SHmax Orientation Research Articles

Three-Dimensional Coupled Temporal Geomechanical Model for Fault-Reactivation and Surface-Deformation Evaluation during Reservoir Depletion and CO2 Sequestration, Securing Long-Term Reservoir Sustainability

Assessing reservoir subsidence due to depletion involves understanding the geological and geophysical processes that lead to ground subsidence as a result of reservoir fluid extraction. Subsidence is a gradual sinking or settling of the Earth’s surface, and it can occur when hydrocarbons are extracted from underground reservoirs. In this study, a time-integrated 3D coupled geomechanical modeling incorporating the fourth dimension—time—into traditional 3D geomechanical models has been constructed utilizing seismic inversion volumes and a one-dimensional mechanical Earth model (1D MEM). The 3D geomechanical model was calibrated to the 1D MEM results. Geomechanical rock properties were derived from the density and sonic log data that was distributed with conditioning to the seismic inversion volumes obtained from running pre-stack inversion. The standard elastic parameter equations were used to generate estimates of the elastic moduli. These properties are dynamic but have been converted to static values using additional equations used in the 1D MEM study. This included estimating the Unconfined Compressive Strength. In situ stresses were matched using different minimum horizontal principal stress gradients and horizontal principal stress ratios. The match is good except where the weak carbonate faults are close to the wells, where the Shmin magnitudes tend to decrease. The SHmax orientations were assessed from image log data and indicated to be 110° in the reservoir section. A time-integrated 3D coupled simulation was created using the finite-element method (FEM). The effective stresses increase while there is depletion in all directions, especially in the Z direction. The predicted compaction in the reservoir and overburden was 350 mm. Most of the compaction occurs at the reservoir level and dissipates towards the surface (seabed). Furthermore, the case displayed no shear failure that might cause or fault reactivation in the reservoir interval (Kangan–Dalan Formations) located in the simulated area. In this study, we applied an integrated and comprehensive geomechanical approach to evaluate subsidence, fault reactivation and stress alteration, while reservoir depletion was assessed using seismic inversion, well logs, and experiment data. The deformation monitoring of geological reservoirs, whether for gas storage or hazardous gas disposal, is essential due to the economic value of the stored assets and the hazardous nature of the disposed materials. This monitoring is vital for ensuring the sustainability of the reservoir by maintaining operational success and detecting integrity issues.

Stress Field Dynamics and Fault Slip Potential in the Paradox Basin

AbstractThe Paradox Basin, straddling Utah, Colorado, Arizona, and New Mexico is characterized by an intricate amalgamation of evaporites and clastic layers and is dominated by prominent salt walls and related subsurface structures. Our research offers a new examination of the stress distribution across the basin, deriving from continuous and discrete stress measurements conducted in boreholes in the region and focal mechanism analysis, emphasizing variations over salt structures. Integrating Coulomb failure criteria with probabilistic methods, we assess potential fault movements resulting from fluid pressure alterations. Our approach provides a comprehensive understanding of the Paradox Basin's state of stress, showing a continuous change of the maximum horizontal stress orientation from N‐S at the Wasatch Fault Zone to WNW‐ESE in the northern part of the Paradox Basin and to WSW‐ENE in the southern part of the basin. Further East, into the Colorado Plateau and the Uncompahgre Uplift, the SHmax orientation becomes E‐W. Decoding stress orientation dynamics has enabled critical insights into fault slip potential, especially in the basin's northern region. The salt wall faults are less likely to slip, and the Paradox Formation's evaporite and clastic rock sequence can serve as a potential low seismic risk target for carbon storage and hydrocarbon extraction.

Constraining maximum horizontal stress using wellbore breakouts — A case study from the Ordovician tight reservoir of the northeastern Oued Mya Basin, Algeria

In this study, we interpret the maximum horizontal stress (SHmax) azimuth from the breakout positions of the wellbore and attempt to constrain the SHmax gradient based on the interpreted breakout width. A cumulative of 110 m of breakouts are deciphered within the Ordovician Hamra Quartzite interval of the Oued Mya Basin from a 138 m acoustic image log. These breakouts are ranked as “A-Quality” following the World Stress Map ranking guidelines. We infer a mean SHmax orientation of N28°E ± 8°. Following the frictional faulting mechanism and stress polygon approach, measurement of the minimum horizontal stress (Shmin) from minifrac tests and observations of the compressive failures from the acoustic image log provide strong constraints on the SHmax magnitude in the reservoir interval in the absence of core-measured rock strength. Interpreted breakout widths exhibit a range between 32.6° and 90.81°, which indicates a SHmax range of 24.4–34.7 MPa/km. The average breakout width of 62.58° translates to a narrower SHmax gradient range, varying between 27.2 and 31.2 MPa/km. The relative magnitudes of the principal stresses indicate a strong strike-slip tectonic stress state. Considering all the uncertainties, we infer a SHmax/Shmin ratio of 1.41–1.81 within the Ordovician interval.

