Direction Of Horizontal Principal Stress Research Articles

Research on the Interaction Mechanism of Multi-Fracture Propagation in Hydraulic Fracturing

During the hydraulic-fracturing process, stress interference occurs among multiple wells and fractures, potentially affecting the trajectory of hydraulic fracture propagation. Previous studies have largely overlooked the influence of proppant support stresses on the trajectories of fracture propagation. This paper establishes a mathematical model, grounded in the boundary element method, designed to compute the propagation of multiple fractures, considering both proppant support on the fracture surface and dynamic perturbations within the local stress field. The findings of this research reveal that the stress field induced by hydraulic fracturing exhibits dynamic evolution characteristics, necessitating a comprehensive study of the fracture initiation and extension across the entire fracturing time domain. The effect of the residual fracture width under proppant action on the in situ stress field cannot be ignored. During simultaneous fracturing, hydraulic fractures are inclined to propagate in the direction of the maximum horizontal principal stress, particularly as the in situ differential stress escalates. Staggered fracturing between wells has been proven to be more effective than head-to-head fracturing. Simply increasing the well spacing cannot solve the problem of inter-well fracture interference. In zipper fracturing, adjusting the fracturing sequence can inhibit the fracture intersections between wells, thereby controlling the trajectory of fracture propagation. The aforementioned research has considerable significance in guiding the control of fracture morphology during hydraulic-fracturing processes.

The shrinkage behavior of boreholes drilled through deepwater creeping salt formations in West Africa

Introduction: Salt formations are complex and pose significant risks during oil and gas drilling. Creep behavior in salt formations under geostress can jeopardize drilling safety.Methods: This study analyzes the shrinkage behavior of boreholes drilled through salt formations in West Africa’s Block B, with emphasis on the differential creep rates in two horizontal principal stress directions and the evolution of wellbore shape over time. The impact of drilling fluid density on shrinkage rates is also investigated.Results: After drilling through salt formations, the creep rates differ between the two horizontal principal stress directions. Shrinkage is faster in the direction of minimum horizontal principal stress and slower in the direction of maximum horizontal principal stress. Over time, shrinkage rates converge, resulting in a transition from elliptical to circular wellbore shape. Higher drilling fluid density leads to reduced shrinkage rates.Discussion: These findings contribute to the theoretical guidance for drilling fluid density selection in salt formations.

Quantitative Prediction of the Development and Opening Sequence of Fractures in an Ultradeep Carbonate Reservoir: A Case Study of the Middle Ordovician in the Shunnan Area, Tarim Basin, China

Summary Quantitative prediction of reservoir tectonic fracture development characteristics, opening pressures, and opening sequences is critical in the exploration and development of oil- and gas-bearing reservoirs and thus has received widespread attention. Using numerical simulations of the paleostress field during the formation of tectonic fractures and the rock fracture criterion, we predict the development and occurrence of fractures in the Middle Ordovician Yijianfang Formation in the Shunnan region of the Tarim Basin, China. The local paleostress fields reflected by the mechanical properties and occurrence of tectonic fractures obtained from core descriptions, acoustic emission (AE) experiments, paleomagnetic experiments, sound velocity measurements, and borehole breakouts were used to determine the regional paleostress and in-situ stress. We established a geomechanical model by combining the mechanical parameters of the rocks with the finite element method (FEM), optimizing the boundary conditions with a self-adaptive constraint algorithm, and conducting numerical simulations of the in-situ stresses. Fracture occurrence and numerical simulation results of the in-situ stress field were used to determine the opening pressure (Pk) and opening sequence of the fractures. The level of fracture development decreases away from the strike-slip fault in the study area. Fracture development is positively correlated with the Young’s modulus, paleostress difference, and paleostress difference coefficient of the rock. The direction of the maximum horizontal principal stress is from north-northeast (NNE) to northeast (NE). Initially, shear fractures and tensional fractures oriented NNE 30°–35° and NE 40°–45°, respectively, open during the water injection process. Pk is positively correlated with the horizontal stress difference coefficient and the angle between the fracture strike and the maximum horizontal principal stress. At the structural highs (burial depths shallower than 6450 m) and the structural lows (burial depths deeper than 6450 m), the burial depth correlates negatively and positively with Pk, respectively. This investigation of the development, occurrence, Pk, and opening sequence of tectonic fractures and their principal controlling factors will have a positive impact on the future exploration and production opportunities of similar fractured reservoirs.

