Strike-slip Faulting Stress Regime Research Articles

Constraint strategies for estimating in-situ stress from borehole measurements

In this study, we describe strategies to reduce uncertainties in the estimation of in-situ stresses. The strategies are based on the commonly used poro-elastic model with strain corrections for in-situ stress estimation. In addition to the calibration of the minimum horizontal stress magnitudes from measurement data, we further constrain the magnitude of maximum horizontal stress quantitatively with (i) a critical stress state model, (ii) (non)-occurrences of breakout and/or drilling-induced fracture observed from image logs, and (iii) qualitatively with elliptical borehole shapes observed from multi-arm caliper logging data. The methodology is demonstrated using wells near Dawson Creek, Northeast British Columbia and Northwest Alberta, in Western Canada Sedimentary Basin (WCSB). The uncertainty of the estimated maximum horizontal stress magnitude was reduced and the range of the maximum horizontal stress was narrowed after the application of these constraint strategies. It is also observed that in the east of the study area, the in-situ stress regime is a normal fault stress regime. The presence of a normal fault stress regime is unexpected since a strike–slip fault stress regime is typically considered for the entire region. Yet, analysis of caliper data in two horizontal wells has confirmed its presence.

Present-day in-situ stress prediction in the Duvernay shale of Simonette Block, Western Canada Sedimentary Basin

The Duvernay Formation of Simonette Block is rich in shale oil and gas resources. The present-day in-situ stress field is a crucial parameter during the exploration and development in this block, a better understanding of which can assist with drilling and completion engineering, fracturing stimulation, and well deployment. In this study, the present-day in-situ stress in the Duvernay shale is predicted and analyzed based on well calculations and geomechanical modeling. The findings indicate that, I, NE-SW-trending is the dominant S Hmax orientation. The S Hmax and S hmin are the maximum and minimum principal stress, indicating that the Duvernay shale of Simonette Block is under a strike-slip faulting stress regime. II, The present-day in-situ stress in the Duvernay shale is heterogeneously distributed, which is controlled by lithology difference, fault development, and distribution. High stress values in the upper shale member D layer are mainly in the northwestern and central parts of Simonette Block. III, The present-day differential stress in the Duvernay shale mainly ranges from 15 MPa to 30 MPa. Bedding-parallel fractures are relatively developed, aiding in the creation of complex fracture networks. The results are expected to offer geological references for further development of shale oil and gas in the Simonette Block.

Seismicity and state of stress in the north-central region of the Gulf of California, Mexico

We compile a catalog of well-located earthquakes to estimate focal mechanisms in the north-central region (27.5 N–31.5 N) of the Gulf of California (GoC) to determine the state stress. The catalog consists of 81 relocated earthquakes from 2016 to 2021 with magnitudes between M4 and M5.7 and depths <25 km. We selected 47 earthquakes recorded by the Gulf of California Broadband Seismological Network (RESBAN) and the National Seismological Service (SSN) to obtain focal mechanism solutions by waveform inversion using the ISOLA software (Sokos and Zahradnik, 2008). Our database consists of 80 focal mechanisms including 33 reported in other catalogs. The fault plane solutions indicate that 36.25% correspond to normal faulting, 33.75% strike-slip, 25% oblique-slip, and 5% thrust faulting. We formed 3 groups of earthquakes with known focal mechanisms to estimate the orientation of the principal stresses using the Bayesian method of right trihedral (BRTM). Each group contains similar focal mechanisms and that align with the main faults in the region. We find that in the northernmost region of the GoC predomine a normal faulting stress regime while in the central region persists a strike-slip faulting stress regime, typical of transform boundaries. We conclude that the state of stress is heterogeneous in the north-central region of the GoC; that the orientation of maximum horizontal stress (σHmax) is compatible with the world stress map (WSM) in some zones; and that the friction coefficient estimated indicates that the faults are strong in the central region of the GoC.

