. The main results show that the dual-porosity medium displays a higher borehole instability potential than the single-porosity one. This increasingly cooling effect increases the higher risk of the tensile transverse fracturing when the constant temperature-perturbation boundary condition chooses a smaller temperature difference than that of the dynamic case at a later time. The drilling mud-pressure window narrows with increasing time when the coupled porothermoelastic model is considered. It suggests that the drilling engineer takes into consideration the dynamic temperature-perturbation boundary effect, fracture spacing, and fracture width into the predrilling design of the time-dependent safe mud-pressure window (SMPW).

Discrete element model for the failure analysis of partially saturated porous media with propagating cracks represented with embedded strong discontinuities

A novel, fully coupled hydro-mechanical model for the failure analysis of partially saturated porous media is presented. Failure mechanisms pertain to propagating discontinuities, i.e. tensile cracks and shear bands influenced by matric suction or capillary pressure, that arises in the partially water saturated conditions. In order to simulate propagating discontinuities, a discrete model based on lattice domain representation is used, where embedded strong discontinuities serve for mesh objective propagation of discontinuities. The full coupling is provided with Biot porous media theory, Bishop effective stress concept and nonlinear Richards’s equation for the unsaturated fluid flow. The relation between suction and saturation state is given through the van Genuchten constitutive model. The capabilities of the model are presented with the three representative simulations. The first one is related to unsaturated flow in one dimensional sand column. The second one deals with mechanically influenced propagation of dilatant and compactive shear bands with the goal of examining how these mechanisms influence the pore pressure field, suction development and saturation state of the porous media. The third one explores the important geo-environmental engineering application in which development of tensile cracks is triggered and propagated by drying mechanism of the porous media, also known as desiccation cracking of soils.

Pore Fluid Pressure Imaging of the Mt. Pollino Region (Southern Italy) From Earthquake Focal Mechanisms

AbstractFocal mechanisms of selected earthquakes, recorded in the Mount Pollino region (southern Italy) from 2010 through 2014, are used to infer the pore fluid pressure at hypocenter depths. The 3‐D excess pore pressure field provides evidence that the sequence occurs in a fluid‐filled volume with values reaching 35 MPa. The mechanisms underlying this swarm‐like sequence and the triggering of earthquakes are investigated by computing the cumulative static Coulomb stress change at hypocenter depths and analyzing the pore‐pressure diffusion mechanism. The results indicate that static Coulomb stress change was lower than 0.01 MPa, which is the value generally assumed as threshold for the triggering, and seismicity distribution was actually driven by pore‐pressure diffusion with relatively low diffusivity value. This latter mechanism could also explain the delayed triggering of the two larger eventsML4.3 andML5.0, respectively, that occurred about 150 days apart.

Drained triaxial testing of shales: insight from the Opalinus Clay

The investigation of the mechanical behaviour of swelling claystones and shales is challenging because of their very low permeability and of their high sensitivity to changes in water content. The former makes it difficult to carry out triaxial tests with controlled homogeneous water pore pressure fields, and the latter results in some possible effects of swelling when unsaturated extracted specimens are re-saturated prior to being tested. This work presents some data from drained triaxial tests performed on specimens of Opalinus Clay, designated as host rock for radioactive waste disposal in Switzerland, extracted at shallow depth close to the city of Lausen. The data are compared to those recently published from undrained triaxial tests on specimens from the same place, and also to two sets of recent data independently obtained on Opalinus Clay specimens sourced from greater depth at the Mt Terri Underground Research Laboratory. By comparing the data of our drained tests on Lausen specimens to those from specific undrained tests in which swelling has been prevented, quite a good comparability in shear strength is observed at confining effective stresses larger than 5–6 MPa. This is related to the small magnitude of the swelling occurring above this stress during the specimen hydration in drained tests. The question of the possible linearity of the shear strength criterion at low stress is also discussed with respect to both our data and other published ones. It is suggested that undrained tests be also carried out at low confining stresses to investigate whether some perturbations due to hydration swelling occur in this area that could result in a nonlinear shape of the criterion. The change in Young modulus and Poisson coefficient with stress is also discussed, and the data on Opalinus Clay are compared to those of the Callovo–Oxfordian claystone from France, evidencing interesting similarities in terms of shear strength properties and Young’s moduli.