Influence of principal stress orientation on stress distribution and plastic zone evolution of rock surrounding tunnels

During the excavation of tunnels, the principal stress orientation changes, with a significant impact on the stress distribution, mechanical properties, and plastic zone evolution of rocks surrounding tunnels, causing severe deformation control, and monitoring problems in the stability of tunnels. Currently, biaxial compression tests were conducted to explore the influence of principal stress orientation on mechanical properties of rocks surrounding tunnels. The analytical solution of stress and the model of plastic zone of rocks considering the principal stress orientation and the distance from the excavation boundary were established to reveal the failure mechanism of surrounding rock under different principal stress orientations. With an increase in the angle between the principal stress orientation and the long axis of tunnel, the maximum tangential stress around the tunnel gradually changed to the minimum horizontal principal stress direction, and its value gradually increased, leading to quicker failure of surrounding rocks, and reducing the strength enhancement effect of the same section size of the tunnel. However, the increase in the angle reduced the damage range and the range of the plastic zone around the tunnel and caused the plastic zone to gradually approach the bottom and roof. The maximum depth of the plastic zone remained parallel to or nearly parallel to the minimum horizontal principal stress direction. When the principal stress orientation was kept constant, the maximum depth of the plastic zone shifted to the minimum horizontal principal stress direction with an increase in the vertical principal stress.

Stress background and rock fractures revealed by ultrasonic borehole television in the Fankou Lead-Zinc Mine

The stress background and rock fractures are essential factors affecting the stability of mines. In order to better understand the in situ stress background and rock fractures in the Guangdong Fankou Mine, we use ultrasonic borehole television scanning to measure rock fractures. The results indicate that rock fractures are intensively distributed at depths of −360 m to −450 m below the surface, suggesting the effect of intensive mining activities. The present maximum horizontal principal stress direction is NWW, which is consistent with the regional tectonic stress field direction. Systematic measurement of rock fractures is fundamental for further three-dimensional geological modeling and is significant for mining engineering.

Characteristics and control factors of tectonic fractures of ultra-deep tight sandstone: Case study of the Lower Cretaceous reservoir in Bozi-Dabei area, Kuqa Depression, Tarim Basin, China

The Bozi-Dabei area in the Kuqa Depression host high-quality reservoirs in the Bashijiqike Formation and Baxigai Formation sandstones of the Lower Cretaceous, in which reservoirs yield significant industrial gas flow despite being situated at a considerable burial depth of 8200 m. The geological history of the target formation involves multiple phases of tectonic movements, resulting in the development of multi-genetic fractures that enhance the reservoir's storage and seepage capacity. Based on the results of the drilling core, field profile survey, imaging logging, and experimental analysis, this study presents an analysis of fractures in the Lower Cretaceous dense sandstone reservoir of the Bozi-Dabei area, andclarifies the characteristics and controlling factors of the multi-genesis and multi-period fractures. Additionally, it proposes an effective fracture development model that accounts for geo-stress control. In the Bozi-Dabei area, the prevailing high extrusion stress environment has led to the development of predominantly regional tectonic fractures and fault-related fractures, with relatively gentle deformation-related fractures. The results of a combination of multi-attribute data determination techniques, including fracture filling, inter-cutting relationship, fracture filling isotope, inclusions, and cathode luminescence tests, this study reveals that the reservoir fractures have experienced three major periods of tectonic movement. The regional tectonic fracture development is mainly controlled by stratigraphic lithology and thickness, while the proximity influences fault co-derived fractures to the fault and the relative positions of the upper and lower plates of the fault. The shift in the direction of the late horizontal maximum principal stress leads to the opening or closing of early fractures under different conditions in the Bozi-Dabei area, consequently affecting the degree of fracture opening and effectiveness. Notably, when the horizontal maximum principal stress is deflected to intersect with early fractures at a smaller angle or even superimpose, the fracture effectiveness of the related group system in the deflection direction improves, resulting in an overall coordination. The distribution characteristics of the fracture system in this highly productive reservoir are the result of dominant configurations from multi-phases of geological activities.