Variations in the present-day stress field and its implications for hydrocarbon development in the Hassi Terfa field, Hassi Messaoud, Algeria

Understanding the contemporary stress field and its variations is important in hydrocarbon production. These fluctuations are significant for borehole stability and trajectory, hydraulic stimulation, fault reactivation, and optimal well completion. In this study, we provide the first geomechanical model to determine the orientation and magnitude of the present-day stress of the Hassi Terfa field using geophysical logs to understand the origin of the spatial variation of the stress state. The breakouts from acoustic image logs provide an average maximum horizontal stress (SHmax) orientation of N115°, consistent with the Africa–Eurasia plate motion, which imposes the largest first-order stress source. However, our analysis demonstrates local stress perturbations near faults. This may mean that the stress field in the Hassi Terfa is superimposed by a third-order source of stress (<100 km). Our model demonstrates variations in stress magnitude with depth interpreted to be lithology-dependent, which act as a natural barrier against fracture propagation. We propose a new approach to quantify these perturbations via the stress regime index R′, using the magnitude of the principal stress components. The results demonstrate different layer-to-layer faulting regimes. These perturbations will limit the vertical propagation of hydraulic fracturing to the overlying formation, which enables it to access a larger reservoir volume and improve production. In general, a strike–slip regime with a slight normal slip component is interpreted for the study area. We develop the first-ever quantitative stress map using the relative principal stress magnitude parameter (Aϕ) to present the lateral variation of the stress regime throughout the Saharan platform. Results demonstrate a consistent strike–slip to normal/strike–slip regime with a uniform SHmax orientation. This map will help operators achieve optimal well direction and optimize hydraulic stimulation. Our results have considerable implications for hydrocarbon production in the central and southeastern Saharan platform.

Stress field and tectonic regime of the Eastern Mediterranean from inversion of earthquake focal mechanisms

The formal stress inversions of 1258 earthquake focal mechanism solutions reveal the presence of six distinct stress regimes. However, the predominant tectonic regimes comprise normal faulting (39%), strike-slip faulting (29%), thrust faulting (25%), and unspecified oblique faulting (7%). The division of the study area into zones based on structural and deformation patterns has provided valuable insights into the variations in the stress field and tectonic processes within this seismically active region. For each designated zone, reduced stress tensors were derived using the improved right dihedron method and the rotation optimization technique implemented by the Win-Tensor software.The stress field derived from the entire dataset is characterized by R′ = 1.0 and an NW-SE SHmax orientation, strongly influenced by the tectonic stress regime of the Hellenic Arc. In specific regions such as CYS, CEM, and WEM, the prevailing tectonic regimes is characterized by a predominant compressive setting with a stress index (R' = 2.0 to 2.5). This pattern aligns with neotectonic deformation associated with the subduction of the African Plate beneath the Eurasian Plate. The EMZ Zone exhibits a strike-slip tectonic regime with an R' of 1.50, primarily influenced by the Dead Sea Transform Fault. This fault accommodates differential motion between the African and Arabian plates in the NNE direction, resulting in left-lateral strike-slip motion along the plate boundary. The mean slip tendency values, varying between 0.62 and 0.66, suggests that the faults in the study area are appropriately oriented for slip or shear displacement, indicating an elevated probability of reactivation.