Magnitudes and orientations of present-day in-situ stresses in the Kurdistan region of Iraq: Insights into combined strike-slip and reverse faulting stress regimes

The study area is in the Kurdistan Region of Iraq, including the northwestern extension of the Zagros suture zone, where present-day stress data derived from actual measurements is rare. The magnitudes and orientations of principal in-situ stresses were determined using well logging data. The deepest drilled formation in this study was the Albian upper Qamchuqa Formation in northeastern Iraq, which is a carbonate hydrocarbon reservoir. In the present study, several wireline logs including borehole image logs, conventional wireline logs, and six-arm caliper logs for two vertical wells provided detailed borehole breakout and drilling-induced tensile fracture (DITF) data between depths of 1600 and 2240 m. Two oil wells (wells A and B) were evaluated, and in well A, several breakouts occurred in eight distinct zones. Below the depth of 1600 m, 26 pairs of breakouts with considerable length were observed. The standard deviation of breakout azimuths was 11°, and the mean azimuth of the breakouts was N162°E. Using breakout and DITF data, the orientations of the maximum and minimum horizontal stresses (SH and Sh) were determined, and the results were validated by six-arm caliper measurements and lithological evaluation from wireline log data. The mean azimuth of maximum horizontal in-situ stress was N72°E, which showed relative consistency with the NE–SW to E–W direction of tectonic movement and previous studies in the nearby Zagros suture zone. Ultimately, the magnitudes of the three principal in-situ stresses were determined by two methods: poroelastic strain theory and breakout analysis. The stress regime below the depth of 1600 m was a reverse faulting stress regime, and above this depth, it likely changed to a strike-slip faulting stress regime. The determined stress regime was consistent with the nature and dynamics of the tectonic plate movement of the Arabian and Eurasian (Iranian) plates. Consequently, results suggest that the pattern of present-day tectonic stress is controlled mainly by the collision between the Arabian and Eurasian plates.

Complex in situ stress states in a deep shale gas reservoir in the southern Sichuan Basin, China: From field stress measurements to in situ stress modeling

The deep shale gas reservoir (3,500 ∼ 5,000 m) of the Silurian Longmaxi Formation in the southern Sichuan Basin of China has great potential for exploration and development. Complicated by the fault zones and other geostructures, the in situ stress state of shale gas formations in the southern Sichuan Basin is poorly understood, and field in situ stress measurement data are rare in these deep formations. This study provides vital insights into the in situ stress state based on in situ stress measurements and into the development of a 3D in situ stress model. To accurately model the distributions of in situ stress in the Longmaxi shale gas formation, multiple diagnostic fracturing injection tests (DFIT) were conducted on a 310-km2 shale gas development block. The minimum horizontal principal stress (σhmin) and pore pressure were interpreted from the DFIT data by using pressure transient analysis (PTA) methods. The maximum horizontal principal stress (σhmax) was estimated from multiple sources. The 3D in situ stress model was constructed by using the finite element (FE) method based on a site-specific 3D geological model. The stress model was calibrated and constrained by the results from field DFIT stress measurements.The in situ stress modeling revealed a complex in situ stress state in this deep shale gas formation. The presence of faults changes both the magnitudes and orientations of in situ stress. There is a general decrease in the angle between the σhmax direction and the fault strike in the vicinity of the faults. The stress magnitudes changed significantly near some faults, and the stress state transitioned from a strike-slip faulting stress regime to a reversed faulting stress regime. The reservoir is highly overpressured, with pore pressure gradients ranging from 17 to 20 kPa/m within the reservoir, which causes some faults to have a higher risk of slipping. Based on the 3D stress model, the fault stability and pore pressures that are required to slip the faults were estimated. Because many previous studies about in situ stress modeling have been based on well log-interpreted stresses, this study emphasizes the importance of constructing in situ stress models based on direct field stress measurements and of constraining the stresses through multiple sources and methods.

Contemporary state of stress in a stable plate interior (northern Poland): The integration of satellite geodesy, borehole and seismological data

In our study we examined the possibility of using strain rate data, determined from geodetic satellite measurements for estimations of magnitude of contemporary stress in a relatively stable area of northern Poland, comprised of fragments of the East European Craton, the Teisseyre-Tornquist Tectonic Zone and the Palaeozoic Platform. The strain rates and their principal directions were determined from 63 GNSS stations lacking special stabilization. The method for filtrating the unstable stations allowed us to eliminate half of these stations from the final strain rate calculations that included data averaging in a correlation radius of 150 km. A comparison of the horizontal strain directions determined by geodetic methods with the directions of contemporary stress based on borehole data (four of which were determined in this study) showed a high degree of convergence. Also the trends of strain rate variations showed consistency with integrated lithospheric strength of main tectonic units. The strain rates obtained are consistent with those published for the adjacent areas. Stress magnitudes were estimated by calibrating the timing of elastic strain accumulation by earthquakes in the Sambia Peninsula (Kaliningrad Oblast) and assuming a transient state of elastic strain on the craton. Our results indicate that the crystalline upper crust in the analysed part of the craton is in the strike-slip faulting stress regime prevailing over the normal faulting stress regime. The present state of an elastic strain has been accumulated over the last few million years causing stress being far from the critical state in the Mohr-Coulomb frictional criteria, except of the seismically active Sambia Peninsula.

