Drilling-induced Tensile Fractures Research Articles

How to estimate the 3D stress state for a deep geological repository

Abstract. When stresses yield a critical value, rock breaks or pre-existing faults are reactivated, generating pathways for fluid migration. Thus, the contemporary undisturbed stress state is a key parameter for assessing the stability of deep geological repositories and for quantifying whether stress changes induced by sub-surface constructions and storage of heat-generating waste lead to a critical state. The prediction of the contemporary 3D stress state is achieved with geomechanical–numerical models that are based on the static geological models. These are based on 3D seismic imaging and borehole data and describe the 3D structure and distribution of rock properties. Furthermore, pointwise measurements of the stress state are needed to fit the model to these data. The workshop is divided into three parts. Each part will have a short introduction to the specific topic, followed by dedicated hands-on exercise in small groups and discussions of the exercise outcome and its implications. 1. How do we formally describe the stress field? Here you will learn that the stress field is different from other physical field quantities such as temperature or displacement. The latter can be described with a scalar or a vector at a point. In contrast, the stress state at a point is described with the so-called Cauchy stress tensor that has nine components. We will give a quick tour through this concept, starting from very basic physics, and explain why the tensor is needed. In particular, you will learn in the exercises that stress cannot be measured directly with most methods but that strain is observed, from which stress is inferred.2. How do we actually measure stress? We present a range of methods that determine individual components of the stress state. In the exercise you will analyse borehole image logs to identify borehole breakouts and drilling-induced tensile fractures at the borehole wall. These are used to derive the orientation of the minimum and maximum horizontal stresses. Furthermore, we will present methods for the estimation of stress magnitudes. These are essential data for the calibration of a 3D geomechanical model. We will give a brief overview of different methods and in the exercise on the analysis of mini-hydraulic fracture tests as well as on the sleeve-fracturing and sleeve re-opening methods.3. From a viewpoint to a 3D description. If rock properties in the sub-surface were homogenous and isotropic and were to follow a simple linear behaviour between stress and strain, we could estimate the stress state analytically. However, this is not the case, and we will discuss three questions. a. How many pointwise stress data are needed to efficiently constrain a 3D model?b. How do I know whether a data point taken from less than a cubic metre is a good representation of a large rock volume, i.e. a whole lithological layer?c. How can we quantify the uncertainties of the model stress state, and what is the typical range? You will use a case study in northern Switzerland to explore these questions.

Geophysical characteristics of a fault system in the northern Central Range of Taiwan and its applications for geothermal energy exploration

The northern Central Range of Taiwan is a high-potential geothermal region. Since the formations are mainly tight metasandstone and slate, permeable structures associated with faults are commonly considered as conduits of geothermal fluids. This study determines the characteristics and orientations of the permeable fault zones by analyzing the geophysical logs and microresistivity formation image log (FMI) of the JT-4 well in Jentse, an important geothermal area in the northern Central Range. Between 720 and 1480 m measured depth (MD), the effective porosity of the intact host rock is mostly below 3% calculated by the geophysical log. Zones with porosity greater than 5% are only clustered within a few thin intervals. The FMI interpretations show these porous zones are in the interior of the fractured and faulted intervals. These porous fault zones comprise fault damage zones with a high density of open fracture planes and fault cores with porous fault breccias. There is a highly brecciated fault core in 1334–1339 m MD, which would be the most permeable interval of the well. Additionally, some healed fault zones with sealed fractures are observed. The picked drilling-induced tensile fractures signify that the direction of the present-day maximum horizontal principal stress is N40–50°E, and most of the open fractures also strike parallel to the NE–SW direction. The study results show that the open fractures are concentrated in the four fault zones belonging to one major normal fault system. After integrating the orientations and locations of the fault zones, we propose that the permeable normal fault system is about 200 m wide, trends N50–70°E, and dips 70–80° to the NW. The development of the open fractures and the permeable fault system in the northern Central Range may be controlled by the current rifting of the Okinawa Trough offshore northeastern Taiwan. The study exhibits the characteristics of fractured fluid conduits of the regional geothermal system, which will benefit future geothermal exploration in northeastern Taiwan.

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.

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.