Interplay of large-scale tectonic deformation and local fluid injection investigated through seismicity patterns at the Reykjanes Geothermal Field, Iceland

SUMMARY Occurrence of seismicity sequences as consequence of fluid injection or extraction has long been studied and documented. Causal relations between injection parameters, such as injection pressure, injection rates, total injected volumes and injectivity, with seismicity derived parameters, such as seismicity rate, cumulative seismic moment, distance of seismicity (RT-plot), b-values, etc. have been derived. In addition, reservoir engineering parameters such as permeability/porosity relations and flow types play a role together with geology knowledge on fault and fracture properties, influenced by the stress field on different scales. In this paper, we study observed seismicity related to water injection at the Reykjanes Geothermal Field, Iceland. The region near the injection well did not experience seismicity before the start of injection. However, we observed continued seismic activity during the 3 months of injection in 2015, resulting in a cloud of about 700 events ranging in magnitude from Mw 0.7 to 3.3. We re-located these events using a modified double-difference algorithm and determined focal mechanism of event subsets. Characteristic for the site is that the events are bound to about 4 km distance to the injection point, and moreover known faults seem to act as barrier to fluids and seismicity. Several repeating sequences of seismicity, defined as bursts of seismicity have hypocenter migration velocities larger than 4 km d–1 and their dominant direction of propagation is away from the injection point towards larger depths. The seismic events within the bursts lack larger magnitude events, have elevated b-values (∼1.5) and consist of many multiplets. Except from the coinciding onset of seismicity with the start of fluid injection, no correlation between injection rates and volumes could be identified, neither could hydraulic diffusivity models explain observed seismicity patterns. Comparison of our results with investigations on background seismicity from 1995 to 2019 and from a seismic swarm in 1972 revealed similar focal mechanism patterns and burst-like seismicity patterns. We finally present a conceptual model where we propose that the observed seismicity patterns represent a stress release mechanism in the area close to the injection well, controlled by an interplay of local pore pressure and stress field changes with continued extensional stress build up at the Reykjanes Ridge.

Solid-Fluid Coupled Numerical Analysis of Suction Caisson Installation in Sand

Suction caissons are widely used foundations in offshore engineering. The change in excess pore pressure and seepage field caused by penetration and suction significantly affects the soil resistance around the caisson wall and tip, and also affects the deformation of the soil within and adjacent to the caisson. This study uses Arbitrary Lagrangian–Eulerian (ALE) large deformation solid-fluid coupled FEM to investigate the changes in suction pressure and the seepage field during the process of the suction caisson installation in sand. A nonlinear Drucker-Prager model is used to model soil, while Coulomb friction is applied at the soil-caisson interface. The ALE solid-fluid coupled FEM is shown to be able to successfully simulate both jacked penetration and suction penetration caisson installation processes in sand observed in centrifuge tests. The difference in penetration resistance for jacked and suction installation is found to be caused by the seepage and excess pore pressure generated during the suction caisson installation, highlighting the importance of using solid-fluid coupled effective stress-based analysis to consider seepage in the evaluation of suction caisson penetration.

Numerical simulation of unstable suction transients in unsaturated soils: the role of wetting collapse

AbstractFluid injection is one of the major triggering factors of instability in unsaturated soils. Robust simulation tools are therefore required to examine hydrologic transients in such a class of inelastic materials. In this paper, the governing equations imposing the balance of mass and momentum in deformable unsaturated porous media are inspected from an analytical perspective. Our main goal is to examine the role of volume collapse on the stability of suction transients. Distinct scenarios are considered in terms of material behavior, distinguishing the case of elastoplastic and viscoplastic materials. It shows that in elastoplastic materials the mathematical ill‐posedness of the partial differential equations leads to a diffusive instability of the pore pressure field, signaled by the loss of stress controllability under water injection. By contrast, the introduction of viscosity restores the mathematical well‐posedness and ensures the positive diffusivity. Numerical implications of these findings have been explored through simulations of water injection with a 1D finite element solver. It shows that an increase of the soil hydraulic sensitivity (i.e., a marked deterioration of the yield stress upon suction removal) can deteriorate the diffusive stability and prevent the computation of suction changes across a collapsible soil layer. While the incorporation of minor amounts of viscosity can restore numerical stability by suppressing the constitutive singularity. Such numerical results have been corroborated by the computation of local stability indicators based on controllability theory, which proved to be robust diagnostic tools to identify the culprit of runaway instability emerging from coupled hydro‐mechanical processes.