A study on the evaluation of geological sweet spots in the super deep strike slip fault controlled reservoir

There are complex strike-slip fault systems in the Ordovician carbonate strata in the T-sh area of Tarim Basin. They are the primary storage space and flow channel of hydrocarbon resources. Scientific evaluation of internal space and conductivity of different parts along the main faults is the decisive factor for efficient development of the T-sh reservoir. Based on the 3-D seismic and the coherent body data, the fault structure models in the reservoir of the S1 and S5M fault zones are established. The original regional geomechanical parameters are determined according to the core testing results and logging data. The local stress field near the fault is studied based on the numerical simulation method. The sliding trend coefficient, expansion coefficient, and comprehensive conductivity coefficient of faults in the reservoir of S1 and S5M fault zones are quantitatively evaluated. The results show that the Ordovician strata in S1 and S5 fault zones are in a strike-slip faulting stress regime. The vertical stress (SV) gradient is 0.0243 MPa/m, the maximum horizontal stress (SH) gradient is 0.0246 MPa/m, the minimum horizontal stress (Sh) gradient is 0.0177 MPa/m, and the orientation of Sh is 166.27 °–197.25°. Based on the simulation results, it is found that there is an apparent correlation between the sliding trend coefficient of the faults and mud loss. The expansion coefficient is positively correlated with venting and production. The research results have practical significance for geological sweet spots prediction and drilling construction in the T-sh oilfield.

Contemporary background stress field in the eastern Tibetan Plateau: Insights from 3D geomechanical modeling

Knowledge of the crustal stress state is important for understanding tectonic processes and seismic hazard assessment as well as for various practical applications. In this study, we present a model describing the contemporary 3D crustal background stress field of the eastern Tibetan Plateau. The model incorporates a complex fault system and an appropriate initial stress state that matches both model-independent stress observations and kinematic data well. Our results show that the modeled maximum horizontal stress SH is generally oriented in NW-SE direction. The Bayan Har Block and the Chuan-Dian Block are dominated by a strike-slip faulting stress regime. However, in some local areas, the crust is governed by a normal faulting regime. We infer that the local normal faulting stress regime in the Bamei-Kangding area is caused by the clockwise rotation of the strike of the Xianshuihe fault at Kangding. This normal faulting stress regime would promote the ascent of intrusions and provide the space necessary for the emplacement of the Zheduoshan granite. The stress state on the Longmen Shan fault zone is rather inhomogeneous. For the Yingxiu-Beichuan fault, the northeastern segment has a lower normal stress but a higher shear stress than the southwestern segment. This inhomogeneous stress state on the Yingxiu-Beichuan fault could provide an explanation for the northeastward propagation of the ultra-long unilateral rupture in the Wenchuan Ms8.0 earthquake. According to the stress state on the Xianshuihe fault, we conclude that the Kangding-Shimian segment has a high potential to produce a strong earthquake due to its high normal stress and a fast slip rate and should be carefully monitored. On the Bamei-Kangding segment, which is governed by a normal faulting stress regime and may be lubricated by fluids, slip seems to be facilitated favoring more frequent small to moderate magnitude earthquakes to occur on this segment.

Geomechanical analysis of horizontal wells in terms of stability for drilling — A case study from the Peri-Baltic Syneclise

This paper presents a shale gas field geomechanics case study in the Peri-Baltic Syneclise (northern Poland). Polish Oil and Gas Company drilled a vertical well, W-1, and stimulated the Silurian target. Next, a horizontal well, W-2H, drilled the Ordovician target and partially collapsed. The remaining interval was stimulated, and microseismic monitoring was performed. A second horizontal well, W-3H, was drilled at the same azimuth as W-2H, but the well collapsed in the upper horizontal section (Silurian). A geomechanical earth model was constructed that matches the drilling experiences and well failure observations found in wells W-1, W-2H, and W-3H. The field was found to be in a strike-slip faulting stress regime, heavily fractured, with weak bedding contributing to the observed drilling problems. An analysis of safe mud weights, optimal casing setting depths, and optimal drilling directions was carried out for a planned well, W-4H. Specific recommendations are made to further enhance the model in any future studies. These recommendations include data acquisition and best practices for the planned well.