Assessment of deep shale fracture toughness using nanoindentation tests

In the past decade, the extraction of shale gas has become economically feasible due to innovations in horizontal drilling and multistage hydraulic fracturing. This has led to increased interest in deeper understanding of the mechanical properties of shale rocks. Currently, the deep Longmaxi shale with a burial depth of more than 3500 m has been receiving increasing interest in the Sichuan Basin. The first task of large scale development of these reservoirs is to understand the mechanical properties of deep shale rocks. Usually, these properties, such as the Young’s elastic modulus E, hardness H, fracture toughness K IC and other parameters are commonly obtained from laboratory tests with core plugs and logging data interpretations. However, these methods have their own limitations. In recent years, the nanoindentation test has been widely used to predict mechanical properties of shale and other rocks. The advantage of this test is the small sample volume required, which means that drilling cuttings or fragments, which are easily obtained during drilling, can be used. In this study, the nanoindentation method is adopted for the accurate measurement of K IC , which is of vital importance for the better understanding of the drilling-induced tensile fractures and for the design of hydraulic fracturing. The specimens retrieved from a deep well in the Sichuan Basin are examined using nanoindentation tests to obtain K IC . The results show that K IC is strongly dependent on mineralogy and is anisotropic. Usually, KIC values measured parallel to the bedding planes are generally higher than the corresponding values measured normal to the bedding planes. The obtained fracture toughness can be used to predict the fracability and brittleness of shale, which have instructional significance for drilling and hydraulic fracturing.

Stress heterogeneity in the Changning shale-gas field, southern Sichuan Basin: Implications for a hydraulic fracturing strategy

To evaluate the risk of induced seismicity due to hydraulic fracturing, a detailed stress analysis in a seismically active region, i.e., the Changning shale-gas field in the southern Sichuan Basin, was conducted utilizing sonic wave velocity logs and high-resolution electrical resistivity image logs of shale gas wells. Wells with relatively higher magnitudes of horizontal stresses are spatially located in the vicinity of thrust faults, in agreement with the stress concentration at the upper tip of the basement-involved thrust fault. The prevailing orientation of maximum horizontal principal stress (SHmax) is N110°–120°E on the southwestern Changning anticline, which aligns with the motion of the Sichuan Basin, as driven by the continuous eastward extrusion of the Tibet plateau, which initiated at approximately 50 Ma. However, the SHmax orientation is N30°–70°E along the northern Changning anticline, implying an inheritance of the N–S geodynamic compression during the Jurassic to Late Cretaceous. Drilling-induced tensile fractures primarily appear in compliant layers that are rich in clay or organic matter in wells located adjacent to thrust faults, suggesting that stiff layers can efficiently inhibit the hydrofracture growth height owing to more intense tectonic stress near thrust faults. A natural fault with a strike of approximately 30° to the SHmax orientation can be more easily activated by extreme pumping pressure in wells that are weakly affected by the stress concentration along the thrust fault. Nevertheless, a cyclic injection of fracturing fluid with a gradually increasing target pressure may effectively reduce the magnitude of induced seismicity.

In Situ Stress Prediction in Subsurface Rocks: An Overview and a New Method

Methods for determining in situ stresses are reviewed, and a new approach is proposed for a better prediction of the in situ stresses. For theoretically calculating horizontal stresses, horizontal strains are needed; however, these strains are very difficult to be obtained. Alternative methods are presented in this paper to allow an easier way for determining horizontal stresses. The uniaxial strain method is oversimplified for the minimum horizontal stress determination; however, it is the lower bound minimum horizontal stress. Based on this concept, a modified stress polygon method is proposed to obtain the minimum and maximum horizontal stresses. This new stress polygon is easier to implement and is more accurate to determine in situ stresses by narrowing the area of the conventional stress polygon when drilling-induced tensile fracture and wellbore breakout data are available. Using the generalized Hooke’s law and coupling pore pressure and in situ stresses, a new method for estimating the maximum horizontal stress is proposed. Combined it to the stress polygon method, a reliable in situ stress estimation can be obtained. The field measurement method, such as minifrac test, is also analyzed in different stress regimes to determine horizontal stress magnitudes and calibrate the proposed theoretical method. The proposed workflow combined theoretical methods to field measurements provides an integrated approach for horizontal stress estimation.