Evaluation of in-situ stress orientations within the Central Kenyan rift, through analysis of borehole images of wells drilled in Menengai Geothermal field, Kenya

The Menengai Caldera is in a complex structural zone within the Kenyan Central Rift and hosts the Menengai Geothermal Field. In assessing the subsurface characteristics of many geological resources, knowledge of in-situ stress orientations and magnitudes at depth is essential. Acoustic televiewer logging campaigns carried out in Menengai Geothermal Field between 2018 and 2019 in the production zones of three geothermal wells, have provided an on-site assessment of horizontal in-situ stress orientation in an area lacking such data. Data generated from drilling-induced features in the Menengai Geothermal Field revealed that the average SHmax orientation for MW-34 is NW-SE, whereas the average SHmax orientation for MW-20B is NE-SW, and the average SHmax orientation for MW-15A is NNE-SSW. Subsurface variations of in-situ stress orientations are related to large-scale features such as fractures, fissures, and fault architecture in the geothermal field. The NW-SE SHmax in-situ stress orientation around MW-34 agrees with the Molo (TVA) structural orientation, the Menengai pre-caldera shield orientation, and the Aswa shear zone. The NNE-SSW strike direction of the southern Kenyan Rift Valley, which changes orientation at the triple junction, is compatible with the NE-SW and NNE-SSW SHmax orientations of MW-20B and MW-15A. MW-34 is aligned to the Solai TVA/Graben because of its NE-SW Shmin orientation. In addition, the MW-15A's almost E-W Shmin orientations coincide with the Kenyan Rift's extension direction, which corresponds to the orientation of the E-W striking horsetail fracture inside the Menengai caldera. Localized stress perturbations manifest as rotations of BOs or DITFs orientations along the well as a function of depth.

Contribution of mine borehole data toward high-resolution stress mapping: An example from northern Bowen Basin, Australia

Most of in-situ stress data in the Australian continent comes from wellbore stress analysis in deep hydrocarbon reservoirs, and earthquake focal mechanism solutions near the Australian plate boundaries, where geophysical tools facilitate understanding of the present-day stress patterns. This resulted in a paucity of stress information in many other regions such as the northern Bowen Basin, which is an active mining province, but with low seismicity rates and limited deep petroleum exploration. The mining industry runs several hundred kilometres of image logs annually to characterise geotechnical attributes. These logs provide an image from the borehole wall, which facilitates analysis of stress-related borehole deformations for in-situ stress characterisation. This paper examines the orientation of horizontal in-situ stress using different types of image logs in mine boreholes across the northern Bowen Basin. Analyses of 128 km of image logs in 680 vertical boreholes resulted in the interpretation of 9046 pairs of stress-related indicators including 735 drilling induced fractures and 8311 borehole breakouts. Our comprehensive database comprises 890 quality-ranked data records for the orientation of maximum horizontal stress (SHmax), which makes the Bowen Basin as a basin with the highest data density in the world in terms of quality-ranked stress information according to the World Stress Map. Statistical analysis of SHmax orientation reveals that the mean SHmax orientation in northern Bowen Basin is N018° ± 16°. The results show that this orientation is consistent over long distances, which is in contrast with several eastern Australian basins. This uniform stress pattern agrees well with plate-scale geomechanical model predictions, which further highlights the impact of plate boundary forces in the contemporary stress pattern of this region. Detailed image log investigation did not show any systematic rotation of stress; however, some small-scale stress perturbations were observed in the vicinity of sharp stiffness contrasts and geological structures.

Variations and causes of in-situ stress orientations in the Dibei-Tuziluoke Gas Field in the Kuqa Foreland Basin, western China

The Kuqa Foreland Basin, adjacent to the southern Tianshan Mountains, is an important gas-producing basin in northwestern China. The development of the basin is dominated by a regional stress field with orientations of the maximum horizontal stress (SHmax) in the north-south direction, as a result of the collision between the Indian and Eurasian Plates. Previous studies have noted variations in the stress state in the Kuqa Foreland Basin; however, little attention has been paid to understanding the mechanisms causing such variations. In this study, a systematic analysis of in-situ SHmax orientations and fractures has been carried out using image logs from eight wells and XMAC/DSI logs from five wells. These wells are located in an East-West (E-W) trending fold-thrust in the foreland basin. The SHmax orientations (having quality ranked between A and D) obtained from borehole breakouts and drilling-induced fractures are highly variable and occur as two evidently different stress patterns on a scale of less than 10 km. The first stress pattern with SHmax orientations between NNW and NNE (165.08 and 188.59 °N) is broadly comparable to the regional stress field inferred from earthquake focal mechanism solutions in northwestern China from the World Stress Map (WSM). The stress pattern is interpreted to be controlled by plate boundary forces related to the collision between the Indian and Eurasian Plates. The SHmax orientations for the second stress pattern, however, ranging from NNE to ENE (26.65–57.77 °N), are clearly inconsistent with the regional stress filed and have been influenced by local geological factors. Faults and hinge-parallel fractures (trending in an approximately W-E direction) formed during the syn-folding stage probably resulted in heterogeneity of elastic properties and thus deflected SHmax orientations, both laterally and vertically, to generate the second stress pattern. The NE-striking hinge-oblique fractures are interpreted to have formed in response to the stress reorientation. Observations in this study suggest that there may exist a close relationship between fracturing (faulting) and stress variations, and pre-existing faults and fractures can influence subsequent fracturing by regulating local stress fields on a scale of several kilometers. The coupling between fracturing or faulting and stress produced fracture swarms, which can significantly enhance fluid flow and hence petroleum production.