Characteristics and mechanism of rockburst at five deep gold mines in Jiaodong Peninsula of China

Rockburst in deep rock engineering poses serious threat to personnel safety and engineering. In order to reveal the rockburst characteristics in deep engineering, the rockburst investigations and rockburst mechanisms based on in-situ stress were carried out at five deep gold mines in Jiaodong Peninsula, China. At the cover depth between 400 m and 600 m, the ground pressure hazards are mainly local spalling. At the cover depth between 600 m and 1000 m, the ground pressure hazards are mainly rockburst, accompanied by serious spalling. When the cover depth is more than 1000 m, in addition to rockburst, the large area collapse occurred in some tunnels. The maximum horizontal principal stress in the northwest of Jiaodong Peninsula is the largest, indicating that the regional in-situ stress field is dominated by horizontal tectonic stress. The average maximum horizontal principal stress direction is about 105°, which is approximately perpendicular to the strike of regional metallogenic fault zone. The increase of in-situ stress is the main factor leading to the serious ground pressure hazards. The tangential stress threshold for rockburst is 8 times the tensile strength of rock at the five gold mines, which is consistent with the stress threshold (8 times tensile strength) leading to brittle failure of rock in Griffith criterion. This indicates that the strain rockburst is caused by the concentration of tensile stress in the micro-cracks of surrounding rock.

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.

Stress interference and interaction between two fractures during their propagation: insights from SCDA test and XFEM simulation

Understanding and predicting the propagation and disturbance of multiple fractures is the key to the successful implementation of hydraulic fracturing. In this study, the two-hole fracturing tests are carried out on granite samples by using soundless cracking demolition agent (SCDA). Combined with the extended finite element method (XFEM) simulation and the induced stress analysis, the interaction and stress interference during the propagation of two fractures are systematically investigated. The relative position of two holes determines whether attraction or repulsion effect occurs between adjacent fractures, the pattern of which can be classified into three modes: the fracture collinear, staggered propagation and parallel propagation. The fractures in collinear pattern are prone to coalescence, the staggered fractures are easy to attract each other and the parallel fractures tend to propagate independently, which ultimately lead to the formation of single, spiral and multiple main fractures, respectively. The stress near the fracture tip controls the possibility and direction of fracture propagation. The superposition of tensile stress is prone to promote fracture propagation, and the stress interference tends to make fractures deflection and mutual exclusion. The fracturing results are quantitatively evaluated by using fractal dimension and fracture surface characteristics. It is found that increasing perforation spacing makes the degree of internal rupture of the sample increase first and then decrease, and the fracturing scheme is optimal when the perforations are arranged perpendicular to the direction of maximum horizontal principal stress.