Fluid flow and fault-related subsurface fractures in slate and metasandstone formations: A case study of the Jentse Geothermal Area, Taiwan

Permeable structures associated with faults are commonly treated as geothermal fluid conduits in most tight geothermal reservoirs. However, the properties of permeable structures within fault zones remain unclear due to the lack of subsurface geological data, e.g., the geothermal systems in the tight slate and metasandstone formations in Northeastern Taiwan. This study presented standard open-hole and Fullbore Formation MicroImager (FMI) logs acquired in the 8.5-inch-diameter section of the JT-3 well (800–1475 m measured depth) in the Jentse geothermal area, Taiwan, to identify the subsurface fracture structures and the possible pathway of fluid flow. Using open-hole log data, we generate a lithology interpretation for the borehole and model the associated effective porosity. Based on the FMI images, four types of natural open fractures, two types of natural sealed fractures, and drilling-induced tensile fractures were identified. Integration of the lithological model, the effective porosity, and the types of fractures reveals one major permeable fault system comprising three permeable fault zones. The fault system is composed of both permeable fault cores marked by porous fault breccia and damage zones characterized by a large number of open-fracture planes. The static borehole temperature log also shows deflections in these interpreted permeable fractures zone. Deflections in these fracture zones that, based on petrophysical and borehole image logs, are interpreted as permeable. According to the orientations of the bed boundaries and fault planes obtained from FMI images, the fault system may have an orientation of N50–70 °E and dips 70–90˚ to the NW with a width of around 116 m. This study illustrates how subsurface structures control geothermal conduits. The concept and workflow of this study may be useful for other fault-controlled geothermal areas in Taiwan.

Influence of stress field anisotropy on drilling-induced tensile fracture

Drilling-induced tensile fracture under anisotropic stress conditions is investigated numerically with an elasto-viscoplastic constitutive model, in which the plastic strain rate can be decomposed into deviatoric and volumetric components. Both the deviatoric and volumetric invariants of the plastic strain rate are assumed to follow an incremental power-law relationship with an Arrhenius dependence. Thermal equilibrium is incorporated into the system, which extends the conventional understanding of stress-induced tensile fracture into an energy-based problem. The rock around the borehole is modelled as impermeable and pressure insensitive (i.e. von Mises material) for simplicity. Anisotropic borehole scenarios characterised by various stress anisotropy ratios are investigated with a focus on the distribution of circumferential stress and its evolution. Simulation results have demonstrated that stress field anisotropy fundamentally promotes shear banding and hence borehole instability.

Deterministic and Statistical Modeling of a New Thermal Breakout Technology for Measuring the Maximum Horizontal In-Situ Stress

Summary The current state-of-the-art technology for in-situ stress measurements involves an integrated approach that combines borehole breakout observations, drilling-induced tensile fractures, and hydraulic fracturing tests (i.e., “mini-fracs”). This methodology has achieved wide application in the oil and gas industry but has several limitations that often prevent successful in-situ stress measurements. One major limitation is that breakouts do not appear in all boreholes and are generally only a natural occurrence that cannot easily be controlled. Because borehole breakouts are used to directly measure the maximum horizontal in-situ stress magnitude, the absence of borehole breakouts presents a major data gap for in-situ stress measurements. In response to this data gap, a new US Department of Energy (US DOE)-sponsored thermal breakout technology that will provide a method for thermally inducing borehole breakouts and allow the consistent measurement of the maximum horizontal stress magnitude is currently in development. This thermal breakout technology involves heating the borehole and increasing the thermoelastic compressive stress in the rock until a breakout develops, which can be directly correlated to the maximum horizontal stress magnitude. The first step in this project was an analytical modeling study of the thermal breakout process. Based on the Kirsch solution (Kirsch 1898), a deterministic and statistical analysis was performed on the pertinent parameters that influence the maximum horizontal stress calculation. As a result of the analysis, the findings indicate that the thermal breakout technology is feasible and provides improved accuracy and/or an enhanced ability to measure the maximum horizontal stress. Future work as part of this US DOE-sponsored project includes additional validation through more detailed numerical modeling, laboratory testing, and field testing of the thermal breakout technology.

