Fault Reactivation Potential Research Articles

Normal fault reactivation induced by hydraulic fracturing: Poroelastic effects

Numerous surface-felt earthquakes have been spatiotemporally correlated with hydraulic-fracturing operations. Because large deformations occur close to hydraulic fractures (HFs), any associated fault reactivation and resulting seismicity must be evaluated within the length scale of the fracture stages and based on the precise fault location relative to the simulated rock volumes. To evaluate the changes in Coulomb failure stress (CFS) with injection, we conduct fully coupled poroelastic finite-element simulations using a pore-pressure cohesive zone model for the fracture and fault core in combination with a fault-fracture intersection model. The simulations quantify the dependence of CFS and the fault reactivation potential on the host rock and fault properties, spacing between the fault and the HF, and the fracturing sequence. We find that fracturing in an anisotropic in-situ stress state does not lead to fault tensile opening but rather dominant shear reactivation through a poroelastic stress disturbance over the fault core ahead of the compressed central stabilized zone. In our simulations, poroelastic stress changes significantly affect fault reactivation in all the simulated scenarios of fracturing 50–200 m away from an optimally oriented normal fault. Asymmetric HF growth due to the stress shadowing effect of the adjacent HFs leads to (1) a larger reactivated fault zone following the simultaneous and sequential fracturing of multiple clusters compared with single-cluster fracturing and (2) a larger unstable area ([Formula: see text].1) over the fault core or a higher potential of the fault slip following sequential fracturing compared with simultaneous fracturing. The fault reactivation area is further increased for a fault with lower conductivity and a higher opening-mode fracture toughness of the overlying layer. To reduce the risk of fault reactivation by hydraulic fracturing under the reservoir characteristics of the Barnett Shale, Fort Worth Basin, it is recommended to (1) conduct simultaneous fracturing instead of sequential and (2) maintain a minimum distance of approximately 200 m for HF operations from known faults.

Post-mining related reactivation potential of faults hosted in tight reservoir rocks around flooded coal mines, eastern Ruhr Basin, Germany

The cessation of hard coal mining in the Ruhr Basin in 2018 marked the region's transition to the post-mining phase. Controlled mine water rebound induces changes in the subsurface stress conditions, as pore pressure increases locally. Presently, mine water rebound is observed in the eastern Ruhr Basin (water province “Haus Aden”) along with associated microseismicity. Furthermore, post-mining challenges might comprise the potential risk of fault reactivation, which is addressed in this study by conducting a fault slip assessment. Based on subsurface coal seam mapping data, a 3D structural model for the NE part of the “Haus Aden” water province has been constructed to serve as the basis for identifying the most vulnerable fault trends and types of the structural inventory. Slip tendency analysis, considering normal faulting conditions, revealed NW-SE to NNW-SSE trending normal faults to be most susceptible to reactivation. Probabilistic fault slip assessment, focused on NW-SE to NNW-SSE trending normal faults mapped within the “Heinrich-Robert” colliery, show no fault reactivation potential for a mine water rebound up to a level of 640 m below ground. Assuming hydrostatic conditions in the vicinity of the faults, friction coefficients are only partially exceeded for high differential stresses. In addition, a novel workflow is used to model the spatial variability of the frictional fault strength as input for a fault stability analysis, exemplified for a selected NNW-SSE trending normal fault. For considering hydrostatic pore pressure, results show that the fault consists mainly of stable, but also unstable, horizontally elongated patches. These findings question the conventional simplified approach of using a single constant friction coefficient for fault stability analysis.

3D mechanical analysis of geothermal reservoir operations in faulted sedimentary aquifers using MACRIS

