Normal Faulting Stress Regime Research Articles

In situ stress analysis in the Yinggehai Basin, northwestern South China Sea: Implication for the pore pressure-stress coupling process

The Yinggehai Basin is located in the triangle between the Sunda block, South China block, and the South China Sea and is an interesting target for in situ stress investigation, not only because of its specific tectonic position but also due to its abnormally high pore pressures. Magnitudes and orientations of in situ stress were directly constrained by measurements operated in the vertical boreholes that were drilled in the Yinggehai Basin. Gradients and magnitudes of the least horizontal stress remarkably increase with overpressure enhancement, indicating that the least principal stress and pore pressure are in fact coupled. The ratios of pore pressure/stress coupling (ΔƠh/ΔPp) are similar in the north and south central Yinggehai Basin, despite the burial depths of overpressure top are different. However, a deeply buried strike-slip faulting regime under WNW-ESE oriented horizontal compression was identified in the central Yinggehai Basin, where the ΔƠh/ΔPp ratios have intermediate values, ranging from that for an idealized normal faulting stress regime to that for the southeastern South China (a strong strike-slip or even reverse faulting stress regime). Additionally, the WNW-ESE orientations of maximum horizontal stress determined from drilling-induced tensile fractures in the Yinggehai Basin are inappropriate for the present maximum horizontal stress derived from earthquake focal mechanism solutions in the northern segment of the Red River Faults, suggesting that the dextral movement of Red River Faults has slightly influenced the present stress state in the Yinggehai Basin. The present stress state of the Yinggehai Basin was primarily generated by the ESE absolute horizontal motion of the Sunda and South China blocks.

Optimal Fracturing Parameters Condition of Integrated Model Development for Tight Oil Sands Reservoir with 2D Fracture Geometry Using Response Surface Methodology

In this study, oil production in tight oil reservoir was declined due to high heterogeneity, complicated and complexity of reservoir of the low permeability reservoir ranges from 0.1 md to 5 md and reservoir porosity ranges from 10 to 18% lead to low fracture conductivity among the fractures. The challenge deal with this problem is to stimulate the reservoir of hydraulic fracturing for maximum oil production is necessary for the study. The application of the central composite design and response surface are the best tool in order to optimize the operating parameters based on the maximum net present value and through the series calculation, the optimal operating parameters of hydraulic fracturing have been determined of 46 bpm of injection rate, 88.5 min of the injection time and 0.002 ft/min0.5 of the leak-off coefficient and the maximum net present value of 46.5 $mm. Finally, the integrated model development of hydraulic fracturing includes of the normal faulting stress regime, fracturing fluid selection and fluid model, proppant selection, fracture geometry, pressure model, material balance, fracture conductivity and simulation production of fractured well and unstimulated well that have been presented in this study. With the using two dimensional Perkins-Kern-Nordgren fracture geometry models coupled with carter leak-off as the 2D PKN-C has been used to account for leak-off coefficient, spurt loss in term of power law parameters to propagate the fracture half-length, fracture width. The result of the fracture conductivity of fractured well at the fracture half-length of 1,940 ft and average fracture width of 0.32 in by series calculated propped fracture concentration of 1.63 lb/ft2 and fracture closure pressure of 4,842.59 psi to fracture conductivity of 6,200 md-ft, which value is measured by the laboratory experiment. The post-fracture production has been shown the fold of increase oil of 18.7 and the oil production rate of fractured well demonstrated much rising compared to oil production rate of unstimulated case.

