Direction Of Horizontal Principal Stress Research Articles

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.

Numerical simulation on spatial steering rule of directional perforation hydraulic fractures in low-permeability reservoir

For purpose of clearing the spatial pattern of hydraulic fracture during directional perforation in low-permeability unconventional reservoirs, RFPA2D-Flow software is used to numerically calculate their spatial steering trajectory and deflection distance. The accuracy of numerical calculation results is verified by theoretical and experimental results. Then the influences of horizontal principal stress difference, perforation length, and azimuth on the hydraulic fractures’ spatial steering trajectory are studied. It is shown by the results that 1) the software can accurately predict the space steering trajectory and deflection distance of directional perforation hydraulic fractures, 2) both deflection distance and spatial steering trajectory of hydraulic fractures are quantitatively evaluation indexes, which are used to evaluate the hydraulic fractures’ spatial steering effects, and 3) under different horizontal principal stress differences, perforation azimuths, and lengths, the same hydraulic fracture’s propagation trajectories are presented. They initiate from the perforation end and gradually deflect along the maximum horizontal principal stress direction and finally represent curved fractures like both wings. With the increase in horizontal principal stress difference, the fractures’ deflection distance decreases. However, it increases with the increase in perforation azimuth and length. Their deflection amplitude increases first and then decreases. Initiation pressure of hydraulic fracture rises with the decrease in perforation length and increase in perforation azimuth and horizontal principal stress difference.

Numerical Simulation on Borehole Instability Based on Disturbance State Concept

This paper carries out a study on the numerical simulation of borehole instability based on the disturbance state concept. By introducing the disturbance damage factor into the classical Mohr–Coulomb yield criterio, we establish a finite element hydro-mechanical coupling model of borehole instability and program the relevant field variable by considering elastic–plastic deformation in borehole instability, the distribution of the damage disturbance area, the variation of porosity and permeability with the disturbance damage factor, etc. Numerical simulation shows that the borehole stability is related to the action time of drilling fluid on the wellbore, stress anisotropy, the internal friction angle of rock, and borehole pressure. A higher horizontal stress difference helps suppress shear instability, and a higher rock internal friction angle enhances shear failure around the borehole along the maximum horizontal principal stress. When considering the effect of the internal friction angle of rock, the rock permeability, disturbance damage factor, and equivalent plastic strain show fluctuation characteristics. Under the high internal friction angle of rock, a strong equivalent plastic strain area and disturbance damage area occur in the direction of the maximum horizontal principal stress. Their cloud picture shows the mantis shape, where the bifurcation corresponds to the whiskers of the shear failure area in borehole instability. This study provides a theoretical basis for solving the problem of borehole instability during drilling engineering.

Hydraulic fracture initiation for perforated wellbore coupled with the effect of fluid seepage

Hydraulic fracturing technology is one of the most effective well stimulation techniques for low-permeability reservoirs, however, the mechanism of the fracture initiation and propagation is not fully understood. Based on Biot’s consolidation theory and linear elasticity theory, this paper develops a general analytical model to investigate the hydraulic fracture initiation from arbitrarily spiral perforation or oriented perforation coupled with the effect of fluid seepage. The analytical solution can be used to determine formation breakdown pressure, the location and direction of an initial crack, and to investigate the factors that affect the formation breakdown pressure and the fracture initiation position. The calculation results show that the direction of fracture initiation does not always align with the direction of the maximum horizontal principal stress and may deviate from the 0° direction of the perforation circumferential angle to the 90° direction. The seepage effect of fracturing fluid can significantly reduce the formation breakdown pressure and increase the possibility of fracture initiation from the tip of perforation. The smaller the formation depth and transfer factor (TF) are, the more favorable the fracture initiation at the tip of the perforation is. With the increase of the Biot coefficient, the formation breakdown pressure decreases significantly. To avoid the failure of cement sheath induced by the fracture initiation from the bottom of perforation, the low-viscosity fracturing fluid with a stronger seepage effect and the casings with harder materials and lower transfer coefficient are recommended.