Accurate and efficient predictions of three-dimensional subsurface stress changes are required for the assessment of geothermal operations with respect to fault stability and the potential risk for induced seismicity. This work extends the model capabilities of Mechanical Analysis of Complex Reservoirs for Induced Seismicity (MACRIS) to account for high-resolution thermo-elastic stress evaluations in structurally complex (i.e. faulted) and matrix permeability dominated geothermal systems. By adopting a mesh-free approach suitable to industry standard flow simulation models, MACRIS is capable of preserving the complex 3D hydraulic development of the injected cold-water volume and the 3D geometrical complexities of the reservoir model. The workflow has been applied to three-dimensional models with clastic reservoir characteristics representative for low enthalpy geothermal exploitation in the Netherlands. The models are marked by a single fault, subject to no and normal offset. Comparison of simulated stress evolutions in MACRIS with alternative analytical solutions highlight the effects of stress arching involved in the poro- and thermo-elastic stress developments on complex faults intersected by or in direct contact with the cold-water volume. Results are in agreement with previous studies and show the effect of thermal stressing to be dominant, arching of stresses to occur at the rim of the cold-water volume, and in cooling reservoirs, the intersection area of the cold-water volume in direct contact with the fault plane to be the main driver for fault reactivation and subsequent seismic potential. Moreover, results show the effects of stress arching (i) to be enhanced in the case of reservoir throw and flow compartmentalization, and (ii) to be reduced by a relative increase in conductive heat transfer between the reservoir and surrounding formations.

The slip tendency of 3D faults in Germany

Abstract. Fault reactivation potential is a crucial aspect for many underground utilizations, such as the construction and long-term safety of a nuclear waste repository, as seismic events can endanger these operations. An estimation of the fault reactivation potential requires information about the stress field, but stress data are only available pointwise and are not evenly distributed throughout Germany. Geomechanical–numerical modeling can be used to derive a spatially continuous description of all six independent components of the stress tensor as shown by the model of Germany by Ahlers et al. (2022). Information about the geometry of faults extending several kilometers in depth is provided for most areas in Germany by the geological models of the federal states and geological models created in the framework of projects such as GeoMol (Assessing subsurface potentials of the Alpine Foreland Basins for sustainable planning and use of natural resources) or GeORG (Geopotenziale des tieferen Untergrundes im Oberrheingraben). We use the 3D fault geometries provided by such models and map the stress data from the Germany model by Ahlers et al. (2022) onto these faults. Then, assuming hydrostatic pore pressure, we calculate the so-called slip tendency (TS), the ratio between resolved shear stress and the effective normal stress on the fault plane as a measure of fault reactivation potential. A fault is considered critical when its TS value exceeds its coefficient of friction. In general, TS ranges between 0 and 0.7 for the analyzed faults. The highest overall TS values are observed along the NNE–SSW-striking Upper Rhine Graben, where TS routinely reaches and exceeds values of 0.7. In the North German Basin, the Ore Mountains and Saxony only very few TS values exceed 0.7. The area with the lowest overall TS is the Molasse Basin, where the TS of the mostly WSW–ENE-striking faults only rarely exceeds values of 0.4. In general, N–S- to NNE–SSW- and NW–SE-striking faults show the highest TS values, whereas WSW–ENE-striking faults show the overall lowest values. With increasing depth, TS decreases. Pore pressure and overpressure have the potential to significantly influence the resulting TS.

Fault reactivation potential in a carbonate field in Brazil based on geomechanical analysis

The reactivation of faults may directly modify their permeability and the integrity of the petroleum system seal. Thus, this understanding is fundamental for decision-making in the petroleum industry at different levels while developing, for example, hydraulic fracturing projects, fracture networking, Enhanced Hydrocarbon Recovery (EOR), and CO2 storage. The identification of the stress field based on elastic properties, failure criterion, rock strength, and in-situ principal stresses allow to understand the geomechanical behavior of stratigraphic units. This process becomes complex in carbonate reservoirs because they present high geomechanical variability related to the sedimentary and diagenetic processes they undergo. In this context, the present work aims to study the potential for fault reactivation in the pre-salt Barra Velha and Itapema Formations and in the Camboriu Formation in the study area, Santos Basin, Brazil, based on geomechanical modeling. Data from 28 wells with basic geophysical well-logs and wellbore image logs, absorption and formation pressure tests, and 3D pre-stack seismic data were used. The results indicate an abnormally high fluid pore pressure (9.49 ppg) concerning hydrostatic pressure (8.34 ppg). Three structural compartments delimited by interpreted seismic horizons and faults were defined. Three fault regimes were defined based on minimum and maximum horizontal stresses gradients: a double normal-transcurrent regime for the Barra Velha and Itapema Formations; a normal regime for the Piçarras Formation; and a double transcurrent-inverse regime for the Camboriu Formation. Finally, based on the stress polygon, scenarios of fault reactivation in the structural compartments were estimated. With increases between 16% and 28.1% of the pore pressure by fluid injection, the transcurrent movement of the faults can be generated. The reverse faults of a shallower structural compartment have the highest probability of reactivation and movement.