가스하이드레이트 개발과정에서의 단층 재활성화 해석

본 논문에서는 해저 하이드레이트 퇴적층에서의 메탄가스 생산 과정에서 발생 가능한 생산정 주변 단층의 재활성화 가능성을 수치해석을 통해 평가하고 재활성화에 따른 미소지진 규모를 예측한 결과를 소개하였다. 가스 생산에 의한 하이드레이트 퇴적층의 유효응력 변화 및 역학적 변형은 TOUGH+Hydrate 코드와 FLAC3D 코드를 순차적으로 연계해석함으로써 시뮬레이션하였다. 단층면 재활성화 기준은 모어쿨롱(Mohr-Coulomb)법칙이 유효한 것으로 가정하였다. 30일간의 시험생산 해석 결과, 감압에 의한 공극압력 감소 및 유효응력의 증가가 주변 단층의 활성화를 일으킬 가능성은 크지 않은 것으로 나타났다. 초기응력 조건에 따른 활성화 가능성을 활동마찰각으로 평가한 결과로부터 수평응력에 비해 수직응력이 상대적으로 큰 정단층 응력조건(normal fault stress regime)에서 단층 재활성화 가능성이 상대적으로 큰 것으로 파악되었다. 또한, 정단층 응력조건에서 단층 재활성화에 기인한 유도지진 발생규모를 모멘트 크기(moment magnitude)로 추정할 경우, 모두 음(-)의 값을 보여 인간이 감지하지 어려운 수준의 미소지진에 해당하는 결과를 보였다. 다만, 본 해석은 하이드레이트 생산과정에서의 단층재활성화 가능성 평가를 목적으로 한 해석기법 구축 및 그 적용성을 소개할 목적으로 상당히 단순화된 지질구조 모델을 가정한 결과이므로, 향후 하이드레이트 시험 생산 및 상업 생산 지역에서의 상세 지질구조, 입력 물성 및 생산 설계조건을 반영한 해석에서는 상이한 결과를 보일 수 있을 것이다. In this paper, we perform a numerical analysis to evaluate the potential of fault reactivation during gas production from hydrate bearing sediments and the moment magnitude of induced seismicity. For the numerical analysis, sequential coupling of TOUGH+Hydrate and FLAC3D was used and the change in effective stress and consequent geomechanical deformation including fault reactivation was simulated by assuming that Mohr-Coulomb shear resistance criterion is valid. From the test production simulation of 30 days, we showed that pore pressure reduction as well as effective stress change hardly induces the fault reactivation in the vicinity of a production well. We also investigated the influence of stress state conditions to a fault reactivation, and showed that normal fault stress regime, where vertical stress is relatively greater than horizontal, may have the largest potential for the reactivation. We tested one simulation that earthquake can be induced during gas production and calculated the moment magnitude of the seismicity. Our calculation presented that all the magnitudes from the calculation were negative values, which indicates that induced earthquakes can be grouped into micro-seismic and as small as hardly perceived by human beings. However, it should be noted that the current simulation was carried out using the highly simplified geometric model and assumptions such that the further simulations for a scheduled test production and commercial scale production considering complex geometric conditions may produce different results.

Impacts of Thermally Induced Stresses on Fracture Stability During Geological Storage of CO2

Geomechanical stability issues may arise due to induced thermal stresses because CO2 will generally reach the storage formation at a temperature lower than that of the reservoir. Cold injection will form a cold region around the injection well, which will induce thermal stress reduction. We simulate cold CO2 injection in deep saline formations in a normal faulting stress regime and investigate under what conditions thermal stresses may jeopardize the caprock sealing capacity by studying the effect of the heterogeneity of the thermal expansion coefficient between the reservoir and caprock. Furthermore, we use an elastoplastic constitutive model to account for inelastic deformation related to fracture instability. We find that the temperature difference should be limited in the presence of very stiff reservoirs, because the thermal stress reduction is proportional to the product of the rock stiffness, the temperature difference and the thermal expansion coefficient. Simulation results show that inelastic strain occurs in the cooled region within the reservoir, but fracture instability does not propagate into the caprock in the considered normal faulting stress regime. However, the cooled region of the lower portion of the caprock may experience yielding if the thermal expansion coefficient of the caprock is larger than that of the reservoir, because the thermal stress reduction in the caprock becomes larger than in the reservoir, which increases the deviatoric stress. Nevertheless, irreversible strain caused by cooling within the caprock are limited to a small region of the lower portion of the caprock and thus, the overall sealing capacity of the caprock is not compromised, so CO2 leakage is unlikely to occur because of cooling.

Geomechanical simulation of the stress tensor rotation caused by injection of cold water in a deep geothermal reservoir

AbstractWe present a three‐dimensional thermohydromechanical numerical study of the evolution and distribution of the stress tensor within the northwest part of The Geysers geothermal reservoir (in California), including a detailed study of the region around one injection well from 2003 to 2012. Initially, after imposing a normal faulting stress regime, we calculated local changes in the stress regime around injection wells. Our results were compared with previously published studies in which the stress state was inferred from inverting the focal plane mechanism of seismic events. Our main finding is that changes in stress tensor orientation are caused by injection‐induced progressive cooling of the reservoir, as well as by the seasonal variations in injection rate. Because of the gravity flow and cooling around a liquid zone formed by the injection, the vertical stress reduction is larger and propagates far below the injection well. At the same time, the horizontal stress increases, mostly because of stress redistribution below and above the cooling area. These two phenomena cause the rotation of the stress tensor and the appearance of a strike‐slip regime above, inside, and below the cooling area. The cooling and the associated rotation of the stress regime can play a significant role in the observed long‐term deepening of the microseismicity below active injection wells.

