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ABSTRACT We investigate horizontal stress variation in the vicinity of the deep-seated Balarud Lineament in the northern part of Iran's Dezful Embayment in the Zagros Fold and Thrust Belt (ZFTB). Both petrophysical data from drilled oil and gas wells (3-4 km deep) and earthquake focal plane solutions (6 to 25 km deep) are used to constrain the orientation and relative magnitudes of the local and regional stresses. The stress orientations in the sedimentary rock strata and the basement are of two entirely distinct types. In the basement, constant regional NE-SW SHmax orientation is observed in the northern ZFTB. The seismologically determined local SHmax direction from 25 focal mechanisms around the Balarud Lineament is 29.3°±8.5°. However, observations of borehole breakouts and tensile-induced fractures indicate that the dominant SHmax orientation is N-S near the Balarud Lineament. The consistent stress direction in the basement indicates a high differential horizontal stress magnitude, whereas the principal stress orientation rotates 35° counter-clockwise in the sedimentary cover where the state of stress is extensional. The Lineament's second-order stress patterns are discussed in terms of wellbore placement and completion decisions. INTRODUCTION Anomalous relative stress magnitudes and orientations have been observed in the world's various uniform lithospheric stress fields. Stress deflection may be observed due to lateral density/strength contrasts, flexural stresses, or superimposed geological structures such as faults (Sonder, 1990) and salt diapirs (Dusseault et al., 2004). In the east-west-trending Transverse Ranges (California), the horizontal stress orientation is different by 25° from the reference stress state in the NW-SE San Andreas fault (Sonder, 1990). The regional NE-SW SHmax is reoriented to N-S in the area overlying the Peace River Arch in the Western Canadian Sedimentary Basin (Bell & McCallum, 1990). The same phenomenon is reported for the Amazon rift in central Brazil (Zoback & Richardson, 1996) and in the Swiss Alps and the northern Alpines foreland (Kastrup et al., 2004). Stress deflections can also be caused by reservoir depletion (Yale et al., 1994) and by earthquakes (Hauksson, 1994). Hauksson (1994) observed a 15° (±10°) rotation of local stress axes due to the 1992 Mw = 7.3 Landers's earthquake sequence. The scale at which second-order stress patterns occur depends on the degree of lateral density/strength contrasts and the size of geological structures and their orientation relative to regional stress fields (Sonder, 1990; Zoback, 1992).
- North America > Canada > British Columbia (0.54)
- North America > Canada > Alberta (0.54)
- Asia > Middle East > Iran (0.48)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (1.00)
- Geology > Structural Geology > Tectonics > Compressional Tectonics > Fold and Thrust Belt (1.00)
- Geophysics > Seismic Surveying (0.68)
- Geophysics > Borehole Geophysics (0.68)
Stress State Change and Fault-Slip Tendency Assessment Associated with Gas Injection and Extraction in the Hutubi (China) Underground Gas Storage
Gao, Guiyun (National Institute of Natural Hazards (MEMC), Chungnam National University (Corresponding author)) | Chang, Chandong (Chungnam National University) | Wang, Chenghu (National Institute of Natural Hazards (MEMC)) | Li, Yanyong (Earthquake Agency of Xinjiang Uygur Autonomous Region) | Jia, Jin (Xi’an Research Institute Co. Ltd)
Summary The Hutubi (HTB) gas reservoir in northwest China was active in production from 1998 to 2012; beginning in 2013, it was used for gas storage. Several earthquakes occurred during the early operation of the Hutubi underground gas storage (HTB-UGS) project, so it is important to explore their relationship with gas injection and extraction from the viewpoint of stress state change and faulting susceptibility. We study the evolution of the stress state and slip tendency associated with the operation of the HTB-UGS to understand the relationship between pore pressure variation and induced seismicity. We constrain the stress state in the reservoir at the time of cessation of production in 2012 using image-logged wellbore breakouts. We then estimate stress state variation over time as a result of pore pressure changes, based on the assumption of poroelastic coupling between horizontal elastic stresses and pore pressure. The maximum principal stress in the reservoir region before the HTB-UGS operation is oriented N23°±6°E, which is consistent with that of the prevailing tectonic stress derived from earthquake focal mechanisms. Our results show that the vertical stress (Sv) and the minimum horizontal principal stress (Shmin) were initially similar in magnitude before petroleum production; the magnitude of Shmin gradually decreased during petroleum production. Gas injection, initiated in 2013, raised the Shmin magnitude, returning it to approximately that of Sv. We use the estimated variations in the reservoir stress state over time to calculate temporal changes in slip tendency of the main faults in the reservoir. The fault-slip tendency decreased continuously with petroleum production and then increased with gas injection. The first earthquake swarm associated with gas injection occurred approximately 2 months after the start of injection, possibly due to the slow pore pressure diffusion. Thereafter, earthquakes were induced whenever gas was injected. Our assessment of slip tendency suggests that earthquake swarms were induced during increasing pore pressure phases when slip tendency reached a value of 0.43 ± 0.04. The maximum allowable pore pressure for avoiding earthquakes (ML > 1.5) is 29.0 ± 4.5 MPa, which might increase through stress adjustments and a newly attained balance. This study provides a possible method for induced earthquake analysis associated with gas injection and extraction from the perspective of stress state change and faulting susceptibility.