3D structural model and slip-dilation tendency analysis of the Chiweta Zone: Geothermal system implications and fault reactivation potential

A 3D structural model and slip-dilation tendency analysis are used to assess the reactivation potential of shear and dilatational structures to investigate structurally controlled geothermal systems in the Chiweta Zone. The Chiweta Zone is in the northern part of the Malawi Rifted Zone, where an elevated heat flow anomaly and one of the hottest hot springs occur. Structurally controlled geothermal systems are poorly understood in Malawi and the entire Western Branch of the East African Rift System. The objectives of this study were to (1) construct a three-dimensional structural geometry of the faults identified with remote sensing and aeromagnetic data, (2) perform a slip-dilation tendency analysis to assess the fault reactivation potential under a stress state condition at depths between ∼700 and 1000 m.b.g.l. of the estimated reservoir, (3) identify, based on the results, which faults are critically stressed and serve as fluid conduits or barriers of the geothermal system, and (4) combine geological, geochemical, and geophysical data with our results to improve the conceptual model of the Chiweta Geothermal System (CGS). Our findings indicate that ∼ NW- and ∼NNE-striking faults southwest- and northwest dipping, respectively, show segments with the highest dilation values of Td ∼0.6 to Td ∼0.8 and probably behave as active fluid pathways. This is supported by the Chiweta Fault, a NW-striking and southwest-dipping fault that controls the only thermal manifestations in the Chiweta Zone. NW- and NE-striking faults dipping to the northeast and southeast, respectively, show lower dilation values of Td ∼0.1 to Td ∼0.5 and probably act as fluid barriers. These results allowed us to define, for the first time, possible structural boundaries of the Chiweta geothermal reservoir and to improve its geothermal conceptual model. The results of this study may help identify favorable areas to target the first exploratory wells and where further subsurface studies (e.g., MT surveys and seismic reflection) should be conducted in the Chiweta Zone. Finally, our methodology is expected to be applied to other geothermal prospects in the Western Branch of the East African Rift System and help advance geothermal energy production in this region.

In-situ stress analysis of Ahnet Basin, South western Algeria: A 1D geomechanical approach

This work aims to determine the present-day in-situ stress and pore pressure in the Ahnet Basin, Algeria, through a 1D geomechanical approach. We investigated the drilling-induced fracture (DIF) from FMI log data to ascertain the direction of maximum horizontal stress (SHmax) from the Ahnet Field. A mean orientation of N140° ( ± 10°) has been interpreted, which is NW-SE (N140°-N320°), with a local variation of ( ± 20°) compared to fields such as Illizi, Hassi Messaoud, and North Algeria, which can be explained by depth variation and intrinsic rock properties. A 1.05 psi/ft gradient of overburden stress (Sv) has been obtained from density. Pore pressure has been estimated from the sonic log by a normal compaction trend, indicates a hydrostatic regime from the surface to the top of the Silurian unit with an average pore pressure gradient of 0.43 psi/ft and an overpressure regime against the hot shale unit with a gradient of 0.58 psi/ft caused by the high in situ temperature in the study area and possible activity of the mega-shear zone. The poroelastic approach under transverse isotropic vertical conditions (VTI) has been used to calculate the minimum and maximum horizontal stress magnitudes. The outlines indicate a high-stress gradient close to 0.82 psi/ft, for Shmin calibrated with MDT stress points and 1.10 psi/ft for SHmax. The stress magnitudes results, suggest a present-day normal to strike-slip stress regime in the Ahnet Basin. Fault reactivation potential at two Silurian units has been inferred from the frictional theory analysis. The results indicate that increased pore pressure in hot shale formations due to by fluid injection and hydraulic fracturing causing shear slippage of the pre-existing faults, resulting in induced seismicity. Our study has contributed to the understanding of stress state origin in Ahnet Basin, the relationship between in situ temperature and pore pressure, and the fault stability analysis in such unconventional reservoir development.