Localized extensional tectonics in an overall reverse-faulting regime, Northeast Japan

In Northeast Japan, it has been recognized that trench-normal compressional stresses, aligned in the approximate direction of plate convergence, tend to dominate stress fields over a broad region. However, a particularly notable event was the shallow, normal-faulting earthquake swarms with a T-axis oriented in the E–W or NW–SE directions that occurred immediately after the 2011 Tohoku-Oki earthquake near the Pacific coast in the Southeast Tohoku district. The stress tensor inversion represents the pre-Tohoku-Oki earthquake stress field in this area as a normal-faulting stress regime with the minimum principal horizontal stress oriented in a roughly NW–SE direction. Additionally, the stress regime varies with depth from normal faulting at shallow depths (<15 km) to thrust faulting at greater depths. Seismic tomography and magnetotelluric soundings defined a geophysical anomaly with low seismic velocity and low resistivity clearly visible beneath the swarm activity, strongly supporting the existence of an interconnected network with fluid-filled porosity. The upper boundary of the conductor is in good agreement with an extensional–compressional stress transition zone. A plausible explanation for these drastic changes in the stress regime is upward flexure of the upper crust due to partly anelastic deformation in the weakened lower crust. Additionally, remarkable upwarping and localized extensional tectonics during the late Pleistocene reflect the long-term rheological heterogeneities in the crust beneath the seismic source region.

A model for analysis of wellbore stability considering the effects of weak bedding planes

In conventional analysis of wellbore stability, rock is usually assumed to be isotropic, but errors occur when isotropic theory is used if there are weak bedding planes that represent the properties of rock anisotropy. Therefore, in this paper, based on circumferential stresses and a new analysis of geometrical relationships of weak bedding planes and wellbore, a new wellbore stability model that considers the effects of weak bedding planes is introduced; then, a comparison is made by comparing the new model with the intrinsic rock failure model based on actual field data, which reveals that wellbore is less stable when weak bedding planes are considered, that the wellbore of cross-dip wells is less stable than that of up-dip wells and down-dip wells in the strike-slip stress regime and that wellbore quality is better in up-dip wells. Furthermore, results show that dip angles and dip directions of weak bedding planes have significant effects on wellbore stability for different inclination angles and wellbore azimuths. Regarding the effects of in-situ stress regimes, it is found that the wellbore of down-dip wells is overall more stable and that the maximum equivalent collapse density (MECD) of wells drilled along the wellbore azimuth θIA = 270–300° is relatively smaller in the normal fault stress regime; also, in the reverse fault stress regime, the MECD of both up-dip wells and wells drilled along θIA = 90–150° is relatively small. Finally, the influence of the anisotropic coefficient of horizontal stresses is also analyzed.

Numerical modeling of the stability of horizontal multidrain oil wells

The stability of the horizontal multidrain wells is a crucial issue and several factors are involved in this matter, including in-situ stresses, magnitude and distribution as well as the mainbore trajectories. In this paper, this issue is evaluated by assuming different circumstances for the above mentioned factors, based on finite difference threedimensional modeling by using the finite difference numerical software, FLAC3D. The stability of the mainbore and lateral branches is analyzed based on the Normalized Yielded Zone Area (NYZA) criterion, i.e. the ratio of the surrounding yielded cross-sectional area to the initial area of the well. Optimum mud pressures are obtained in the mainbore and lateral branches in different mainbore trajectories under three in-situ stress regimes. In addition, the stability of the junction where the lateral branches are bifurcated from the mainbore is assessed in those situations. The optimum trajectory of the mainbore, in which the junction has obtained the most stable condition, is selected in each stress regime. It was concluded that in the Normal Faulting (NF) stress regime, the mainbore and junction stability varies in relation to the mainbore trajectories, inversely. However, in the other two stress regimes, i.e. Strike Slip (SS) and Reverse Faulting (RF), the variations of the mainbore and junction stability are in the same trend with respect to the mainbore trajectory deviations.