Effect of In-Situ Stress on Hydraulic Fracturing of Tight Sandstone Based on Discrete Element Method

The tight sandstone reservoir in the Qianfoya formation of well PL-3 of the Puguang gas field in Sichuan, China, obtained a high-yield gas flow after a volume fracturing treatment. However, the stimulated reservoir volume (SRV), fracture morphology, scale and formation law still remain unclear. Based on particle flow discrete-element theory in this paper, we carried out a few trials of the Brazilian splitting test, uniaxial compression and triaxial compression of rock mechanics. Meanwhile, the research also testified to the conversion relationship between macroparameters and microparameters, established the numerical simulation on hydraulic fracturing through PFC2D discrete element software, and finally analyzed the influence of difference coefficients on the fracturing effect, in terms of different in-situ stresses. The conclusions are as follows: firstly, the influence of in-situ stress is essential for the direction, shape and quantity of fracture propagation, and the fractures generated by hydraulic fracturing are mainly tension fractures, accounting for over 90% of the total longitudinal fractures. Secondly, it is indicated that when the difference coefficient is small in the in-situ stress, the fractures formed by hydraulic fracturing expand randomly around the wellbore. When the difference coefficient Kh of in-situ stress is above 0.6, the development of hydraulic fractures is mainly controlled by in-situ stress; as a result, the fractures tend to expand in the vertical direction of the minimum horizontal principal stress and the fracture shape is relatively singular. When the difference coefficient of in-situ stress was 0.3, in total, 3121 fractures were generated by fracturing, and the fractal dimension D value of the fracture network complexity was 1.60. In this case, this fracturing effect was the best and it is the easiest to achieve for the purpose of economical and effective development on large-scale volume fracturing.

Damage of reservoir rock induced by CO2 injection

The study of reservoir rock damage induced by gas injection is of great significance to the design of reservoir stimulation and the improvement of oil and gas recovery. Based on an example horizontal well in the Hudson Oilfield of the Tarim Basin and considering the multi-physics coupling effects among high-pressure fluid, rock deformation, and damage propagation during CO 2 injection, a three-dimensional finite element model for CO 2 injection in deep reservoir considering seepage-stress-damage coupling was developed. The evolution of reservoir rock damage under different CO 2 injection conditions was systematically investigated. The results show that tensile damage and shear damage are concentrated in the vertical direction and the horizontal maximum compressive principal stress direction, respectively, and the tensile damage is the main damage mode. At higher CO 2 injection rate and pressure, the damaged areas near the wellbore are mainly distributed in the direction of the maximum compressive principal stress, and the development of the damaged area near the wellbore will be inhibited by the formation and evolution of far-field damage. CO 2 injection aggravates the extension of tensile damage, but inhibits the initiation of shear damage, and eventually leads to the gradual transition from shear damage to tensile damage. Under the same injection conditions, CO 2 injection has superior performance in creating rock damage compared with the injection of nitrogen and water. The results in this study provide guidance for enhanced oil recovery in deep oil and gas reservoirs with CO 2 injection.

New prediction method of horizontal principal stress of deep tight reservoirs

As the tight reservoir of Lucaogou Formation in Jimsar sag is characterized as large burial depth and poor physical properties, hydraulic fracturing technology is greatly needed to increase the production. Firstly, based on elastic theory, the borehole displacement formula under stress is derived, then the borehole quasi-ellipse characteristic model is proposed, on this basis, the in-situ stress prediction model based on borehole deformation is constructed. Secondly, the force analysis of diamond bit during drilling is performed, and the continuous prediction formula of elastic modulus based on weight on bit (WOB) and torque parameters is derived. Combined with these two steps, a new model of continuous in-situ stress prediction based on borehole deformation and drilling parameters is established. Compared with the values measured by the acoustic emission method in the laboratory, the deviation between the predicted in-situ stress values and those measured by the acoustic emission method is less than 4%, which meets the accuracy requirements of the fracturing in the oil field. The prediction results show that the minimum horizontal principal stress of Lucaogou Formation is 51.1–62.7 MPa, and the maximum horizontal principal stress is 58.9–69.1 MPa, respectively. The maximum horizontal principal stress direction is NW–SE direction with an angle of about 159°. The results show that the fracture direction in the study area is in accordance with the present in-situ stress direction, which is beneficial to the secondary reconstruction of natural fractures and keeps good opening property of fractures. This study provides a theoretical basis for in-situ stress prediction and compressibility evaluation of tight reservoir.

Study on hydraulic fracturing initiation model based on nonlinear constitutive equation

In this article, the nonlinear characteristics of rock deformation were characterized based on the Power-enhanced constitutive equation, and the piecewise nonlinear constitutive equation was established by using the piecewise approximation method. Based on the constitutive equation, an elastic-plastic stress field model around wells was established to analyze the stress field around wells during hydraulic fracturing. The initiation mode of elastic-plastic formation was analyzed and optimized. The results showed that the plastic yield of rock was determined by the in-situ stress field and the initial yield stress. The smaller the yield stress was, the greater the in-situ stress was, and the more easily the plastic yield occurred. Under the combined of rock mechanics parameters and in-situ stress, the tension failure occurred when the wellbore was under elastic state, and there were two failure modes of plastic tension failure and plastic shear failure after plastic state. The initiation directions of elastic tension, plastic tension, and plastic shear failure were the direction of maximum horizontal principal stress, while there was a failure angle between the fracture plane and the maximum horizontal principal stress under the shear failure.