Numerical simulation of three-dimensional fracturing fracture propagation in radial wells

A fracture propagation model of radial well fracturing is established based on the finite element-meshless method. The model considers the coupling effect of fracturing fluid flow and rock matrix deformation. The fracture geometries of radial well fracturing are simulated, the induction effect of radial well on the fracture is quantitatively characterized, and the influences of azimuth, horizontal principle stress difference, and reservoir matrix permeability on the fracture geometries are revealed. The radial wells can induce the fractures to extend parallel to their axes when two radial wells in the same layer are fractured. When the radial wells are symmetrically distributed along the direction of the minimum horizontal principle stress with the azimuth greater than 15°, the extrusion effect reduces the fracture length of radial wells. When the radial wells are symmetrically distributed along the direction of the maximum horizontal principal stress, the extrusion increases the fracture length of the radial wells. The fracture geometries are controlled by the rectification of radial borehole, the extrusion between radial wells in the same layer, and the deflection of the maximum horizontal principal stress. When the radial wells are distributed along the minimum horizontal principal stress symmetrically, the fracture length induced by the radial well decreases with the increase of azimuth; in contrast, when the radial wells are distributed along the maximum horizontal principal stress symmetrically, the fracture length induced by the radial well first decreases and then increases with the increase of azimuth. The fracture length induced by the radial well decreases with the increase of horizontal principal stress difference. The increase of rock matrix permeability and pore pressure of the matrix around radial wells makes the inducing effect of the radial well on fractures increase.

Shear-Wave Anisotropy Measurements in the Crust from Receiver Functions: An Interplay of Lower and Upper Crustal Anisotropy

We report a study using teleseismic P-wave receiver functions to infer the orientation of the maximum horizontal principal stress from the direction of upper crustal shear-wave velocity anisotropy. We apply an inverse approach using the Neighborhood Algorithm to conduct a nonlinear search, attaining a best-fitting crustal model that includes shear velocity anisotropy. Unlike previous methods reported in the literature, this method is able to distinguish anisotropy in the upper, brittle crust from that in the lower, ductile crust in certain instances. We apply this method to teleseismically recorded earthquakes in the Central Valley of California, the Permian Basin, Texas, northern Oklahoma and sites near the San Andreas Fault in California. Of the forty-one stations to which we apply this method, twenty have a good apparent signal. A misfit calculation is performed by calculating a zero-lag cross-correlation coefficient for each modeled receiver function with the data for a given back azimuth range. While the fast polarization direction in the upper crust of some of these stations aligns with independent indicators of the direction of the maximum horizontal principal stress, the fast direction in the upper crust at other stations does not, apparently indicating that the anisotropy was resulting from a different mechanism.

A multi-fracture balanced extension control method for horizontal wells in anisotropic shale

Shale has anisotropy in elastic parameters and permeability coefficient in parallel and perpendicular directions to the bedding plane. In the study of fracture propagation during hydraulic fracturing, it is necessary to consider the influence of shale anisotropy. Through the development of fluid pipe element, fluid pipe connector element, cohesive damage element, and rock matrix element with pore pressure, a fluid–structure coupling numerical model with comprehensive consideration of dynamic flow distribution, dynamic fracture propagation and extension, and stress interference are established. The anisotropic characteristic parameters of shale reservoirs are given to study the balanced propagation control method of multi-fracture in horizontal shale Wells. The results show that under anisotropic conditions, hydraulic fractures are easy to expand in the direction parallel to the bedding plane but not in the direction perpendicular to the bedding plane. Finally, It forms a long extension in the length direction and a short extension in the height direction. The stress interference along the direction of the minimum horizontal principal stress is gradually intense with the increase of the degree of anisotropy. During multi-fracture propagation in horizontal shale wells, the stress interference generated in the direction of minimum horizontal principal stress can be balanced by effectively controlling the perforation friction so as to promote the balanced extension of multiple fractures. At the same time, with the increase of injection flow and the decrease of the distance between clusters, the stress interference degree gradually becomes serious. Therefore, it is necessary to strictly design how to control the perforation friction in the multi-fracture fracturing process of shale horizontal Wells with large flow rates and small cluster distances. The research conclusions provide a theoretical reference for the efficient development of shale petroleum resources.