Finite element modeling of spherical indentation in a poro‐elasto‐plastic medium via step displacement loading

AbstractA hydromechanically coupled finite element method (FEM) algorithm following a mixed continuous Galerkin formulation for displacement and pore pressure is adopted for modeling of spherical indentation in a poro‐elasto‐plastic medium. The fully saturated porous medium is assumed to be isotropic and elasto‐perfectly plastic, obeying a Drucker‐Prager yield criterion with an associative or non‐associative flow rule. The Newton‐Raphson method with the tangent stiffness scheme is employed to deal with plasticity in the solid skeleton. A stabilization scheme, which permits equal‐order interpolation for the displacement and pore pressure fields and suppresses pore pressure oscillation in the incompressible or nearly incompressible limit, is incorporated in this FEM algorithm. The FEM algorithm is extensively benchmarked with poroelastic analytical solutions to the problems of Terzaghi, Mandel, Cryer, and De Leeuw and an analytic solution for one‐dimensional poro‐elasto‐plastic consolidation. Numerical simulations of poroelastic spherical indentation via step displacement loading are conducted to show that the normalized indentation force as a function of dimensionless time has only relatively weak dependence on material properties through a single derived parameter . Such universality is shown for three types of surface drainage conditions. For indentation in a poro‐elasto‐plastic medium, it is shown that even though plasticity occurs immediately at the undrained limit, if cohesion is within a certain range, there is no plastic strain accumulation during the transient period and the normalized force relaxation behavior could be approximated as poroelastic.

Effect of partial drainage on the pullout behaviour of a suction bucket foundation

Suction-bucket-supported jacket foundations are being used increasingly for offshore wind turbines, the advantages being large bearing capacity and efficient installation. For a large-capacity wind turbine, the pullout capacity and displacement of the bucket in the tripod or tetrapod jacket can be critical to the safety of the structure. The loading rate, which is commonly dominated by the offshore environmental conditions, has a significant effect on the ultimate capacity and stiffness of the bucket foundation. In this paper, a numerical model is developed to investigate the response of a bucket foundation under vertical pullout loading at different rates. A bounding-surface model based on critical state soil mechanics is introduced to account for the soil behaviour under different stress states and drainage conditions. The model is modified and calibrated against a comprehensive set of laboratory tests, including both triaxial compression, triaxial extension and direct simple shear tests in drained and undrained conditions. How partial drainage affects the load–displacement response and pore pressure field are thus presented. A parametric study on the selection of soil parameters is also conducted to shed light on practical foundation design.

The 2019 Mw 5.8 Changning, China earthquake: A cascade rupture of fold-accommodation faults induced by fluid injection

Industrial production activities associated with salt mining, hydraulic fracturing, or geothermal exploitation can induce pore pressure and stress field changes, thus inducing seismic activity. Most of the events are not disastrous due to limited energy radiated. Here we report a Mw 5.8 damaging event that occurred in Changning, China on June 17, 2019, probably the largest induced event by industrial exploitation ever recorded. The earthquake is located within the Changning anticline, and it raises great concerns on why and how the event occurred in a relatively quiescent area in terms of tectonic activity. Moreover, it has been debated whether this earthquake ruptured solely on the fold-accommodation faults or also involved a deep-rooted fault under the anticline. By using InSAR data from two satellites, we investigate the detailed rupture and slip pattern of this earthquake. We propose a cascade rupture mode of the fold-accommodation faults, through poroelastic modeling of excess pore pressure by fluid injection and pumping operations and static Coulomb stress calculation on the pre-stressed faults. We argue that water injections may have triggered this event. The result highlights the importance of reassessing seismic hazard over similar tectonic environments with intensive industrial exploitation.