Hydraulic fractures simulation in non-uniform stress field of horizontal shale gas well

Shale reservoirs have low porosity and ultra-low permeability. A significant increase in gas well production requires horizontal well fracturing. In this view, the propagation behavior of hydraulic fractures during the fracturing process is critical to the effect of shale gas reservoir stimulation. However, some shale formations have developed faults and the in-situ stress field has a non-uniform distribution. Also, the maximum horizontal principal stress direction varies with the spatial position, and non-planar deflection propagation behavior in hydraulic fractures is possible. The purpose of this article is to reveal the effect of the development of faults in the shale formation on the propagation of hydraulic fractures. A calculation model for the non-uniform tectonic stress field near the fault was established, and on this basis, we constructed a shale gas horizontal well-staged multi-cluster fracturing hydraulic fracture deflection propagation model under the influence of faults. Furthermore, the fracture propagation law was simulated and analyzed in the presence of a non-uniform in-situ stress field. The findings demonstrate that hydraulic fractures near the strike-slip fault tend to run parallel to the fault direction, whereas hydraulic fractures near the normal fault tend to run perpendicular to the fault direction. The greater the fault throw and the greater the hydraulic fracture propagation deflection angle, the closer the hydraulic fracture is to the fault. The deflection behavior of hydraulic fractures increases flow resistance in the fracture, and proppant transport does not flow smoothly, increasing the difficulty and risk of the fracturing operation. The findings of this study could provide an important theoretical foundation for the design of staged multi-cluster fracturing of horizontal wells near faults in shale formations.

A Random‐XFEM technique modeling of hydraulic fracture interaction with natural void

In this paper, a Random-XFEM technology was employed to simulate hydraulic fracture interaction with the natural void in a random field of Young's modulus. The random distribution of Young's modulus is characterized by random field theory, while the stress and pressure fields fracturing were solved by the extended finite element method. And a Random-XFEM iteration algorithm was proposed to solve the hydraulic fracture propagation problem. Then the proposed model was verified against the KGD model and numerical study. A series of numerical examples were presented to analyze the interaction mechanism of hydraulic fracture and void in a random field. The numerical results show that the random field of Young's modulus has a great impact on the hydraulic fracture propagation path. Under the random distribution of Young's modulus, the hydraulic fracture would deviate from the direction of maximum horizontal principal stress. And the random field parameters (Young's modulus mean and point variance) have different effects on the hydraulic fracture propagation path. Also, the interaction patterns of hydraulic fracture and void are greatly affected by random fields.

Distribution Law of Deep Complex In Situ Stress and Influence of Underground Engineering in Huanghuai Area

Huanghuai area is rich in coal resources, but due to the increasingly complex geological environment faced by coal mining, the complexity of structural stress is one of the main problems. In order to find out the distribution law of deep in situ stress in the Huanghuai area, the in situ stress measurement data of 81 effective measuring points in 24 mines with depths ranging from −100 m to −1200 m are analyzed in the present study base on the in situ stress measurement data. Furthermore, numerical simulation and field observation are used to analyze the deformation and failure characteristics of the surrounding rock. The research results show that the deep mining area of Huanghuai exhibited a high stress level, and the vertical and horizontal principal stress increases with increasing depth. The ratio of the lateral pressure coefficient ranges from 0.90 to 2.70, and the in situ stress field presented a trend of transition to the quasihydrostatic pressure field type as the depth extended, in which 80.23% of the measuring points are distributed between 1.20 and 2.10, belonging to the typical tectonic stress field type in which tectonic stress is absolutely dominant. The observation results of the surrounding rock and numerical simulation reveal that when the layout axis of the roadway is approximately perpendicular to the direction of the maximum horizontal principal stress, a high stress concentration area is present on the roof and floor of the roadway, the deformation increases sharply, and the support pressure of the roof and floor increases.