Recent temporal changes in the stress state and fault reactivation assessment of HTB underground gas storage in China associated with gas injection and extraction

The stress state variations due to seasonal injection and extraction of gas in HTB underground gas storage (HTB-UGS) were estimated, as well as the fault-slip analysis of the main fault, to understand how gas injection and extraction result in earthquakes. The stress state at different depths at the time of a complete depletion in 2012 was first estimated using image-logged borehole breakout data and stress inversions of focal mechanism solutions. Further, the temporal variations of the stress state caused by changes in the pore pressure due to gas injection and extraction were estimated based on a simple pore pressure-stress coupling assumption. The fault reactivation/slip tendency temporal changes and their possible relation with seismicity in this area were analyzed. Results revealed that the stress orientation in the reservoir layer was around N21°E−N23°E, which was consistent with the tectonic stress prevailing direction. The stress regime varied between reverse and strike-slip faulting stress regimes during the gas injection and extraction cycles. The main fault-slip tendency initially decreased continuously with gas production and then increased with gas injection. Earthquake sequences appeared when the slip tendency increased to a critical value of 0.45 ± 0.03.

In Situ Stress and Stress Regime in the Onshore Part of the Northeast Java Basin

In situ stress is importance in the petroleum industry because it will significantly enhance our understanding of present-day deformation in a sedimentary basin. The Northeast Java Basin is an example of a tectonically active basin in Indonesia. However, the in situ stress in this basin is still little known. This study attempts to analyze the regional in situ stress (i.e., vertical stress, minimum and maximum horizontal stresses) magnitude and orientation, and stress regime in the onshore part of the Northeast Java Basin based on twelve wells data, consist of density log, direct/indirect pressure test, and leak-off test (LOT) data. The magnitude of vertical ( and minimum horizontal ( stresses were determined using density log and LOT data, respectively. Meanwhile, the orientation of maximum horizontal stress ( was determined using image log data, while its magnitude was determined based on pore pressure, mudweight, and the vertical and minimum horizontal stresses. The stress regime was simply analyzed based on the magnitude of in situ stress using Anderson’s faulting theory. The results show that the vertical stress ( ) in wells that experienced less erosion can be determined using the following equation: , where is in psi, and z is in ft. However, wells that experienced severe erosion have vertical stress gradients higher than one psi/ft ( . The minimum horizontal stress ( ) in the hydrostatic zone can be estimated as, while in the overpressured zone, . The maximum horizontal stress ( ) in the shallow and deep hydrostatic zones can be estimated using equations: and , respectively. While in the overpressured zone, . The orientation of is ~NE-SW, with a strike-slip faulting stress regime.

In Situ Stress and Stress Regime in the Onshore Part of the Northeast Java Basin

In situ stress is importance in the petroleum industry because it will significantly enhance our understanding of present-day deformation in a sedimentary basin. The Northeast Java Basin is an example of a tectonically active basin in Indonesia. However, the in situ stress in this basin is still little known. This study attempts to analyze the regional in situ stress (i.e., vertical stress, minimum and maximum horizontal stresses) magnitude and orientation, and stress regime in the onshore part of the Northeast Java Basin based on twelve wells data, consist of density log, direct/indirect pressure test, and leak-off test (LOT) data. The magnitude of vertical ( and minimum horizontal ( stresses were determined using density log and LOT data, respectively. Meanwhile, the orientation of maximum horizontal stress ( was determined using image log data, while its magnitude was determined based on pore pressure, mudweight, and the vertical and minimum horizontal stresses. The stress regime was simply analyzed based on the magnitude of in situ stress using Anderson’s faulting theory. The results show that the vertical stress ( ) in wells that experienced less erosion can be determined using the following equation: , where is in psi, and z is in ft. However, wells that experienced severe erosion have vertical stress gradients higher than one psi/ft ( . The minimum horizontal stress ( ) in the hydrostatic zone can be estimated as, while in the overpressured zone, . The maximum horizontal stress ( ) in the shallow and deep hydrostatic zones can be estimated using equations: and , respectively. While in the overpressured zone, . The orientation of is ~NE-SW, with a strike-slip faulting stress regime.