In-situ stress prediction in ultra-deep carbonate reservoirs of Fuman Oilfield, Tarim Basin of China

The Fuman Oilfield in Tarim Basin has great potential for ultra-deep carbonate oil and gas resources, and is an important area for future storage and production increase. The present-day in-situ stress field is critical during the exploration and development. However, no systemic investigations have been carried out in this oilfield. Therefore, in this study, the present-day in-situ stress field in the Ordovician carbonate reservoir is predicted and analyzed based on well log calculation and geomechanical numerical modeling. The results indicate that, 1) NE-SW-trending is the dominant horizontal maximum principal stress (SHmax) orientation. The vertical principal stress is the maximum principal stress, showing the Ordovician reservoir is under a normal faulting stress regime. 2) The distribution of in-situ stresses in the Ordovician carbonate reservoir is heterogeneous, which is mainly controlled burial depth and fault/fracture development. High stress magnitudes in the Yingshan Formation are mainly in the southeastern part of Fudong area, Fuman Oilfield. The present-day horizontal differential stress mainly ranges from 27 MPa to 30 MPa in the Yingshan carbonate reservoir. iii) Natural fractures are generally stale under the present-day in-situ stress state. Fractures that parallel to the SHmax orientation with high fracture dip angle are easier to be reactivated. The results are expected to provide geomechanical references for further oil and gas development in the Fuman Oilfield of Tarim Basin.

S Hmax orientation in the Alpine region from observations of stress-induced anisotropy of nonlinear elasticity

SUMMARY The orientation of SHmax is commonly estimated from in situ borehole breakouts and earthquake focal mechanisms. Borehole measurements are expensive, and therefore sparse, and earthquake measurements can only be made in regions with many well-characterized earthquakes. Here, we derive the stress-field orientation using stress-induced anisotropy in nonlinear elasticity. In this method, we measure the strain derivative of velocity as a function of azimuth. We use a natural pump-probe (NPP) approach which consists of measuring elastic wave speed using empirical Green’s functions (probe) at different points of the earth tidal strain cycle (pump). The approach is validated using a larger data set in the Northern Alpine Foreland region where the orientation of maximum horizontal compressive stress is known from borehole breakouts and drilling-induced fractures. The technique resolves NNW-SSW to N-S directed SHmax which is in good agreement with conventional methods and the recent crustal stress model. We confirm that the NPP method can be applied to dense large-scale seismic arrays. The technique is then applied to the Southern Alps to understand the contemporary stress pattern associated with the ongoing deformation due to counterclockwise rotation of the Adriatic plate with respect to the European plate. Our results explain why the two major faults in Northeastern Italy, the Giudicarie Fault and the Periadriatic Line (Pustertal–Gailtal Fault) are currently inactive, while the currently acting stress field allows faults in Slovenia to deform actively. We have demonstrated that the pump-probe method has the potential to fill in the measurement gap left by conventional approaches, both in terms of regional coverage and in depth.

SpannEnD – a 3D geomechanical model of Germany for the prediction of the recent crustal stress state