Paleo stress contribution to fault and natural fracture distribution in the Cooper Basin

The contribution of the unconventional reservoirs to the global oil and gas production made it important to address the main factors that control high production from these reservoirs. Hydraulic fracturing that intersect natural fractures results in a high stimulated rock volume and high production. Over a decade of effort to use elastic dislocation and different types of restoration to predict fracture network didn't succeed fully in addressing this factor.We used image log fractures and fault network within an iterative boundary element method (iBEM3D) to predict the paleo-tectonic events and the fracture network in the Cooper Basin. The methodology was able to predict only the major tectonic events that occurred after the deposition of the Cooper Basin sediments and contributed to the formation of the natural fractures. As the methodology does not include fault elastic properties, fracture orientations near the faults showed unrealistic results and should not be considered as indicative for the actual natural fractures.The main trend of the Cooper Basin fractures was attributed to post Triassic inter-seismic relaxation after major tectonic compressional events, which resulted in a normal fault stress regime. However, the current day stress regime is believed to be also a major factor in forming some of the natural fractures. Hunter Bowen orogeny in the Late Triassic contributed less to the existing fractures. Whereas, Cainozoic compressional forces played no role in the formation of the Cooper Basin natural fractures.

Potential fracture propagation into the caprock induced by cold [formula omitted] injection in normal faulting stress regimes

Thermal effects are an important component in the analysis of geologic carbon storage because the injected CO2 reaches the storage formation at a lower temperature than that of the reservoir rock. The main fear is related to the possibility that the shear slip that may occur within the reservoir due to cooling could propagate into the caprock, which could result in CO2 leakage. We model a baserock–reservoir–caprock system in a normal faulting stress regime using an axisymmetric model in which we inject cold CO2 and use an elasto-plastic constitutive model to simulate inelastic deformation. CO2 forms a cold region around the injection well of a significantly smaller extension than the CO2 plume. Within this cold region, inelastic strain is yielded due to the thermal stress reduction caused by the thermal contraction of the rock. This inelastic strain occurs only within the reservoir and does not propagate into the caprock. Actually, the stability of the lower portion of the caprock improves in the cooled region as a result of a stress redistribution that occurs to satisfy stress equilibrium and displacement compatibility caused by the thermal stress reduction of the reservoir. This stress redistribution tightens the caprock and prevents CO2 leakage. Thus, thermally induced stresses are not likely to generate fracture propagation into the caprock in normal faulting stress regimes.

In-situ stress analysis using the anelastic strain recovery (ASR) method at the first offshore gas production test site in the eastern Nankai Trough, Japan

To evaluate in-situ stresses by applying the anelastic strain recovery (ASR) method to drilling cores, four cores retrieved from different depths between 265 and 313 m below seafloor in a well at the first offshore gas production test site in the eastern Nankai Trough, Japan were utilized to measure anelastic strains onboard the drilling ship. The onsite ASR measurements suggest this depth range in the study area is a normal faulting stress regime dominated by gravitation. In subsequent laboratory experiments, the cores were returned to their in-situ stress states via the K0 consolidation procedure, then elastic and anelastic strains were measured during and after the unloading process to verify the onboard measurements and quantify in-situ stresses. These laboratory calibration experiments verified that the ASR measurements following the stress release could provide reasonable estimates for the artificially applied preload stress.

Kinematics of the Neogene Terror Rift: Constraints from calcite twinning strains in the ANDRILL McMurdo Ice Shelf (AND-1B) core, Victoria Land Basin, Antarctica

We report new strain analyses of mechanically twinned calcite in veins hosted by Neogene (13.6–4.3 Ma) sedimentary and volcanic rocks recovered from the Terror Rift system in the southern Ross Sea, Antarctica, by the ANDRILL (ANtarctic geological DRILLing) McMurdo Ice Shelf (MIS) Project. Strain analyses of the ANDRILL MIS AND-1B drill core samples yield prolate and oblate ellipsoids with principal shortening and extension strains ranging from −7% to 9%, respectively. The majority of samples show ≤25% negative expected values, indicating homogeneous coaxial strain characterized predominantly by subvertical shortening. We attribute the subvertical shortening strains to mechanical twinning at relatively shallow depths in an Andersonian normal faulting stress regime induced by sedimentary and ice sheet loading of the stratigraphic sequence and characterized by low stress magnitudes. Oriented samples yield a northwest-southeast average extension direction that is subparallel to other indicators of Neogene extension. This northwest-southeast extension is consistent with strain predicted by Neogene orthogonal rifting in a north-northeast–trending rift segment, as well as models of right-lateral transtensional rifting. The overall paucity of a noncoaxial layer-parallel shortening signal in the AND-1B twin populations favors orthogonal extension in the Neogene Terror Rift system, but could also be due to spatial partitioning of strain in a transtensional rift regime.