Understanding present-day stress in the onshore Canning Basin of Western Australia

Western Australia’s Canning Basin is an underexplored prospective basin with proven petroleum systems and small-scale production. Recently, several formations within deeper depocentres have been investigated for unconventional hydrocarbon resources. Modern petroleum resource evaluation generally depends on an understanding of local and regional stresses that are a primary control over the formation and propagation of induced fractures. There are significant gaps in our understanding of these factors within the Canning Basin. This study characterises the regional stress regime of the onshore Canning Basin and presents modelled present-day stress, allowing for the identification of significant stress heterogeneity and natural fracture barriers. Interpretation of wireline data reveals a present-day state of stress with variation in magnitude and faulting-type. An approximately northeast–southwest regional present-day maximum horizontal stress orientation is interpreted, which is in broad agreement with the Australian Stress Map and previously published data. One-dimensional mechanical earth models, constructed for intervals from 15 Canning Basin wells, highlight the relationship between lithology and stress. Significant changes in stress within and between lithological units, owing to the existence of discrete mechanical units forming numerous inter- and intra-formational stress boundaries, likely to act as natural barriers to fracture propagation, particularly within those units currently targeted for their unconventional resource potential, are interpreted. A strike-slip faulting stress regime is interpreted broadly through the basin. However, when analysed in detail there are three distinct stress zones identified: (1) a transitional reverse- to strike-slip faulting stress regime in the top ∼1 km, (2) a strike-slip faulting stress regime from ∼1 km to ∼3.0 km and (3) a transitional strike-slip to normal faulting regime at depths greater than ∼3.0 km. This study is a component of the Australian Government’s Exploring for the Future initiative, which focusses on gathering new data and information about northern Australia’s resource potential. KEY POINTS New stress data, including 15 mechanical earth models, are presented for Western Australia’s onshore Canning Basin. Broadly, a strike-slip stress regime with a NE–SW-oriented maximum horizontal orientation is described. Significant stress heterogeneities and natural fracture barriers are identified.

Dependence of earthquake faulting type on fault strike across the Korean Peninsula: Evidence for weak faults and comparison with the Japanese Archipelago

We re-evaluate the stress state across the Korean Peninsula by analyzing the stress tensor inversion results, strike–rake relationships, and faulting-type indicator–magnitude relationships of published earthquake focal mechanism data. Previous studies have shown that the stress state is characterized by a strike-slip faulting stress regime with ENE–WSW maximum compression, NNW–SSE minimum compression, and vertical intermediate stress. The differential stress magnitudes are estimated to be on the order of 100 MPa under the assumption of the strong-fault earthquake with a high Coulomb friction coefficient of 0.85. Conversely, other studies have shown that the weak-fault model is applicable across the Japanese Archipelago, where the differential stress level is only a few tens of MPa. We, therefore, review previous studies, evaluate the focal mechanism data across the Korean Peninsula, and infer the absolute stress level. Our comparison of the geological and tectonic histories of the Korean Peninsula and Japanese Archipelago suggests that the NNW–SSE tensional stresses in the Korean Peninsula are caused by slab rollback of the Philippine Sea Plate at around 5 Ma, west of Kyushu Island, southwestern Japan. We also find that the earthquake faulting types across the Korean Peninsula are closely related to their fault strikes, confirming a spatially uniform distribution of the strike-slip stress regime with nearly equal tensional and compressional tectonic stresses. However, this situation leads to the apparent paradox of strike-dependent fault strength. We eliminate this paradox by proposing a model whereby reverse- and normal-faulting events are caused by local stress heterogeneities around the edges of strike-slip faults under a uniform low-strength strike-slip stress state for all faulting types. We also highlight the usefulness of strike–rake and faulting-type indicator–magnitude diagrams in considering stress states.

Characteristics of in-situ stress and its controls on coalbed methane development in the southeastern Qinshui Basin, North China