Abstract. The recent crustal stress state is a crucial parameter in the search for a high-level nuclear waste repository. It is relevant not only for site selection and short-term stability during operation but also for predicting long-term safety. However, the level of knowledge in this regard is still limited in Germany. It is based on stress orientations and stress regime information from the World Stress Map (WSM) project and pointwise stress magnitude records. We present the results of a 3D geomechanical numerical model that improves the state of knowledge by providing a continuum-mechanics-based prediction of the recent crustal stress field in Germany. The model extends over an area of 1000×1250 km2, covering Germany and nearby regions. It contains 22 geological units, each parameterized with individual properties (density, Young's modulus and Poisson's ratio). We assume linear elasticity, and the finite element method is used to solve the equilibrium of forces. The model is calibrated with horizontal stress magnitude records and validated with additional data, for example, vertical stress magnitudes or data of the WSM project. Our model results are in a good agreement with a mean orientation of the maximum horizontal stress (SHmax) derived from the WSM, as indicated by a mean of the absolute differences of 12∘. Furthermore, the model results lie entirely within the standard deviation of the derived orientation of SHmax. Mean values of the absolute stress differences between calibration data and model results of 4.6 MPa for the minimum horizontal stresses (SHmin) and 6.4 MPa for the SHmax also show a good agreement. The model results can be used, for example, for the calculation of fracture potential, for slip tendency analyses or as boundary conditions for smaller local models. For smaller model volumes, where little or no stress information is available, it provides synthetic stress magnitude data that can be used for calibration. As such, the model provides a valuable starting point for the detailed assessment of stability at yet-to-be-determined siting regions for a high-level nuclear waste repository.

In Situ Stress Determination Based on Acoustic Image Logs and Borehole Measurements in the In-Adaoui and Bourarhat Hydrocarbon Fields, Eastern Algeria

The study of in situ stress from image logs is a key factor for understanding regional stresses and the exploitation of hydrocarbon resources. This work presents a comprehensive geomechanical analysis of two eastern Algerian hydrocarbon fields to infer the magnitudes of principal stress components and stress field orientation. Acoustic image logs and borehole measurements were used in this research to aid our understanding of regional stress and field development. The studied In-Adaoui and Bourarhat fields encompass a combined thickness of 3050 m of Paleozoic and Mesozoic stratigraphy, with the primary reservoir facies in the Ordovician interval. The Ordovician sandstone reservoir interval indicates an average Poisson’s ratio (v) of 0.3, 100–150 MPa UCS, and 27–52 GPa Young’s modulus (E). Direct formation pressure measurements indicate that the sandstone reservoir is in a hydrostatic pore pressure regime. Density-derived vertical stress had a 1.1 PSI/feet gradient. Minimum horizontal stress modeled from both Poisson’s ratio and an effective stress ratio-based approach yielded an average 0.82 PSI/feet gradient, as validated with the leak-off test data. Drilling-induced tensile fractures (DITF) and compressive failures, i.e., breakouts (BO), were identified from acoustic image logs. On the basis of the DITF criterion, the maximum horizontal stress gradient was found to be 1.57–1.71 PSI/feet, while the BO width-derived gradient was 1.27–1.37 PSI/feet. Relative stress magnitudes indicate a strike-slip stress regime. A mean SHMax orientation of N130°E (NW-SE) was interpreted from the wellbore failures, classified as B-quality stress indicators following the World Stress Map (WSM) ranking scheme. The inferred stress magnitude and orientation were in agreement with the regional trend of the western Mediterranean region and provide a basis for field development and hydraulic fracturing in the low-permeable reservoir. On the basis of the geomechanical assessments, drilling and reservoir development strategies are discussed, and optimization opportunities are identified.

Evaluating uncertainty in empirically derived unconfined compressive strength (UCS) estimates and implications for drilling applications; a case study from the Cooper Basin

Accurate estimates of Unconfined Compressive Strength (UCS) are essential for a range of subsurface applications, including drilling wells for subsurface fluid extraction or injection. However, measuring UCS of subsurface rock samples through laboratory-based uniaxial and triaxial testing is time consuming, expensive, and potentially subject to individual sample variance. P-wave velocity logs are routinely obtained in petroleum basins and are commonly used to estimate UCS using empirically-derived correlations. In this study we analysed P-wave velocity data from 43 wells in the Cooper Basin, Australia and created mean ranges of expected UCS through the Tirrawarra, Toolachee, Patchawarra, Murteree Shale and Epsilon formations, using literature-derived P-wave velocity-UCS correlations, and also estimated UCS based on selected available laboratory-determined P-wave velocity measurements. These two suites of P-wave velocity-derived estimates of UCS are then compared to available results from laboratory uniaxial testing of core samples. Our analysis indicates that P-wave velocities, and subsequent derived estimates of UCS from P-wave wireline logs, are typically lower than those from laboratory-analysed samples. This may reflect a sampling bias towards the selection of strong rocks for laboratory UCS testing or lower estimates of UCS due to wellbore damage and pore space content (air or liquids) from P-wave log derived velocities. However, the laboratory-derived P-wave velocities generally agree with laboratory-derived UCS data from the Patchawarra, Murteree Shale and Epsilon formations when using published empirical velocity-strength correlations. A retrospective case study presents the impacts of different UCS estimates on mud weight required to produce observed borehole breakouts in the Epsilon Formation. Breakouts were observed 90 degrees to the orientation of SHmax (117° N) with a mean width of 51°, the breakouts were produced during drilling using a 9.7 ppg mud weight. The back-calculated estimate of UCS from the observed breakout widths at 9.7 ppg is 154 MPa. This sits between the UCS estimate of 171 MPa from uniaxial testing and 145 MPa from the mean of laboratory sample-derived P-wave velocities. Estimates of UCS from empirical correlations based solely on p-wave log-derived are far lower and vary between 87.3 and 114.4 MPa.