The analysis of slip tendency of major tectonic faults in Germany

Abstract. Seismic hazard during subsurface operations is often related to the reactivation of pre-existing tectonic faults. The analysis of the slip tendency, i.e., the ratio of shear to normal stress acting on the fault plane, allows an assessment of the reactivation potential of faults. We use the total stresses that result from a large-scale 3D geomechanical–numerical model of Germany and adjacent areas to calculate the slip tendency for three 3D fault geometry sets with increasing complexity. This allows us to draw general conclusions about the influence of the fault geometry on the reactivation potential. In general, the fault reactivation potential is higher in Germany for faults that strike NW–SE and NNE–SSW. Due to the prevailing normal stress regime in the geomechanical–numerical model results, faults dipping at an angle of about 60∘ generally show higher slip tendencies in comparison to steeper or shallower dipping faults. Faults implemented with a straight geometry show higher slip tendencies than those represented with a more complex, uneven geometry. Pore pressure has been assumed to be hydrostatic and has been shown to have a major influence on the calculated slip tendencies. Compared to slip tendency values calculated without pore pressure, the consideration of pore pressure leads to an increase in slip tendency of up to 50 %. The qualitative comparison of the slip tendency with the occurrence of seismic events with moment magnitudes Mw>3.5 shows areas with an overall good spatial correlation between elevated slip tendencies and seismic activity but also highlights areas where more detailed and diverse fault sets would be beneficial.

Fault Reactivation Potential in a Carbonate Field in Brazil Based on Geomechanical Analysis

Fault Reactivation Potential in a Carbonate Field in Brazil Based on Geomechanical Analysis

Assessing the accuracy of fault interpretation using machine-learning techniques when risking faults for CO2 storage site assessment

Generating an accurate model of the subsurface for the purpose of assessing the feasibility of a CO2 storage site is crucial. In particular, how faults are interpreted is likely to influence the predicted capacity and integrity of the reservoir; whether this is through identifying high-risk areas along the fault, where fluid is likely to flow across the fault, or by assessing the reactivation potential of the fault with increased pressure, causing fluid to flow up the fault. New technologies allow users to interpret faults effortlessly, and in much quicker time, using methods such as deep learning (DL). These DL techniques use knowledge from neural networks to allow end users to compute areas where faults are likely to occur. Although these new technologies may be attractive due to reduced interpretation time, it is important to understand the inherent uncertainties in their ability to predict accurate fault geometries. Here, we compare DL fault interpretation versus manual fault interpretation, and we can see distinct differences to those faults where significant ambiguity exists due to poor seismic resolution at the fault; we observe an increased irregularity when DL methods are used over conventional manual interpretation. This can result in significant differences between the resulting analyses, such as fault reactivation potential. Conversely, we observe that well-imaged faults indicate a close similarity between the resulting fault surfaces when DL and manual fault interpretation methods are used; hence, we also observe a close similarity between any attributes and fault analyses made.

The Influence of Loading Path on Fault Reactivation: A Laboratory Perspective

AbstractThe loading path the fault experiences is often neglected when evaluating its potential for reactivation and the related seismic risk. However, stress history affects fault zone compaction and dilation, and thus its mechanics. Therefore, in incohesive fault cores that could dilate or compact, the role of the loading path could not be ruled out. Here we reproduce in the laboratory different tectonic loading paths for reverse (load‐strengthening in the absence of significant fluid pressure increase) and normal gouge‐bearing faults (load‐weakening) to investigate the loading path influence on fault reactivation and seismic potential. We find that, before reactivation, experimental reverse faults undergo compaction, whereas experimental normal faults experience dilation. Additionally, when reactivated at comparable normal stress, normal faults are more prone to slip seismically than reverse faults. We infer that the higher mean stress normal faults experience compacts more efficiently the fault rock, increasing its stiffness and favoring seismic slip.