Numerical simulation of artificial freezing of soils accounting for thermo-hydro-mechanical effects

In the study a thermo-hydro-mechanical model of freezing of saturated soil is presented, with focus on numerical simulation of artificial ground freezing (AGF). Artificial freezing of saturated soils induces such process in the soils as water migration, frost heave and consolidation which can have an effect on the freezing process and surrounding areas. To take into account the important from geotechnical point of view processes the thermo-hydro-mechanical model was developed. The model is based on the fundamental balance equations of continuum media mechanics. The Clausius-Clapeyron equation and constitutive relations of poromechanics are used for describing a relationship between pore pressure, temperature, stress and strain fields. Also an inelastic strain is included accounting for an effect of frost heave. The equations of the model were implemented in Comsol Multiphysics® software and solved using the finite element method relative to variables of porosity temperature and displacement. Numerical simulation of artificial freezing of a soil stratum for a vertical shaft sinking was carried out. A mesh convergence of numerical solution was analyzed. Results of the simulation have shown the model enables to describe a frozen wall formation with a coupled change of porosity, water pore pressure, volumetric strain and mean stress.

A new approach for modeling three-dimensional fractured reservoirs with embedded complex fracture networks

In the last decades, the study of fluid flow through discontinuities such as fractures has received significant attention from the petroleum industry owing to the discovery of highly productive naturally fractured reservoirs. Several approaches based on an explicit or implicit representation of fractures have been adopted for modeling fluid migration in fractured rocks. The explicit approach is the most accurate for such a purpose; however, it is computationally expensive. Alternatively, the implicit representation is very attractive because fracture properties, such as stiffness and permeability can be introduced in the model through dual continuum models. Therefore, the modeling of fluid flow through fractured porous media has been performed through conceptual models, while the modelling of more realistic scenarios is still an open question. This paper proposes a new embedded fracture approach for fluid flow in highly fractured porous media. This approach includes the fracture contribution into the continuum element using a condensation technique based on the multi-constrain method. The main advantage of this approach is the guarantee of compatibility between fracture and porous matrix within a standard finite element mesh. Several fractured rock reservoir cells are analyzed to demonstrate the robustness of the proposed approach and its applicability to highly fractured rocks with random fracture orientation. The pore pressure field obtained with the proposed embedded approach is very close to the predictions provided by the explicit approach with zero-thickness interface elements. Despite the simplicity of the method, the results are promising.

Building 1D and 3D Mechanical Earth Models for Underground Gas Storage - A Case Study from the Molasse Basin, Southern Germany

Hydromechanical models of gas storage in porous media provide valuable information for various applications ranging from the prediction of ground surface displacements to the determination of maximum reservoir pressure and storage capacity to maintain fault stability and caprock integrity. A workflow to set up such models is presented and applied to a former gas field in southern Germany for which transformation to a gas storage site is considered. The workflow comprises 1D mechanical earth modeling (1D MEM) to calculate elastic properties as well as a first estimate for the vertical and horizontal stresses at well locations by using log data. This information is then used to populate a 3D finite element model (3D MEM) which has been built from seismic data and comprises not only the reservoir but the entire overburden up to the earth’s surface as well as part of the underburden. The size of this model is 30 × 24 × 5 km3. The pore pressure field has been derived from dynamic fluid flow simulation through history matching for the production and subsequent shut-in phase. The validated model is ready to be used for analyzing new wells for future field development and testing arbitrary injection-production schedules, among others.

Building 1D and 3D Mechanical Earth Models for Underground Gas Storage—A Case Study from the Molasse Basin, Southern Germany

Hydromechanical models of gas storage in porous media provide valuable information for various applications ranging from the prediction of ground surface displacements to the determination of maximum reservoir pressure and storage capacity to maintain fault stability and caprock integrity. A workflow to set up such models is presented and applied to a former gas field in southern Germany for which transformation to a gas storage site is considered. The workflow comprises 1D mechanical earth modeling (1D MEM) to calculate elastic properties as well as a first estimate for the vertical and horizontal stresses at well locations by using log data. This information is then used to populate a 3D finite element model (3D MEM) which has been built from seismic data and comprises not only the reservoir but the entire overburden up to the earth’s surface as well as part of the underburden. The size of this model is 30 × 24 × 5 km3. The pore pressure field has been derived from dynamic fluid flow simulation through history matching for the production and subsequent shut-in phase. The validated model is ready to be used for analyzing new wells for future field development and testing arbitrary injection-production schedules, among others.