In situ stress database of the greater Ruhr region (Germany) derived from hydrofracturing tests and borehole logs

Abstract. Between 1986 and 1995, 429 hydrofracturing tests have been carried out in six now-abandoned coal mines and two coal bed methane boreholes at depths between 600 and 1950 m within the greater Ruhr region in western Germany. From these tests, stress magnitudes and orientations of the stress tensor are derived. The majority of hydrofracturing tests were carried out from mine galleries away from mine workings in a relatively undisturbed rock mass. These data along with detailed information have been disclosed recently. In combination with already published material, we provide the first comprehensive stress database of the greater Ruhr region. Our study summarises the results of the extensive in situ stress test campaign and assigns quality to each data record using the established quality ranking schemes of the World Stress Map project. The stress magnitudes suggest predominantly strike-slip stress regime, where the magnitude of the minimum horizontal stress, Shmin, is half of the magnitude of the maximum horizontal stress, SHmax, implying that the horizontal differential stress is high. We observe no particular change in the stress gradient at depth throughout the Carboniferous layers and no significant difference between tests carried out in coal mines and deep boreholes. The mean SHmax orientation varies between 133 ± 13∘ in the westernmost located Friedrich Heinrich coal mine and 168 ± 23∘ in the easternmost located Westphalia coal mine. The mean SHmax orientation, based on 87 data records from this and already published studies, of 161 ± 43∘ is in good agreement with the regional stress orientation observed in northwestern Europe. The presented public database provides in situ stress magnitude and stress orientation data records that are essential for the calibration of geomechanical numerical models on regional and/or reservoir scales for, among others, assessing stability issues of borehole trajectories, caverns, and georeservoirs in general. For an application example of this database, we estimate slip and dilation tendencies of major geological discontinuities, discovered during the 700-year-long coal mining activities in the region. The result, although burdened by high uncertainties, shows that the discontinuities striking in the N–S and NW–SE directions have a higher slip tendency compared to the ones striking ENE–WSW and NNW–SSE, whereas a high dilation tendency is observed for discontinuities striking NNW–SSE and a low dilation tendency for the ones striking ENE–WSW. The stress orientation database is available under https://doi.org/10.24406/fordatis/200 (Kruszewski et al., 2022a), the stress magnitude database is available under https://doi.org/10.24406/fordatis/201 (Kruszewski et al., 2022b), whereas the hydrofracturing test reports are available under https://doi.org/10.24406/fordatis/222 (Kruszewski et al., 2022c).

Spatially varying crustal stress along the Zagros seismic belt inferred from earthquake focal mechanisms

We determine the stress field of the Zagros fold-and-thrust belt (ZFTB) in the collisional zone between the Arabian and Iranian plates. Using 898 mainshock focal mechanisms, which occurred along the belt between 1956 and 2021, we calculate stress at 32 locations. The results reveal that the crustal stress in the Zagros belt is heterogeneous with variations in the orientation of SHmax as well as in the stress ratio. The interpolation of the Simpson's index Aϕ points to three distinct domains: northern, central and southern, which are characterized by wrench, compressional, and wrench-compressional to compressional tectonic regimes, respectively. Tectonic stress in the study area is characterized by a horizontal to sub-horizontal maximum compression axis in the ∼NE-SW to ∼NNW-SSE direction. The GPS measurements of the strain tensor and the SHmax direction inferred from the stress inversion indicate that both the deformation pattern and the seismicity along the ZFTB zone are primarily affected by convergence of the Arabian and Iranian plates.