In-situ stress is a key reservoir parameter to evaluate reservoir permeability, hydraulic fractures, coal seam deformation and coalbed methane (CBM) recovery. With limited CBM test wells in the Zhengzhuang field, southeast of the Qinshui basin, North China, the in-situ stress data is inadequate for CBM exploration and development. It is necessary to find a method to predicate in-situ stress through other exploration data such as geophysical well loggings. In this study, we provide a new well-logging data-based model to predicate the in-situ stress based on 17 sets of well test data and comprehensive logging data. As a distinguished characteristic of this model, different structural compartmentalization of CBM reservoirs was considered. A regional adaptive residual strain index was introduced to the model. Based on the model, the in-situ stress distribution in the Zhengzhuang field were evaluated systematically, and the influences of in-situ stress on permeability and the propagation of hydro-fractures were discussed. Results indicate that the magnitude of the maximum (SHmax,14.19–45.40 MPa) and minimum horizontal stresses (Shmin, 10.62–28.38 MPa) and the gravitational stress (Sv, 9.58–30.82 MPa) all show positive correlations with burial depths. The in-situ stress fields in the study area are characterized by 1) SHmax > Sv > Shmin in shallow layers (<700 m), indicating a dominant strike-slip faulting stress regime; 2) Sv ≈ SHmax > Shmin and SHmax > Sv > Shmin in the depth of 700–1050 m, suggesting a transformed regime; and 3) SHmax > Sv > Shmin in deep layers (>1050 m), indicating a strike-slip faulting stress regime. The SHmax in the study area is orientated by NE-SW, with a trend of 40°–49°. Resulted from the change of the in-situ stress regimes from shallow (500 m) to deep layers (∼1000 m), the reservoir permeability variation shows a typical increase followed by decrease. The presence of natural fractures significantly affect the propagation pattern of hydraulic fractures, and the length difference between the major and branch fractures increases with increasing stress anisotropy in the Zhengzhuang field.

A methodology for unstructured damped stress inversion of microseismic focal mechanisms: Application to the Vaca Muerta Formation, Argentina

Current methodologies for stress inversion from microseismic focal mechanisms require the assignment of events to a regular grid and then solving for the stress state at each grid node. This approach can lead to irregularities in the solution because some nodes may contain few or even no events. To address this issue, we modified the algorithm to solve for stresses on an irregular (unstructured) grid. We first use the [Formula: see text]-means algorithm to split the data into suitably sized groups. The centroids of these groups are then considered as the nodes of an unstructured grid, and we simultaneously solve for the stress state in each group using damped inversion. To account for the irregularity of the unstructured grid, we use the reciprocal square distance between nodes as weights, as opposed to the existing method in which a weight of unity is assigned between adjacent nodes on a regular grid. Focal planes are selected from the auxiliary plane using the fault instability criterion. The method is first applied to synthetic data sets in which we simulate and subsequently invert for the stress field around a mode-I fracture at depth, in a strike-slip and in a normal faulting stress regime. Results indicate a stress orientation error of 10° and a stress ratio error between 1% and 10%. We then consider focal mechanism data from an unconventional shale play in the Vaca Muerta Formation in Argentina, and our results suggest the presence of a preexisting strike-slip faulting stress regime. We also find that the unambiguous focal plane picks suggest that the apparent dip-slip focal mechanisms are indeed dip-slip movement along subvertical natural fractures, which correlate well with image log data. We suggest that these dip-slip events are caused by stress changes induced by the opening of the hydraulic fractures.

Characteristics of in-situ stress distribution in Zhengzhuang Region, Southern Qinshui Basin, China and its stress path during depletion

The in-situ stress and its behavior during depletion are critical for investigation of the reservoir permeability, wellbore stability, enhanced oil recovery (e.g. stimulation), fault reactivation, and reservoir management. However, the rules that govern in situ stress variation in coalbed methane (CBM) reservoirs and its influences on reservoir stability are still unclear due to the matrix shrinkage effect. In this study, the distribution characteristics of in-situ stress in the Zhengzhuang (ZZ) region were investigated by multi-loop hydraulic fracturing tests and a theoretical stress-depletion response model was built to reveal the dynamic rules of in-situ stress during CBM depletion. Additionally, considering the redistribution of in-situ stress and the limitation induced by fault friction coefficient, a criterion for stress-based failure was developed to analyze the stability of CBM reservoir. The results suggest that in the ZZ block, the fracturing pressure, closure pressure, and the maximum and minimum horizontal principal pressures are positively correlated with burial depth. During drainage, the horizontal principal stress reduces and the effective horizontal principal stress increases linearly as pore pressure reduction. During gas desorption phase, the horizontal principal stress decreases and the effective horizontal principal stress increases non-linearly under weak desorption, whereas both decline non-linearly in response to strong desorption. In addition, the failure criterion for CBM reservoirs indicates that reservoirs in a normal faulting stress regime are the most easily damaged, which may occur during drainage or desorption. However, reservoir damage can only be induced under strong desorption in a strike-slip faulting stress regime and cannot occur in a reverse faulting stress regime. If faults do develop in reservoirs, the reservoir stability is governed by the fault friction coefficient. Finally, by combining the data, the changes in in-situ stress and the stability of the reservoirs during depletion in the ZZ region were revealed and its implications for CBM development were discussed.