Analysis of borehole collapse and fracture initiation positions and drilling trajectory optimization

Abstract For borehole stability analysis, it is of vital importance to correctly determine the borehole and fracture initiation postions; currently studying their variation rule, most of them are based on borehole stress solutions for plane stress condition, or neglects the influence of shear stresses. This research analyses the failure directions of deviated boreholes in different faulting stress regimes for plane strain condition with the polyaxial strength failure criterion Mogi-Coulomb and develops a new analytical equation of determining the optimum drilling trajectory. The analytical results show that Poisson׳s ratio generally exerts more influence on the angular positions of borehole collapse than fracture initiation positions with the exception of some cases where Poisson׳s ratio has no influence on the angular positions of borehole collapse and the borehole collapse and fracture initiation positions. It is not always correct to neglect the effect of shear stresses on the borehole collapse and fracture initiation positions with respect to particular cases; when drilling orientates along the direction of minimum horizontal in-situ stress in normal faulting stress regime, there is a range of inclinations where the borehole collapse and fracture positions are identical; moreover, the same circumstances will also take place in a direction parallel to the maximum horizontal in-situ stress in the reverse faulting stress regime or the borehole is horizontal with particular drilling orientation in the strike-slip faulting stress regime. The optimum drilling trajectory lies in the critical turning point, which corresponds to the aforementioned three situations, respectively, of angular positions of borehole collapse. Therefore, this study’s result can provide a theoretical basis for the optimization design of borehole trajectory and a safe drilling fluid density window.

Long term impacts of cold CO2 injection on the caprock integrity

Carbon dioxide (CO2) will reach the storage formation at a temperature lower than that of the reservoir, especially for high flow rates. Thus, thermo-mechanical effects might jeopardize the caprock mechanical stability and cause induced seismicity. We perform thermo-hydro-mechanical simulations of cold (liquid) CO2 injection and analyze the impacts on the rock mechanical stability during a 30 year injection period. Injection of cold CO2 develops a cold region around the injection well that induces a thermal stress reduction in the reservoir due to its contraction. Stress redistribution, which occurs to satisfy stress equilibrium and displacement compatibility, causes the horizontal total stress to increase in the lower portion of the caprock. The thermal stress reduction in the reservoir decreases its stability when injecting a constant mass flow rate through a vertical well in normal faulting stress regimes. Such decrease in stability, if sufficiently large, might cause induced seismicity as well as enhancing reservoir permeability and injectivity. However, the caprock tightens due to the increase in horizontal total stress, improving its stability. After a significant improvement in caprock stability during the first years of injection, stability decreases gradually for longer injection times, but the stress state remains more stable than prior to injection, even for stiff caprocks. By contrast, in a reverse faulting stress regime, both the reservoir and the caprock are less stable during the first years of injection, but stability improves subsequently. On the other hand, injecting cold CO2 at a constant mass flow rate through a horizontal well does not significantly affect the caprock stability for the scenarios considered in this study (in both normal and reverse faulting stress regimes). We show that accounting for the thermal expansion of the grains is very important in low porosity formations to avoid simulating artificial porosity and total stress reductions in the cooled region of the caprock that yield unreal high mobilized friction angles in the lower part of the caprock in normal faulting stress regimes. Overall, injecting cold CO2 should not be feared because of the thermal stresses reduction, though care should be taken to avoid excessive induced seismicity.

Determination of three-dimensional in situ stresses by anelastic strain recovery in Wenchuan Earthquake Fault Scientific Drilling Project Hole-1 (WFSD-1)