Geomechanical and Petrophysical Assessment of the Lower Turonian Tight Carbonates, Southeastern Constantine Basin, Algeria: Implications for Unconventional Reservoir Development and Fracture Reactivation Potential

In this study, we assessed the unconventional reservoir characteristics of the Lower Turonian carbonates from the southeastern Constantine Basin. We integrated petrography, petrophysical, and rock-mechanical assessments to infer formation properties and unconventional reservoir development strategies. The studied fossiliferous argillaceous limestones are rich in planktonic foraminifera, deposited in a calm and low energy depositional condition, i.e., deep marine basinal environment. Routine core analysis exhibits very poor porosity (mostly &lt; 5%) and permeability (&lt;0.1 mD), implying the dominance of nano and microporosity. Micritization and calcite cementation are inferred as the major reservoir quality-destroying diagenetic factors. Based on the wireline log-based elastic properties, the upper part of the studied interval exhibits higher brittleness (BI &gt; 0.48) and fracability (FI &gt; 0.5) indices compared to the lower interval. Borehole breakouts indicate ~N-S SHmax orientation and a normal to strike-slip transitional stress state has been constrained based on a geomechanical assessment. We analyzed safe wellbore trajectory and minimum mud weight requirements to ensure stability in the deviated and horizontal wells required for field development. At the present stress state, none of the fracture orientations are critically stressed. We inferred the fracture reactivation potential during hydraulic stimulation required to bring the tight Turonian limestones into production. Additional pore pressure build-up required to reactivate optimally oriented natural fractures has also been inferred to ensure success of hydraulic fracturing.

Natural fractures in the deep Sinian carbonates of the central Sichuan Basin, China: Implications for reservoir quality

The deeply buried carbonates in the Sinian Dengying Formation are one of the crucial hydrocarbon exploration targets in the central Sichuan Basin of China, and are characterized by strong heterogeneity in reservoir quality, which is closely related to natural fractures. To reveal the impacts of natural fractures on the reservoir quality of Deng-4 Member carbonates in the Gaoshiti-Moxi area, the types, characteristics, and factors controlling the development and effectiveness of fractures, were systematically analyzed based on core samples, thin sections, image logs, petrophysical measurements, and drill stem tests. Tectonic shear fractures are the dominant fracture type affecting the petrophysical properties of the Deng-4 Member reservoirs. The distribution and development of shear fractures are mainly controlled by lithology, mechanical layers and faults. Factors influencing the effectiveness of shear fractures include the timing of fracture formation, cementation, dissolution and current in-situ stress. Early-formed shear fractures are more susceptible to filling during subsequent cementation, whereas fractures formed at later stages are rarely filled and, also, have the potential to be solution-enlarged by dissolution. The relatively large apertures of unfilled shear fractures parallel to the orientation of SHmax are associated with the small effective normal stress exerted on fracture surfaces. Shear fractures effectively improve the overall permeability of Deng-4 Member carbonate reservoirs by connecting relatively isolated pores but have only minor impacts on reservoir porosity. The development degree and effectiveness of fractures together determine the contribution of fractures to reservoir permeability. In the Gaoshiti-Moxi area, shear fractures in the NWW-SEE and nearly E-W strikes provide the major flow pathways and contribute the most of permeability of the tight carbonate reservoirs in Deng-4 Member. These results provide insights for deeper understanding of the impact on reservoir quality of natural fractures in tight carbonates, and are significant for the evaluation of high-quality carbonate reservoirs in the Sichuan Basin.

Spatial Variation of Shallow Stress Orientation Along the Hikurangi Subduction Margin: Insights From In‐Situ Borehole Image Logging

AbstractKnowledge of the contemporary in‐situ stress orientations in the Earth's crust can improve our understanding of active crustal deformation, geodynamic processes, and seismicity in tectonically active regions such as the Hikurangi Subduction Margin (HSM), New Zealand. The HSM subduction interface is characterized by varying slip behavior along strike, which may be a manifestation of combined variations in both stress state and the mechanical properties of faults and their hanging walls. Alternatively, these variations in subduction thrust slip behavior may drive heterogeneity in the stress state in space and time. In this study, we analyze borehole image and oriented four‐arm caliper logs acquired from 13 boreholes along the HSM to present a comprehensive stress orientation data set from borehole data within the shallow (&lt;3 km) upper plate of the subduction thrust. Our results reveal a 065°/245° SHmax orientation within the central HSM (Hawke's Bay region) which rotates to 112°/292° and 140°/320° in the southern HSM. This rotation of SHmax orientation correlates spatially with along‐strike variations in subduction interface slip behavior, characterized by creep and/or shallow episodic slip events in the central HSM and interseismic locking in the south. The borehole SHmax orientations suggest that contemporary stress orientations may be caused in part by along‐strike variation in deformation style imposed by clockwise rotation of forearc. In the southern HSM, borehole‐derived SHmax orientations are inconsistent with SHmax orientations derived from focal mechanism solutions in the subducting plate, implying some degree of mechanical decoupling between the shallow hanging wall and subducting slab.