Analytical orientation criteria for drilling and completion-induced fracture initiation considering fluid infiltration from the wellbore

Analytically-developed criteria are presented for the orientation of fracture initiation in porous-permeable rocks considering fluid infiltration from non-perforated and perforated wellbores. For non-perforated wellbores, this is a fully-exact analytical solution representing the initiation of drilling-induced tensile fractures (DITFs). For perforated wells, a set of analytical approximations are derived for fracture initiation representing completion-induced hydraulic fractures (CIHFs). DITF orientation is used to constrain the in-situ stress tensor, especially in the case of transverse DITFs, which are rarer than longitudinal DITFs. Results indicate that transverse CIHF initiation only occurs over a wellbore pressure-at-breakdown window, while longitudinal CIHF initiation occurs at comparatively higher wellbore pressures. Transverse CIHF initiation however, occurs more frequently than transverse DITFs, because the presence of perforations aids transverse fracture initiation. Low Biot's poroelastic coefficient and high Poisson's ratio values also aid transverse fracture initiation. The region of the in-situ stress states where transverse initiation is promoted can be shown in dimensionless plots. Case studies are run on seven prolific shale plays (Barnett, Bakken, Fayetteville, Haynesville, Niobrara, Marcellus, and Vaca Muerta) using breakdown pressures from the literature. Transverse CIHF initiation is found to be a highly likely outcome, as opposed to results from past studies that neglected pore pressure effects. Based on the findings of this study, recommendations are made to practitioners to enhance their chances on achieving transverse CIHF initiation in the field. These involve slow injection rates, oriented perforation strategy and strong bonding between the casing, cement and the surrounding rock to prevent fracturing fluid leakages.

Recent tectonics, geodynamics and seismotectonics in the Ninh Thuan Nuclear Power plants and surrounding regions, South Vietnam

We studied recent tectonics and present-day geodynamics in the Ninh Thuan nuclear power plants and surrounding regions to reveal seismogenic faults, deformation and evaluation of seismotectonics using various methods such as remote sensing, GPS, seismic interpretation, and stress and strain analysis. We based our study on geomorphological investigation, satellite images, fault gouge, drilling core, fault scarps and analysis of offshore seismic profiles to determine the capable faults in the studied region. Using international reference frame ITRF08, we determined the absolute velocities of the GPS stations with a slip rate to the east of 22.5 to 25.3 mm/year and to the south of 4.4 to 8.4 mm/year. The present strain rate was determined from present tectonic velocities that were consistent with the recent strain rates determined from topographic profiles, the slip rate of capable faults, and the thickness of Pleistocene sediments. The present strain rate variation from 10 to 30 nano per year demonstrated that the studied region has been weakly deformed under a stable tectonic regime. The state of stress determined from the fault population, focal mechanism, borehole breakouts and drilling-induced tensile fracture methods indicated that the area was deformed primarily under a strike-slip regime with a small extensive component. The maximum credible earthquake was determined from the dimensions of the capable fault and state of stress using various methods. From these capable faults, we suggest monitoring the faults, which could produce a Maximum Credible Earthquake (MCE) over a range of 5.9–6.5.

Effect of faults and rock physical properties on in situ stress within highly heterogeneous carbonate reservoirs

This study characterizes the reservoir-scale in situ stress state within a carbonate oil and gas field in the southeast United Arab Emirates (UAE) to further our understanding of the effects of faults and rock property heterogeneity on variations in stress. The in situ stress state was constrained using image-logged wellbore wall stress indicators (i.e., borehole breakouts and drilling-induced tensile fractures) observed within several vertical boreholes penetrating layered carbonate rock formations. The orientation and magnitude of the in situ stress state vary significantly with both location and depth. The azimuth of the maximum horizontal principal stress (SHmax) has diverse orientations that vary significantly with depth and span a ~60° range from NW to NNE. The magnitudes of horizontal principal stresses also vary significantly, meaning that no unique stress–depth relations can be defined. Despite considerable fluctuation in stress magnitudes with depth, the stress state in the reservoir is generally in a strike-slip faulting regime, which is consistent with that prevailing throughout onshore UAE. These widespread variations in stress orientation and magnitude appear to be linked to a combination of geomechanical processes associated with the presence of a set of reservoir-scale faults and the highly heterogeneous mechanical properties of the carbonate reservoir. Our analysis indicates that the SHmax azimuth rotates proximal to faults from the prevailing far-field stress orientation toward the strike of the faults, but the angle of this azimuthal rotation is dependent on the Young's modulus characteristics of individual formations. The gradient of the horizontal principal stress tends to increase with increasing Young's modulus values, indicating that stiffer formations transfer higher magnitudes of stress. The observed stress variations also agree well with the results of three-dimensional finite element modelling. Our study demonstrates that the strong mechanical heterogeneity of carbonate reservoirs controls the in situ stress state in a complex but explainable way.