Abstract The Wenchuan Earthquake Fault Scientific Drilling Project was implemented rapidly after the great 12 May 2008 earthquake (Mw 7.9) to better understand rupture mechanisms of the seismic faults. The first borehole of the project, WFSD-1 was located in Hongkou Township, Dujiangyan City, Sichuan Province, China on the hanging wall of the Yingxiu–Beichuan fault, which underwent large dextral and vertical displacement during the earthquake. The near-vertical borehole was 1201 m long. In-situ stresses were measured in rock samples from depths between 424 and 1173 m drilling depth by the anelastic strain recovery (ASR) method. The average trend of the maximum principal stress σ1 was N309° (ranging from N291° to N325°), rotated with increasing borehole depth from NW–SE to WNW–ESE. The magnitude of the in-situ maximum principal stress was estimated to be 35.3 MPa at the depth of 1173 m. The relations between horizontal and vertical stresses are vertical stress σv > maximum horizontal stress σH > minimum horizontal stress σh at depths above 424 m, σH > σh > σv at depths from 424 m to 800 m, and σH > σv > σh below the depth of 800 m. These indicate that the stress states are in a normal faulting stress regime above 424 m; a reverse faulting regime from 424 m to 800 m and a dextral strike-slip regime below 800 m. The stress measurements from WFSD-1 are consistent with the focal mechanism solution in which the mainshock was dominated by thrusting accompanied by dextral strike-slip motion. The orientations of σ1 are roughly consistent with the tectonic displacement direction of the Longmenshan area, which provides further evidence of NW–SE movements representing compression of the Songpan–Ganzi block toward the Sichuan basin.

Current stress and strain-rate fields across the Dead Sea Fault System: Constraints from seismological data and GPS observations

The stress and strain-rate fields characterizing the Dead Sea Fault System are investigated by using seismological and geodetic observations. In order to assess spatial variations in the regional stress field, we compiled a multidisciplinary dataset of well-constrained horizontal indicators, by merging all available data reported in literature with the data obtained in this study through weighted stress inversions of focal plane solutions. Our findings indicate that the state of stress is characterized by the coexistence of a normal faulting stress regime with the primarily strike-slip one, according to the regional frame illustrated by previous geological and seismological observations. An updated velocity field computed from new observations and earlier published data depicts the general left-lateral motion of the Dead Sea Fault System well. In agreement with previous studies, we detected some differences in the slip-rate pattern between the northern and the southern sectors of the fault system. The geodetic strain-rate field highlights how much of the deformation is accommodated along the fault system itself in a narrow region. The comparison between the stress and the strain-rate directions reveals that both orientations are near-parallel, clearly indicating that present-day crustal stress and ground deformation patterns are chiefly driven by the same tectonic processes.

2011年東北地方太平洋沖地震の発生後に活発化した霞ヶ浦南端直下の正断層型地震活動

We have investigated the characteristics of an activated earthquake sequence below the southern extremity of Lake Kasumigaura in the Kanto region, central Japan, after the 2011 Mw 9.0 Off the Pacific Coast of Tohoku Earthquake (Tohoku-oki earthquake). The earthquake locations of this sequence using one-dimensional velocity model suitable for the area reveal that the triggered earthquakes distribute at a depth from 13 to 20 km forming a 25 km-long in the NW-SE direction and 20-km-wide zone. The depth range indicates that they are shallow crustal earthquakes, which are uncommon in the area. The sequence seems to consist of multiple small-sized clusters rather than a clear planar fault plane structure. The double-difference relocation shows that the locations for the periods before and after the Tohoku-oki earthquake are basically complementary to each other. Focal mechanisms were determined from P-wave polarity data as well as body wave amplitudes for thirty-six earthquakes including one event that occurred before the Tohoku-oki earthquake. The pre-Tohoku-oki earthquake of the magnitude 1.9 has a reverse faulting mechanism with P-axis in the NW-SE direction. Conversely, the triggered earthquakes are primarily normal-faulting mechanisms with T-axes in the direction of WSW-ENE to NW-SE. The stress tensor inversion for the mechanisms of triggered earthquakes indicates that the area is characterized by a pure normal-faulting stress regime with the minimum compressive stress in the E-W direction subhorizontaly. The stress drop values estimated for three M3-class triggered earthquakes and their paired smaller earthquakes for empirical Green’s functions range approximately from 0.5 to 15 MPa, suggesting that there is no difference in the underlying physical process between these triggered earthquakes and natural tectonic ones. We computed the static stress changes due to the Tohoku-oki earthquake together with the Mj 7.6 aftershock offshore of Ibaraki so as to examine why such unusual normal-faulting earthquakes were activated in the area characterized by a reverse-faulting stress field. The analysis of Coulomb stress change (ΔCFF) shows that the induced stress favors the activation of normal-fault earthquakes of the present sequence. However, the induced stress cannot change the background reverse-faulting stress field into normal-faulting one by rotating the maximum compressive stress from horizontal to vertical, because both earthquakes produce a strike-slip faulting stress field in the area with the same maximum compressive stress direction as the background one. Therefore, we cannot attribute the activation of the seismicity to coseismic stress rotation. Instead, we suggest the existence of local stress heterogeneity that the pre-Tohoku-oki earthquake stress field in the area was a normal-faulting regime, which is also supported by the comparison of the induced stress magnitude with the stress drop values of triggered earthquakes. We thus conclude that the 2011 Tohoku-oki earthquake and the Mj 7.6 aftershock triggered the normal-faulting earthquake sequence in a limited area in combination with a locally formed pre-shock normal-faulting stress regime.