- Asia > China > Xinjiang Uyghur Autonomous Region (0.28)
- North America > United States > New Jersey (0.28)
- North America > United States > Texas (0.28)
- Europe > United Kingdom > England (0.28)
- Geophysics > Seismic Surveying > Borehole Seismic Surveying (1.00)
- Geophysics > Borehole Geophysics (1.00)
- North America > United States > Texas > Permian Basin > Cogdell Field > Fuller Sand Formation (0.99)
- North America > United States > Texas > Permian Basin > Cogdell Field > Area Formation (0.99)
- Europe > France > Nouvelle-Aquitaine > Lacq Basin > Lacq Field (0.99)
- (2 more...)
Abstract Microseismic monitoring has become necessary as daily operational work for various underground energy development projects, e.g., unconventional oil and gas or enhanced geothermal system projects. The primary purpose of microseismic monitoring is to record the effect of fluid injections and to infer the shape of the created reservoir. Thus, hypocenter location is the first order information. Various seismological analysis technics provide chances to extract more information such as fault orientations from microseismicity. However, the quality of microseismic waveforms is limited, i.e., they have a small amplitude. In this study, we propose the novel concept to introduce information of natural fractures inferred from borehole logging into microseismic analysis as prior information. In seismology, we solve reverse problems by analyzing seismograms. We cannot access in-situ geophysical data in seismogenic zones since basically natural earthquakes occur mostly at much greater depth than we can reach. In the field of induced seismicity, we can access geophysical parameters related to induced seismicity directly through borehole measurements. This offers possibilities to integrate geophysical information in seismological analyses geomechanical and geological theory. We focus on natural fracture information acquired by borehole log analysis and apply it to focal mechanism analysis of induced seismicity. In this work we use data from the Basel EGS project to test our concept. We estimate the focal mechanism of induced seismicity from many first motions of P-wave arrivals, but it is challenging to constrain the solution due to usually small numbers of monitoring stations in real induced seismicity cases. We use the distribution of natural fracture orientations as prior information and build a statistical model of possible fault orientations of induced seismicity. Then, the range of possible focal mechanisms from the seismological analysis is constrained by superposing the statistical model. Introducing such prior information leads to a significant reduction of the range of focal mechanisms.
- Europe > Switzerland > Basel-City > Basel (0.30)
- Asia > Japan > Tōhoku (0.29)
- Energy > Oil & Gas > Upstream (1.00)
- Energy > Renewable > Geothermal > Geothermal Resource (0.35)
ABSTRACT We have developed a novel regularized approach to estimate a composite focal mechanism for microseismic events that share a similar source mechanism. The method operates by minimizing the weighted misfits of the SH/P amplitude ratios (in absolute sense and logarithmic scale) and P-wave polarities, using a regularization parameter determined from the trade-off curve for these values. This approach overcomes the low signal-to-noise ratio (S/N) and single-event azimuthal gaps that may otherwise limit the effectiveness of sparse surface arrays. Compared with focal mechanisms derived from P-wave polarity or amplitude-based methods, our regularized approach reduces the multiplicity of solutions and avoids the use of signed amplitude ratios, which may be ambiguous for data with low S/N. We apply our method to a set of 13 microseismic events recorded during hydraulic-fracture stimulation of the Marcellus Shale in West Virginia and Pennsylvania, USA, yielding a strike-slip focal mechanism accompanied by a minor normal component. Our solution is similar to previously reported focal mechanisms in this area. Jackknife analysis, which tests stability of the inversion based on random sampling of the observation, indicates 95% confidence intervals of 1° and 2°, respectively, for the plunge and azimuth of the P and T axes. By analyzing the event subsets, outliers are identified and the assumption of a single dominant focal mechanism is validated. Numerical modeling demonstrates that our approach is robust in the presence of variations of up to 0°–10° and 0°–35°, respectively, for the plunge and azimuth of P and T axes of the focal mechanisms of these events. Sensitivity analysis using synthetic data also indicates that the algorithm is tolerant to mispicks as well as errors in polarity and amplitude ratio. In the presence of some dissimilar focal mechanisms, the dominant focal mechanism can be reliably estimated if at least 70% of the events have similar source mechanisms.