The recent stress state of Germany – results of a geomechanical–numerical 3D model

Abstract. A decisive criterion for the selection and the long-term safety of a deep geological repository for high radioactive waste is the crustal stress state and its future changes. The basis of any prognosis is the recent crustal stress state, but the state of knowledge in Germany is quite low in this respect. There are stress orientation data provided by the World Stress Map (WSM, Heidbach et al., 2018) and stress magnitude data from a database (Morawietz et al., 2020) for Germany, both providing selective information on the recent stress field. However, these data are often incomplete, of low quality and spatially unevenly distributed. Therefore, a 3D continuous description is not possible with these data so far, at most for the orientation of the maximum horizontal stress (SHmax), but not for the most important magnitudes of the minimum (Shmin) and SHmax. In the course of the SpannEnD project, a geomechanical–numerical 3D model of Germany is created, with which a continuous description of the complete tensor of the recent stress field in Germany is possible. The model covers an area of 1250×1000 km2 from Poland in the east, to France in the west, from Italy in the south to Scandinavia in the north. The depth extent is 100 km. Even though the focus is primarily on Germany, the model area was chosen to be so wide to minimize boundary effects and for a simplified definition of the displacement boundary conditions, which are ideally oriented perpendicular or parallel to the orientation of SHmax. The model contains a total of 21 units: The upper part of the lithospheric mantle, the lower crust, four laterally overlapping units of the upper crust, and 14 stratigraphic units of the sedimentary cover. The stratigraphic subdivision of the sedimentary cover is only done in the core area of the model; because this area is the focus of our study, our calibration data are mainly from this region and well-resolved geometry data are available. Outside of the core area, the sediments are grouped into an undifferentiated unit. The units are parameterized with density and elastic material parameters (Poisson's ratio and Young's modulus). The model has a lateral resolution of 2.5×2.5 km2 and a vertical resolution of a maximum of 240 m; in total it includes 11.1 million hexahedral elements. The equilibrium of forces between body and surface forces is solved by finite element method. The model is calibrated with Shmin and SHmax magnitudes from the WSM and data from the stress magnitude database. First, an initial stress state is generated and in a second step displacement boundary conditions are defined at the model edges, which are adjusted until a best-fit to the calibration data is found. The results show good agreement with both the SHmax orientation data from the WSM and the magnitudes of the two principal horizontal stresses (Shmin and SHmax) from the magnitude database.

Evaluation of the Diametrical Core Deformation and Discing Analyses for In-Situ Stress Estimation and Application to the 4.9\xa0km Deep Rock Core from the Basel Geothermal Borehole, Switzerland

The in situ state of rock mass stresses is a key design parameter, e.g., for deep engineered geothermal systems. However, knowledge of the stress state at great depths is sparse mostly because of the lack of possible in situ tests in deep boreholes. Among different options, core-based in situ stress estimation may provide valuable stress information though core-based techniques have not yet become a standard. In this study we focus on the Diametrical Core Deformation Analysis (DCDA) technique using monzogranitic to monzonitic rock drill cores from 4.9 km depth of the Basel-1 borehole in Switzerland. With DCDA the maximum and minimum horizontal stress (SHmax and Shmin) directions, and the horizontal differential stress magnitudes (∆S) can be estimated from rock cores extracted from vertical boreholes. Our study has three goals: first, to assess photogrammetric core scanning to conduct DCDA; second, to compare DCDA results with borehole breakout and stress-induced core discing fracture (CDF) data sets; and third, to investigate the impact of rock elastic anisotropy on ∆S. Our study reveals that photogrammetric scanning can be used to extract reliable core diametrical data and CDF traces. Locally aligned core pieces showed similar SHmax orientations, conform to borehole breakout results. However, the variability of core diametrical differences was large for the Basel-1 core pieces, which leads to a large spread of ∆S. Finally, we demonstrate that core elastic anisotropy must be considered, requiring robust estimates of rock elastic moduli, to receive valuable stress information from DCDA analyses.