Well trajectory optimization of ultradeep and high-pressure drilling engineering based on high in situ stress as the main control factor: A case study from the Ordovician carbonate reservoir in the Shunbei area of the Tarim Basin

With increasing oil and gas exploration and development, well trajectory optimization has gradually become the focus of the oil and gas industry. Considering the wellbore instability in the Shunbei area of the Tarim Basin, the well trajectory was scientifically optimized under the guidance of rock mechanics, drilling engineering, and mathematical methods, combined with actual geologic data, and with in situ stress as the main controlling factor. In this paper, the stress state of the wellbore is analyzed by linear elastic theory to establish the stress distribution model of the wellbore. The safety window model of wellbore stability is established using different rock failure criteria to calculate the collapse pressure and fracture pressure of the formation. Based on this, the safe mud density window is defined to achieve wellbore trajectory optimization. Finally, the influence factors of wellbore stability are discussed, and the applicability of different rock failure criteria is evaluated. The results indicate that under the normal faulting stress regime condition in the study area, the direction of horizontal minimum principal stress is the best drilling direction, where the borehole inclination angle of [Formula: see text] > 50° is the optimal well trajectory. The wellbore stabilities of high-angle deviated wells and horizontal wells are better than those of low-angle deviated wells and vertical wells. The calculation results of the Mogi-Coulomb criterion can describe the conditions of the in situ stress field more accurately. The safe windows for different well trajectories are obtained directly by the numerical method, which is very practical for optimizing well trajectories and improving wellbore stability.

Fracture Parameters Optimization and Field Application in Low-Permeability Sandstone Reservoirs under Fracturing Flooding Conditions

To solve engineering problems in the production process after fracturing and flooding of low-permeability sandstone reservoirs, such as rapid water-cut rise and low water flooding efficiency, a method for optimizing the fracture parameters of low-permeability sandstone reservoirs under fracturing flooding conditions was proposed. A rock property test experiment was first carried out, the fracturing coefficient was defined, and an evaluation method for the brittleness index of low-permeability sandstone was established to optimize the perforation location of the fracturing reservoir. A productivity numerical model for the two-phase flow of oil–water in matrix–fracture media was established to optimize the fracture morphology under fracturing flooding conditions. The results showed that the quartz content, Young’s modulus, and peak stress mainly affected the fracturing coefficient of rock and are the key indicators for evaluating the brittleness of low-permeability sandstone reservoirs. For production wells in the direction of minimum horizontal principal stress, the swept area of water flooding should be expanded, fracture length should be optimized to 90 m, and fracture conductivity should be 20 D·cm. For fracturing production wells in the direction of maximum horizontal principal stress, the advancing speed of the water injection front should be slowed down to reduce the risk of water channeling in injection-production wells. The optimized fracture length was 80 m, and the fracture conductivity was 25 D·cm. The application of these findings can markedly improve oil production and provide a reference for optimizing the fracture parameters of low-permeability sandstone reservoirs under fracturing flooding conditions.

Study on hydraulic fracture extension of volcanic formation in J230 area, Xinjiang

It is key of “network fracturing” stimulation to correctly recognize the rock failure pattern during hydraulic fracturing treatment. In this paper, basic characteristics of the Carboniferous volcanic reservoirs in J230 area of Xinjiang oilfield are overviewed. Triaxial in-situ stresses of the formation are interpreted by conventional well-logging data, and the natural fissure characteristics are interpreted by imaging well logging information, and intersection of geometry relationship between hydraulic fracture and cross natural cracks is described on approaching angle. Current experimental achievements of hydraulic fracture penetration cross or extension along natural fissure are summarized, mechanics criterion under the different action manner are established for opening and shear failure of natural fractures. Taking Carboniferous volcanic reservoir of J230 area as an example, a hydraulic fracture propagation model is obtained by calculation results under the action of hydraulic-natural cracks synergy. It is shown that the approaching angle in the formation is mainly 50°–60°, shear sliding failure will be firstly induced along natural-hydraulic fracture interface and then natural fissure will open because of going net pressure. With further increase of the net fluid pressure, hydraulic fracture propagated along the natural fissure because of tangential failure and gradually turns to the direction of maximum horizontal principal stress, which is conducive to the formation of network fractures in stimulated reservoir treatment.