The stress field in the frontal part of the Eastern Alps (Austria) from borehole image log data

Stress orientations in the frontal part of the Eastern Alps are inferred from borehole breakouts and drilling-induced tensile fractures depicted in image logs from 62 vertical or slightly deviated wellbores with up to 5km depth. Borehole breakouts (2463 individual features with 1673.9m cumulative length) are observed in all the wells except one, whereas drilling induced fractures only occur in four wells.Microresistivity image log data from the Alpine foreland, the external thrust sheets (allochthonous Molasse, Helvetic- and Flysch nappes) and the autochthonous units in the footwall of the Alpine units reveal consistent in-situ stress orientations with approximately N-S-directed maximum horizontal stresses (SHmax). Indications for stress partitioning across thrusts including the Alpine floor thrust are not observed. In the Western Sector of the study area the mean SHmax is oriented 002° with a standard deviation (SD) of 11.1°. In the central and eastern sector, the mean SHmax is oriented 177° (SD 14°) and 173° (SD 14.3°), respectively, indicating a counterclockwise rotation of SHmax of about 10° from west to east. The observed stress indicators and the analysis of deviated wells suggest a strike-slip regime for the western and Central Sector, whereas a thrust regime is probable in the east of the study area. The thrust regime there coincides with the occurrence of Middle Pleistocene conglomerates which were previously described to be displaced by Alpine thrust faults.The orientations of SHmax fit into the general observation that SHmax directions at the Alpine front are oriented about perpendicular to the strike of the mountain belt and rotate along the Alpine arc. Our stress orientations - including those derived from the autochthonous basement - are, however, markedly different from the NW-SE-directed maximum horizontal stresses reported from the Variscan units in the European foreland north of the study area.

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.

In situ stress states at KURT, an underground research laboratory in South Korea for the study of high-level radioactive waste disposal

The Korea Atomic Energy Research Institute Underground Research Tunnel (KURT) is an underground research laboratory in South Korea, built for investigations into the geological disposal of high-level radioactive waste. We characterize in situ stress states at KURT using data from a series of hydraulic fracturing (HF) tests and borehole image logs to depths of ~700 m in two boreholes. The tensile fractures induced by HF tests, plus several borehole stress indicators (e.g., drilling-induced tensile fractures and borehole breakouts) measured from image logs, consistently indicate an ESE–WNW oriented maximum horizontal principal compressive stress (SHmax). This site-scale SHmax orientation varies slightly from the regional-scale SHmax orientation, likely reflecting a local stress perturbation resulting from a fault network that traverses the site. Estimated magnitudes of the minimum horizontal principal compressive stress (Shmin), determined either from the shut-in pressures recorded during HF tests or from stress indicators on image logs, are comparable to the vertical stress, indicating that the stress regime at the KURT site straddles the boundary between strike-slip and reverse faulting. The depth-dependent trend in estimated SHmax magnitudes deviates at ~500 m depth, which we attribute to variations in the distribution of natural fractures in the granitic rock mass. This depth-dependent variation in SHmax magnitudes has implications for the slip stability of pre-existing fractures at the site. That is, at shallow depths, SHmax lies within the Coulomb stress limit for optimally oriented fractures and faults with a frictional coefficient of 0.6, whereas at greater depths, SHmax exceeds this limit, meaning that pre-existing fractures at shallow depth are more susceptible to slip reactivation. Our stress estimation results suggest that the site-scale stress state is strongly coupled with characteristics of natural fractures, emphasizing the importance of detailed geologic and stress data for subsurface utilization and its stability evaluation.