- North America > United States > Virginia (0.69)
- North America > United States > West Virginia (0.69)
- North America > United States > Pennsylvania (0.69)
- (3 more...)
- Research Report > New Finding (0.34)
- Research Report > Experimental Study (0.34)
- Geology > Geological Subdiscipline (1.00)
- Geology > Structural Geology > Tectonics > Plate Tectonics > Earthquake (0.97)
Abstract Multi-stage, multi-well completions cause pore-pressures to increase around each stage treated, compound from earlier offset treatment stages, then dissipate as the injected fluid leaks off into the rock formation. Rock stresses change in a dynamic fashion from virgin reservoir stress to an altered stress influencing subsequently treated stages which can restrict slurry propagation from these injections into regions experiencing excess stress. Stress shadows are time-dependent and dissipate over time and return to the virgin stress state. Microseismic focal mechanisms detected from a high-fold wide azimuth surface array can be used to observe and calculate stress changes in the reservoir and constrain the time it takes for stresses to return to the virgin reservoir state. Operators can take advantage of stress changes and contain fractures close to the stages by building stress wedges around subsequently treated stages. After stress dissipates fluid propagates into previously opened fractures leading to poor fracture containment. In this paper, we review the effects of time-dependent stress shadows on multi-well completions in the Wolfcamp Formation in Southeast New Mexico. Then radioactive tracer data from the Niobrara Formation in the Denver-Julsburg basin is analyzed to provide further verification of the time-dependent process. Increased stresses from previous treatments remain elevated for ∼7 days which push fluid injected on neighboring wells away from the stress shadow. Production of well-specific tracer corroborates the hypothesis that local stress-shadows are elevated for ∼7 days which can push fluid from subsequent neighboring wells. After stresses dissipate through the fractures created during the initial stimulation, new tracer on offset wells was produced as much as 3,000 ft away on a neighboring well. Introduction Microseismic monitoring is a proven technology for observing and mapping reservoir response to hydraulic fracture stimulations. The event radiation pattern of the P-wave first arrival reveals advanced characteristics of the fracture describing deformation at the source location when detected using a high-fold wide azimuth surface array. The full-moment tensor can be generally decomposed into the relative percentages of isotropic, double couple and compensated linear vector dipole components (e.g. Aki and Richards, 1980) which fully describes the failure process in terms of volume change, amount of shearing, and other complexities related to deformation. The local stress field can be calculated using a set of focal mechanisms by minimizing the misfit angle between the modeled stress field and the observed focal mechanism slip vectors (Angelier, 1989) where the local stress field extent is defined by the spatial extent of the observed focal mechanisms. The local stress field orientation and relative magnitude can be resolved for a group of microseismic focal mechanisms by minimizing the misfit angle between the modeled stress field and the observed focal mechanism slip vectors for the subsets using a method described by Vavrycuk, 2014.
- North America > United States > New Mexico (0.55)
- North America > United States > Texas (0.35)
- North America > United States > Wyoming (0.34)
- (3 more...)
- North America > United States > Wyoming > Laramie Basin > Niobrara Formation (0.99)
- North America > United States > Wyoming > DJ (Denver-Julesburg) Basin > Niobrara Formation (0.99)
- North America > United States > Texas > Permian Basin > Wolfcamp Formation (0.99)
- (7 more...)
- Well Completion > Hydraulic Fracturing (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Seismic processing and interpretation (1.00)
- Reservoir Description and Dynamics > Reservoir Characterization > Reservoir geomechanics (1.00)
- Reservoir Description and Dynamics > Formation Evaluation & Management > Tracer test analysis (1.00)