Study on the Mechanical Properties and Failure Law of Rocks with Interbedded Sand and Mud.

In view of the borehole instability during the drilling process of the thin sand and mud interbedded sections in the Shahejie Formation, the physicochemical and mechanical properties of sand and mud interbed rock were studied through a series of laboratory tests to determine the main factors influencing the formation instability. The effects of fracture development of interbed sand and mud, mechanical weak plane, borehole trajectory, and seepage effect on borehole stability were evaluated and analyzed through the established model. The results indicated that the microcracks are developed on the lithologic interface due to the change of lithology of the sand-mud interbed. The anisotropy of the mechanical properties of the rocks with interbedded sand and mud is obvious, which leads to the great mechanical weak plane effect on the wellbore stability. The bottom-hole pressure difference leads to the seepage effect, which reduces the effective supporting force of the drilling fluid on the borehole wall and promotes rock sliding and failure along the lithologic interface developed by the sand-mud thin interbedded layer. Considering the influence of the borehole trajectory, mechanical weak plane, and seepage effect, the recommended drilling fluid density in the deviated section is 1.69 g/cm3. The wellbore stability is the best when drilling along the direction of the minimum horizontal principal stress in the horizontal section. Further strengthening the plugging performance of the drilling fluid, drilling through the microfractures vertically on the interbedded lithologic interface, and reducing the lateral vibration of the drilling tools as far as possible are necessary. The research results provide a theoretical basis for the safe drilling of thin sand and mud interbedded strata.

Study on the Solid Production Mechanism of the Fractured Granite Reservoirs-Example of YL Area in Qiongdongnan Basin

The granite buried hill gas reservoir in YL area of Qiongdongnan basin faces a serious problem of solid production, which seriously affects gas well production and reduces economic benefits; however, the solid production mechanism of fractured granite reservoirs is still unclear. In this study, the reasons for solid production were revealed and the mechanism was clarified based on the analysis of geological and mechanical characteristics of the granite buried hill reservoir. The solid production of fractured granite reservoirs can be divided into three modes, those being shedding of fracture filling and solid particles on the fracture wall, shear slip failure along the fracture, and shear failure of granite matrix. Take the YL area in the Qiongdongnan Basin as an example, the solid production of fractured granite reservoirs is mainly based on shedding of fracture filling and solid particles on the fracture wall and shear slip failure along the fracture, the possibility of shear failure of granite matrix is less. In addition, the closer the wellbore, the greater the risk of shedding of fracture filling and solid particles on the fracture wall. The high-angle fractures have a greater risk of shear slip failure. In addition, the direction of the minimum horizontal principal stress is higher risk of solid production. The research provides the basis and foundation for safe and efficient development of fractured granite reservoirs and for later measure selection and optimization.

Application of long-reach directional drilling boreholes for gas drainage of adjacent seams in coal mines with severe geological conditions

This study aimed to demonstrate the application of Long Reach Directionally Drilled Boreholes (LRDD) for gas drainage of adjacent seams before and during the longwall face operations of low permeability-high gas content coals Staszic-Wujek Hard Coal Mine in the Upper Silesia Coal Basin (Poland). Five LRDD Boreholes (TM1a-TM5) with a length of 300 and 400 m were located over coal seam 501 in the fractured zone and monitored over six months of longwall face operations. LRDD Boreholes were combined with Cross-Measured Boreholes. Reservoir characterization and geological modeling supported the results obtained from gas drainage. The drainage efficiency of LRDD Boreholes was the approximately 70% level, while conventional Cross-Measured Boreholes were only 30%. The highest goaf gas quality (94% methane concentration) was reported for TM4, placed at an average elevation of 41 m above coal seam 501. The highest goaf gas production (average 6.2 m3/min) was reported for LRDD Borehole TM3. This borehole was placed within the fracture zone (average elevation of 24.4 m) and drilled through the sandstone lithotype with the best reservoir properties, enhancing drainage performance. LRDD Boreholes TM2 and TM4 achieved similar performance. These three LRDD Boreholes were drilled close to the maximum principal horizontal stress direction, providing borehole stability during under-mining. The lowest goaf gas production was reported for LRDD Boreholes TM1a and TM5. Both Boreholes were placed in the rubble zone.