Laboratory study on the potential for fault reactivation of the Draupne caprock during CO2 injection

The successful injection and storage of carbon dioxide into the subsurface relies on an extensive and robust caprock above the storage formation. Even though migration of CO2 through intact caprocks overlying saline aquifers or abandoned hydrocarbon reservoirs is very unlikely, the pore pressure increase accompanying injection can potentially cause reactivation of existing fractures and faults. Consequently, the caprock permeability can increase and provide potential pathways for CO2 migration through the caprock. Specific knowledge about CO2 injection pressures caprocks can undertake without faults or fractures being critically stressed is therefore important. In this study, residual strength of fractured samples of the North Sea Draupne caprock is evaluated from mechanical testing. Samples containing two different types of fractures have been tested in the direct shear box and results are compared with residual strength determinations from undrained triaxial compression tests. The aim is to improve the understanding of how strength of fractured material differs between these two laboratory tests.

Stress perturbation caused by multistage hydraulic fracturing: Implications for deep fault reactivation

Swarms of earthquakes during shale gas exploitation in the Changning area of Sichuan Basin indicate that hydraulic fracturing induces seismicity both within the target reservoir but also to depths of several kilometers below the horizontal well. These remote earthquakes are possibly triggered by total stress perturbations resulting from the hydraulic fracturing. We use a dislocation-based analytical model to simulate multistage hydraulic fracturing of three horizontal wells at a single well pad to explore the spatiotemporal evolution of total stress perturbations. Results show that the number and distribution of fracturing stages affect both the distribution and magnitude of stress changes and that the stress change diminishes with distance. The undrained injection-induced stress change is below 10-3 MPa at distances ≥1 km for first-stage fracturing but reach 10-1 MPa for multistage fracturing of 30 stages in three wells. Undrained stress changes scale linearly with the magnitude of fluid leakoff into the formation – halving the effective fracture width halves the induced stress magnitudes and with an identical distribution – limiting the potential for fault reactivation. Scaling analysis for pressure diffusion distal from the reservoir indicate that the short-term impact is indeed essentially undrained. Estimates for long-term depletion identify a similar induced stress signal of opposite sign but with similar Coulomb potential for reactivation in the long-term. Such magnitudes of Coulomb stress changes suggest the possibility of fault reactivation on critically-stressed faults at kilometer separation from the injection both in the short-term due to stimulation and in the long-term resulting from depletion.

Slip Tendency Analysis, Fault Reactivation Potential and Induced Seismicity in the Val d'Agri Oilfield (Italy)

AbstractThe Val d'Agri basin hosts an oil‐field, the largest in onshore Europe, and it is one of the areas of highest seismic hazard in Italy. In an unproductive marginal portion of the reservoir, wastewater is re‐injected by a high‐rate well. Since the beginning of re‐injection in June 2006, a spatio‐temporal correlation between microseismicity (ML ≤ 2.0) and wastewater injection has been observed (suggesting induced seismicity). In this study, we perform a slip‐tendency analysis on the fault system involved in the induced seismicity through a coupled fluid‐flow and geomechanical numerical model simulating the stress partitioning due to the tectonic forces and to the fluid injection. The model results show that the fluid diffusion is strongly dependent on the active stress field and the geological structure in which fluids are injected, which conditioned the occurrence of seismicity that aligned on a small portion of a NE‐dipping fault. However, another fault located closer to the injection well and where no seismicity was detected, is the better well‐oriented fault with the active stress field and, also, the one more susceptible to the pore pressure increase. These results suggest different types of fault deformation acting in the Val d'Agri oilfield as response to the fluid injection (i.e., a mixed‐mode fault slip behavior). Understanding the stress partitioning in tectonically active regions where underground activities such as fluid injection are ongoing is fundamental to give strong constraints for the discrimination between natural and induced seismicity, and finally for a more reliable and robust definition of seismic hazard.

Fault reactivation potential and associated permeability evolution under changing injection conditions