Complex rotation of maximum horizontal stress in the Wufeng-Longmaxi Shale on the eastern margin of the Sichuan Basin, China: Implications for predicting natural fractures

Abstract The in-situ stress state of the eastern margin of the Sichuan Basin, where there is a complicated fault zone, is poorly understood. This study offers vital insights into the in-situ stress state based on information from 17 wellbores in the Wufeng-Longmaxi Shale near the eastern boundary fault (i.e., the Qiyueshan Fault) of the Sichuan Basin, China. Drilling-induced tensile fractures and borehole breakouts revealed a decrease in the angle between the SHmax (maximum horizontal stress) orientation and fault strike when the wellbores are close to the damage zone of the Qiyueshan Fault. This stress rotation may be caused by the lateral contrast in the Poisson's ratio between the country rock and damage zone of the Qiyueshan Fault. A high angle between the SHmax orientation and fault strike also appeared in the individual fault blocks of the Qiyueshan Fault as a result of local stress concentration. The higher magnitudes of horizontal stresses indicate stronger lateral compression in the Qiyueshan Fault Zone. Owing to the lateral compression of a “sandwich” consisting of a high-E (Young's modulus) roof and low-E shale intervals, natural fractures mainly appeared in the low-E shale intervals when the wellbore was close to the fault damage zone. The low fracture intensity in the low-E shale intervals of the wellbores located at the boundaries of the stress concentration blocks may be associated with an extensional stepover process.

An overview of wellbore methods of investigating stress state of the upper layers of the earth’s crust

The wellbore methods for studying the stress-strain state of the upper layers of the Earth’s crust are overviewed. Some main methods for stress estimating from wellbore data are presented alongside with the analysis of their applicability and uncertainty appearing after their application. The reconstruction of stress profiles along the well trajectory based on the data on wellbore breakouts, drilling-induced tensile fractures, elastic wave propagation in the well surrounding the rock masses, and regularities in the spatial orientation and distribution of natural fractures is discussed. The estimation of stress state parameters based on minifracs and leakoff tests as well as on strain recovery techniques from the oriented core samples from wells is described.

Wellbore stability analysis based on the stress model around boreholes

Wellbore stability analysis plays an important role in the oil and gas drilling. Instability problems during the drilling phase are often the results of a combination of both mechanical and chemical effects. This study aims to assess the mechanical wellbore stability based on the stress model around boreholes. The development of the stress model around boreholes, which is associated with the in-situ stresses, rock properties as well as the wellbore pressure and configuration, are presented. It could visualize the stress distribution around an arbitratily orientated wellbore. Next, lower hemisphere diagrams are presented to demonstrate the wellbore pressure required to initiate borehole tensile and compressive failures. A program for the risk analysis of wellbore (RAoWB) is designed and developed by the Matlab programming language to describe and analyse the risk diagrams of the drilling induced tensile fractures (DITFs) and breakouts (BOs). They help to choice the optimum wellbore trajectories for well planning, as well as to predict the wellbore instabilities caused by inappropriate wellbore pressures.

A reassessment of the stress and natural fracture orientations from analysis of image logs in the Chinese Continental Scientific Drilling Program borehole at Donghai county, Jiangsu province, China

The Chinese Continental Scientific Drilling (CCSD) project has drilled a 5100 m deep research borehole in the Sulu ultra-high pressure (UHP) metamorphic belt, eastern China. The UHP metamorphic belt is thought to be a product of continent-continent collision and has consequently experienced very intensive structural deformation. Based on a more complete well log data set of CCSD borehole, we can have a more detailed and reliable study on the structure features of borehole wall and other rock physical properties than a previous study published in 2009. Abundant data related to borehole breakouts (BOs), drilling induced tensile fractures (DITFs) and natural fractures were collected from the image logs. The BO and DITF data indicate that the average direction of the maximum horizontal principal stress (SH) of the CCSD borehole site is about 79.2° which is consistent with the convergent direction (E-W) of the Pacific Sea Plate with respect to the Eurasian Plate. Analysis of DITFs indicated that in the case of the CCSD borehole, axial drilling induced tensile fractures (ADITFs) occur occasionally in the upper section (0–2300 m) of the borehole with low dipping angle (0–10°), while transverse drilling induced tensile fractures (TDITFs) occur significantly in the lower section (3800–5000 m) with high dipping angle (10–30°). The natural fracture distribution at depth in the metamorphic rocks of the CCSD borehole indicates that (1) the failure strength of rocks and borehole depth are two factors that affects natural fracture frequency, (2) most of the dip azimuth of natural fractures is consistent with the dip azimuth of foliations observed in the core, (3) the development of most of the natural fractures probably was dominated by the development of foliations, and both the natural fractures and foliations developed in response to the subduction and exhumation of the Sulu terrene.