Study on Rock Mechanical Properties and Wellbore Stability of Fractured Carbonate Formation Based on Fractal Geometry.

To mitigate borehole wall instability in fractured carbonate formations in an oilfield, the main factors affecting borehole wall instability were determined by combining the characteristics of underground cores, logging data, and a series of laboratory mechanics experiments. The geometric morphology characteristics of a carbonate rock fracture surface were studied together with an artificial rock fracture surface formed by triaxial mechanics experiments. A relationship between the geometric morphology characteristics of a fracture surface and rock mechanical properties was established based on fractal geometry. Using the Mohr-Coulomb failure and weak plane failure criteria, a rock strength criterion based on the fractal characteristics of a rock fracture surface was established. Finally, the mechanical rock properties characterized by fractal geometry were imported into the established borehole stability evaluation model. The results show that a collapse formation is mainly limestone with relatively developed microfractures, and some fractures are filled with expansive clay. The anisotropy of mechanical properties of bedrocks and microfractured rocks is obvious, whereas drilling-fluid immersion has little effect on the mechanical properties of a rock. 3D scanning experiments of artificial fracture surfaces formed after a triaxial mechanical test of a bedrock and fracture surfaces of a rock with microfractures show that the geometric characteristics of fracture surfaces after bedrock failure were more complex than those of fractured rocks. The geometric characteristics of rock fracture surfaces were numerically expressed through astatistical analysis and fractal geometry. Function relationships among cohesion, an internal friction angle, and fractal dimensions of bedrocks and microfracture rocks were fitted. A numerical simulation of borehole stability based on the fractal model of a carbonate fracture surface shows that different fracture inclinations and borehole trajectories significantly influence the collapse pressure equivalent density of a borehole wall. On drilling a horizontal well along the inclination of a fracture, the collapse pressure equivalent density of the borehole wall is relatively low when the fracture inclination is along the direction of the minimum horizontal principal stress. Unlike that from a conventional borehole stability model, the collapse pressure equivalent density calculated from the fractal model will increase by 0.1-0.2 g/cm3. The study results provide a theoretical basis for safe and efficient drilling in fractured carbonate formations.

Evolution of Near-Well Damage Caused by Fluid Injection through Perforations in Wellbores in Low-Permeability Reservoirs: A Case Study in a Shale Oil Reservoir

Abstract During the development of shale oil resources, fluid injection is usually involved in the process of hydraulic fracturing. Fluid injection through perforations causes near-well damage, which is closely related to the subsequent initiation and propagation of hydraulic fractures. This study is focused on the characterization of the temporal and spatial evolving patterns for near-well damage induced by fluid injection through perforations in the early stage of hydraulic fracturing. A coupled hydromechanical model is introduced in a case study in a shale oil reservoir in northwestern China. The model considers porous media flow during fluid injection. It also considers elasticity in the rock skeleton before the damage. Once the damage is initiated, a damage factor is employed to quantify the magnitude of injection-induced damage. Results show that damage evolution is highly sensitive to perforation number and injection rate in each individual perforation. Damage propagation is more favorable in the direction of the initial maximum horizontal principal stress. The propagation of damage is drastic at the beginning of fluid injection, while the damage front travels relatively slow afterward. This study provides insights into the near-well damage evolution before main fractures are initiated and can be used as a reference for the optimization of perforation parameters in the hydraulic fracturing design in this shale oil field.