Understanding the hydraulic and frictional sensitivity of fault to different injection conditions is one of the efficient ways to provide useful implications for fault reactivation potential. Numerical simulations of fractured reservoir have provided information on how fault behaviour varies under changing hydromechanical properties and injection conditions. A coupled hydro-mechanical model which can represent the elastoplastic behaviour of a fault was employed to predict and quantify the effects of varying injection positions and injection rates on permeability response and potential of fault reactivation under isothermal injection. We examine the sensitivity of seismic event magnitude and timing to variations in both pressure perturbation and stress as injection location changes. We generate results for two scenarios: one with changing injection position but with uniform injection rate, and another scenario with increasing injection rate at the same injection position. We observed that the potential of fault reactivation is affected by the hydraulic diffusivity potential of the fluid pressure, and this mechanism is mediated by a function of the injector position and injection rate. As the velocity of fluid transmission increases, increasing fluid pressure impact pore pressure elevation and reduced effective stress. However, an injector position where there is low diffusivity causes low pore pressure build-up rate, incapable of inducing shear failure, and thus, permeability enhancement is retarded in this case. Accordingly, the injection rate variation influences the rate of pore pressure build-up, the timing and magnitude of induced seismic events. This is also reflected in the permeability evolution as a response to the variations in the magnitude of fault openings and cracks. This changing injection conditions however influences the timing required to reach the critical peak friction point as pore pressure build-up rate and sensitivity to loading response change. Hence, with changing position and rate of injection, the evolution of fault permeability appears to be intrinsically controlled by a condition which favours elastoplastic deformation and fracture failure, with slip distance increasing with high injection rates.

Fault reactivation induced by tunneling activity in clay material: Hints from numerical modeling

Seismic events with magnitude 3 and above have been associated with the removal of rock mass in mining environment since long-time. On the contrary, little is known about the possible seismic events induced by tunneling, although it presents similarities with mining. One great example is the case of the 57 km long Gotthard Base Tunnel excavation, which has been associated more than hundred seismic events, with the largest one having magnitude of ML 2.4, damaging the tunnel infrastructures (e.g. gallery floor or portal area).Different underground structures will be built probably up to 1000 m below ground for the construction of future deep geological for the storage of nuclear waste. While seismic risk will probably not constitute a liability for the storage site construction, it is important to understand the potential for reactivation of seismogenic features located nearby the future location of emplacement tunnels.Here we present numerical simulations aimed at understanding the potential for fault reactivation during tunnel construction in clay material, a potential host rock for nuclear waste repository. We evaluate the evolution of the stress changes during the simulation of the excavation with FLAC3D numerical solver. A strain-softening friction model is used to simulate the occurrence of a sudden slip on a fault zone when critical conditions for reactivation are satisfied. This constitutes a worst-case scenario, given the low seismogenic potential of clay rocks. We also present a sensitivity analysis on several critical parameters including fault frictional properties, stress conditions, as well as different tunnel sizes at varying distance from a nearby failure plane, with the final purpose of evaluating safety of a potential nuclear repository site on the short- and long-term.

Investigation of present-day in-situ stresses and pore pressure in the south Cambay Basin, western India: Implications for drilling, reservoir development and fault reactivation

This work presents the assessment of present-day stresses and pore pressures in a prolific onshore hydrocarbon bearing field located in the southern Cambay Basin, western India, through the development of a comprehensive geomechanical model. A-C quality wellbore breakouts, inferred from the available four-arm caliper logs and anisotropy analyses of dipole shear sonic logs, indicate a NE (N40°E to N50°E) azimuth for maximum horizontal stress (SHMax). Pore pressure characterisation reveals mild overpressure in the Lower Eocene Shale caused by disequilibrium compaction resulting from high sedimentation rates. Vertical stress (Sv) was derived from density logs, and a poroelastic strain model was employed to quantify the minimum (Shmin) and maximum horizontal stress components (SHMax), being later validated with leak-off tests. Based on the relative magnitudes of three principal in-situ stress components, post-rift strata (Upper Eocene and younger) reveal a normal faulting regime (Sv > SHMax > Shmin), whereas a normal fault to strike-slip faulting (SHMax ≥ Sv > Shmin) transition is observed in the syn-rift unit (Lower Eocene and older). The Mohr-Coulomb rock failure criterion was utilised to depict the shear failure gradient to prevent wellbore instability, and results were validated with caliper logs. We have also analysed the fault reactivation potential at reservoir level using frictional fault theory and stress polygons. The results indicate that a minimum of 31% increase in pore pressure by fluid injection and hydraulic fracturing treatment can cause shear slippage of critically oriented faults, resulting in induced seismicity. Our study has helped to understand the interplay between stress states and pore fluid pressure and their impact on reservoir development, drilling and completion design, and fault stability analysis in any given oil and gas field.