Injection-induced seismicity: strategies for reducing risk using high stress path reservoirs and temperature-induced stress preconditioning

SUMMARYIt is suggested that fluid injection in normal faulting stress regimes can stabilize a reservoir if the stress path is high enough. This stabilization is not seen when the reservoir is significantly cooled as a result of injection. Further, a new strategy is suggested for stimulating reservoirs in shear with a reduced chance of inducing a large magnitude seismic event. The version of this methodology presented here is applicable for reverse faulting stress regimes and involves an initial stress preconditioning stage where the reservoir is cooled and the pressure increase is limited. This process reduces the horizontal total stress and thereby also the differential stress. Next, the reservoir is stimulated with a rapid increase in pore pressure, resulting in shear failure at a lower differential stress than was initially present in the reservoir. Due to the connection seen between the Gutenberg–Richter b-value and differential stress, it is suggested that reservoirs stimulated in this fashion will exhibit higher b-values and thereby also have a reduced chance of hosting a large magnitude event. It is suggested that adaptations of this methodology are applicable to both normal and strike-slip faulting stress regimes.

Present-day stress state and fault stability analysis in the capital area of China constrained by in situ stress measurements and focal mechanism solutions

This work proposes to determine the present-day in situ stress state across both known and suspected faults in the capital area of China and to assess fault stability and consequent seismic hazards. The population of main active faults in the capital area is summarized, and 168 sets of hydraulic fracturing and overcoring data and 84 sets of earthquake focal mechanism solutions are gathered and studied. The available data indicate that reverse and strike-slip faulting stress regimes are favored at reservoir depths in this region. The ratio of maximum horizontal stress to vertical stress (KH) and the ratio of minimum horizontal stress to vertical stress (Kh) at a depth of 500 m show complex spatial variations in stress magnitudes. Generally, KH appears to increase southward, which might suggest that stress is accumulating in the southern part of the region. The relative stress magnitude R appears independent of the depth and mainly varies between 0.5 and 1.0. The observation of stress data suggests a relatively uniform direction (nearly E-W) for the maximum horizontal principal stress. The influence of hydrostatic pore pressure on critical stresses is discussed, and the presence of hydrostatic pore pressure makes faults more conducive to reactivation. Most locations in this region are critically stressed based on the Coulomb frictional failure criterion and Byerlee’s law with hydrostatic pore pressure; thus, the present-day stress state appears to be high enough to initiate slip. In addition, the relationship between the tectonic stress field and earthquakes is discussed.

In situ stress and natural fractures in the Carnarvon Basin, North West Shelf, Australia

ABSTRACTFractures are important with regards to permeability within the basin subsurface; thus, understanding their generation within a given stress regime is crucial to the extraction of petroleum resources. A total of 517 naturally occurring fractures are identified on 12 resistivity image logs from the Exmouth and Beagle sub-basins, the Rankin Platform and the Exmouth Plateau in the Carnarvon Basin on the North West Shelf of Australia. The fractures have been identified on 12 image logs and can be divided into two sets: (1) electrically resistive and conductive fractures striking northeast–southwest; and (2) electrically resistive and conductive fractures striking east–west. There were 235 electrically resistive fractures identified that dominantly strike northeast–southwest, and 282 conductive fractures identified that dominantly strike east–west. The latter are considered to be open for fluid flow. The in situ stress field (orientations and magnitudes) is a major control on the ability of fractures to transmit fluid. This study identified 123 drilling-induced tensile fractures and 175 borehole breakouts present in 12 image logs, and a mean maximum horizontal stress orientation of 110°. Density logs and leak-off tests were used to calculate the in situ stress magnitudes with a vertical stress gradient () of 21.7 MPa km−1, a minimum horizontal stress gradient of 16.8 MPa km−1 and a maximum horizontal stress gradient of 23.4 MPa km−1. This defines a strike-slip faulting stress regime for wells in this study in the Carnarvon Basin. The in situ stress and natural factures determined and identified in this study provide further clarity to the exploration and production processes occurring in the Carnarvon Basin.