Mechanical Stability of Horizontal Wellbore Implementing Mogi-Coulomb Law

In this paper, a linear elastic constitutive model is described. The model consists of a three dimensional analyses of stress concentration around an arbitrarily oriented borehole, due to anisotropic in situ stress combined with internal wellbore pressure. Studying the principal stresses around a borehole require the consideration of three possible permutations for the principal stresses: (1) σ z ≥ σ q ≥ σ r , (2) σ q ≥ σ z ≥ s r , and (3) σ q ≥ σ r ≥ σ z . Considering the practical field conditions, in normal faulting stress regime and reverse faulting stress regime, wellbore stability analysis can be simplified by only assuming case 2 ( σ θ ≥ σ z ≥ σ r ) for the principal stresses around horizontal borehole. In strike-slip stress regime, however, all the three possible permutations for the principal stresses should be considered in wellbore stability analysis. The constitutive model in conjunction with Mogi-Coulomb law has been used to introduce a new wellbore stability model for horizontal boreholes. The developed model has improved wellbore stability analysis compared to adopting the classical Mohr-Coulomb criterion that is commonly applied. This has been verified by several typical field case studies. Key words : Wellbore stability; Borehole failure; Collapse pressure; Mogi-coulomb criterion; Horizontal drilling

Plio-Quaternary Stress State in the Burdur Basin, SW-Turkey

This study defines the Plio-Quaternary to present day stress regime in the Burdur Basin, located at the northeastern end of the Fethiye–Burdur Fault Zone in SW Turkey. This fault length, which is considered the landward continuation of the Pliny-Strabo trench, is an important feature in SW Turkey. The inversion slip vectors measured on fault planes indicate a consistent normal faulting stress regime during Plio-Quaternary time, continuing into recent times as indicated by earthquake focal mechanism inversions. Both states have consistent NW–SE trending horizontal minimum stress axes (σ3). The orientation of fault sets is predominantly around the NE–SW direction in the major Fethiye–Burdur Fault Zone, making the extension NW–SE. The mean stress ratio is 0.74 indicating a triaxial stress state, which is clearly different from radial extension. The NW–SE extension is probably responsible for the formation of the Burdur Basin during Plio-Quaternary time. This extension, which is probably caused by slab-pull force due to the subduction process along the Cyprus arc, produces a dominant normal motion along the FBFZ.

Parametric and non-parametric statistical approaches to the determination of paleostress from dilatant fractures: Application to an Early Miocene dike swarm in central Japan

Several methods have been proposed for determining paleostress states from orientations of dilatant fractures such as dikes and veins. Recently a stochastic inversion method was invented to objectively estimate the principal stress axes and the stress ratio. Whether a fracture is dilated or not is controlled by the balance of the fluid pressure and the normal stress acting on it. The magnitude of normal stress depends on the fracture orientation, which causes anisotropic orientation distribution of dilatant fractures. The inversion method assumes that the orientation distribution of fractures can be approximated by a Bingham distribution, an exponential probability distribution on the unit sphere, of which symmetric axes are interpreted as the principal stress axes. However, it is unknown if the exponential type of distribution function is suitable or not. Here, we examine the distribution functions and propose two improved methods. One method uses the shifted power-law function as the shape of probability distribution, which is more flexible than the Bingham distribution and is applicable to various shapes of orientation distributions. Furthermore, an index of the driving fluid pressure can be estimated with a confidence interval. The other is a non-parametric (distribution-free) method, which can avoid the a priori assumption on the shape of distribution function without significantly losing accuracy or precision. The new methods were applied to an Early Miocene dike swarm formed during the back-arc opening of the Japan Sea. A normal-faulting stress regime with the minimum principal stress axis trending roughly perpendicular to the arc was obtained from the dikes. A moderately high stress ratio and a high fluid pressure were also estimated.