Poroelastic models for fault reactivation in response to concurrent injection and production in stacked reservoirs

Concurrent production and injection in stacked reservoirs as commonly conducted in unconventional resource exploitation potentially influences reactivation of nearby faults. Using three-dimensional, fully-coupled poroelastic finite-element simulations, we assessed the potential for reactivation of a barrier normal fault in a normal-faulting stress regime for twelve generic injection–production scenarios that differ in the depth of injection and production, and in the position and distance relative to the dipping fault plane. The simulations display significant variation in the Coulomb failure stress (CFS) with depth along the fault plane for these scenarios, reflecting differences in pore pressure distribution and associated poroelastic changes in normal and shear stress across the fault. Based on the CFS trends with depth we find that 1.) concurrent production and injection reduces or increases the fault reactivation potential in the injection reservoir depending on the lateral position and the distance of the wellbores relative to the fault plane; 2.) the fault is most prone to reactivation with stacked wellbores and injection into the upper reservoir within the hanging wall or the lower reservoir within the footwall, and 3.) the fault is least prone to injection-induced reactivation for stacked wellbores and injection into the lower reservoir within the hanging wall at wellbore-to-fault distances ten times the reservoir thickness. With decreasing wellbore-to-fault distance, induced poroelastic shear stresses and thus CFS increase, making injection only into the lower reservoir, without concurrent production, the most stable configuration at close distance. These simulations demonstrate the importance of the coupled poroelastic effects and of the three-dimensional arrangement of injection and production wellbores on fault reactivation. Our results are intended to provide general guidance for further detailed site-specific geomechanical evaluations needed for induced seismic hazard assessment.

Present day In-situ stress magnitude and orientation of horizontal stress components in the eastern Illizi basin, Algeria: A geomechanical modeling

We analyzed drilling induced tensile fractures from resistivity image log data to ascertain the orientation of maximum horizontal stress (SH) from the eastern Illizi basin, Algeria. An average SH azimuth of 150°N (± 10°) has been interpreted from B-quality induced fractures, as per world stress map guidelines. The overall NW-SE orientation of SH translates to the relative plate motion of the African and Eurasian plates. Vertical stress (Sv) gradient of 1.07 PSI/ft has been derived from density log. Pore pressure estimated from sonic slowness reveals overpressure in Silurian shale, deposited in a transgressive depositional environment, whereas Devonian and Ordovician hydrocarbon reservoirs have been seen to be normally pressured. Poroelastic strain model has been employed to quantify maximum and minimum horizontal stress (Sh) magnitudes. An effective stress ratio of 0.6, interpreted from leak-off test has also been used to model Sh. Using frictional faulting theory, upper limit of SH has been quantified. SH/Sv ratio of 1.04 (1.01–1.26) has been seen in the study area. Based on the relative stress magnitudes (SH > Sv > Sh), a present day strike-slip faulting regime has been inferred in the eastern Illizi basin, Algeria. Fault reactivation potential at reservoir level has been inferred from stress polygon analysis.

Estimation of the current stress field and fault reactivation analysis in the Asmari reservoir, SW Iran

Knowing the current condition of the faults and fractures in a reservoir is crucial for production and injection activities. A good estimation of the fault reactivation potential in the current stress field is a useful tool for locating the appropriate spot to drill injection wells and to calculate the maximum sustainable pore pressure in enhanced oil recovery and geosequestration projects. In this study, after specifying the current stress state in the Gachsaran oilfield based on Anderson’s faulting theory, the reactivation tendency of four faults (F1, F2, F3, and F4) in the Asmari reservoir is analyzed using 3D Mohr diagrams and slip tendency factors. Results showed that all the faults are stable in the current stress state, and F2 has the potential to undergo the highest pore pressure build-up in the field. On the other hand, F3 has the proper conditions (i.e., strike and dip referring to σHmax orientation) for reactivation. Stress polygons were also applied to show the effect of the pore pressure increase on fault stability, in a graphical manner. According to the results, the best location for drilling a new injection well in this part of the field is the NW side of F2, due to the lower risk of reactivation. It was found that both methods of 3D Mohr diagrams and slip tendency factors predict similar results, and with the lack of image logs for stress orientation determination, the slip tendency method can be applied. The results of such studies can also be used for locating safe injection points and determining the injection pressure prior to numerical modeling in further geomechanical studies.