Analysis of Crustal Stress and Its Influence on the Stability of the Deep Tunnel in the Huanaote Mining Area

Crustal stress is a critical parameter utilized to analyze the stability of the tunnel in underground hard‐rock mining sites. In order to analyze the distribution law of crustal stress and its influence on the stability of the deep tunnel in the Huanaote mining area, the in‐situ stress field model of this area was constructed firstly by using the borehole stress relief method. Moreover, the numerical model is established based on three real engineering conditions: the excavation direction is parallel, vertical, and intersection to the measured maximum horizontal principal stress direction, respectively. Results show that, compared with the other two layout schemes, the stress of two side walls reaches peak at 27 MPa, when the excavation direction is intersected to the measured maximum horizontal principal stress direction, which indicates that the support and maintenance of two side walls should be strengthened to ensure the stability of the tunnel and the safety of personnel and equipment in real projects.

In situ stress characteristics of the NE Sichuan basin based on acoustic emission test and imaging logging

This study presents the distribution rule of in situ stress in the northeast Sichuan basin and its relationship with fracture. Sixty-seven sets of core samples of 21 Wells from the terrigenous clastic rock formation (Shaximiao, Qianfoya, Xujiahe) and marine carbonate formation (Jialingjiang, Leikoupo, Feixianguan) in the northeast Sichuan basin were tested by acoustic emission experiment. The in situ stress variation with the depth was established and the corresponding regression analysis was done. The horizontal principal stress direction of terrigenous clastic rock formation and marine carbonate rock formation was obtained by combining the dual diameter data of 6 wells and the imaging logging data of 3 wells. The results show that the vertical stress in the northeast of Sichuan basin has a linear relationship with the depth, and there is little difference between the vertical stress and the overburden weight of rocks. The maximum and minimum horizontal principal stress and horizontal shear stress increase with the burial depth. The divergence degree of horizontal shear stress with depth greater than 3000 m is greater than that of the stratum smaller than 3000 m. The horizontal stress plays a dominant role in the northeast Sichuan basin. With the increase in depth, the influence of tectonic stress field decreases and the vertical stress increases. Impacted by Dabashan and Qinling plate tectonic movement, the direction of in situ stress in marine carbonate strata is nearly east–west. The direction of maximum horizontal principal stress in terrigenous clastic rock formation is basically northwest–southeast. The imaging logging data show that the fracture direction is consistent with the horizontal principal stress direction, and the present in situ stress direction is favorable to the secondary reconstruction of natural fractures, and the fractures keep good opening. The distribution law of in situ stress in northeast Sichuan basin shows σH > σV > σh, indicating that the fault activity in this area is dominated by strike-slip type, the tectonic stress field is dominated by horizontal tectonic stress, in addition that the stress state is conducive to reverse fault activity.

Experimental Study of Hydraulic Fracture Propagation Behavior during Multistage Fracturing in a Directional Well

Due to the limited space of offshore platform, it is unable to implement large-scale multistage hydraulic fracturing for the horizontal well in Lufeng offshore oilfield. Thus, multistage hydraulic fracturing technology in directional well was researched essentially to solve this problem. Modeling of fracture propagation during multistage fracturing in the directional and horizontal wells in artificial cores was carried out based on a true triaxial hydraulic fracturing simulation experiment system. The effects of horizontal stress difference, stage spacing, perforation depth, and well deviation angle on multifracture propagation were investigated in detail. Through the comparative analysis of the characteristics of postfrac rock and pressure curves, the following conclusions were obtained: (1) multistage fracturing in horizontal wells is conducive to create multiple transverse fractures. Under relatively high horizontal stress difference coefficient (1.0) and small stage spacing conditions, fractures tend to deflect and merge due to the strong stress interference among multiple stages. As a consequence, the initiation pressure for the subsequent stages increases by more than 8%, whereas in large stage spacing conditions, the interference is relatively lower, resulting in the relatively straight fractures. (2) Deepening perforation holes can reduce the initiation pressure and reduce the stress interference among stages. (3) When the projection trace of directional wellbore on horizontal plane is consistent with the direction of the minimum horizontal principal stress, fractures intersecting the wellbore obliquely are easily formed by multistage fracturing. With the decrease of well deviation angle, the angle between fracture surface and wellbore axis decreases, which is not conducive to the uniform distribution of multiple fractures. (4) When there is a certain angle between the projection trace of directional wellbore on horizontal plane and the direction of minimum horizontal principal stress, the growth of multiple fractures is extremely ununiform and the fracture paths are obviously tortuous.

Wastewater Disposal Has Not Significantly Altered the Regional Stress State in Southern Kansas

Abstract Wastewater disposal is primarily responsible for the increased seismicity rate since ∼2013 in southern Kansas. Previous work that used shear-wave splitting (SWS) in southern Kansas interpreted an ∼90° temporal rotation in the fast polarization direction and attributed it to increased pore pressures resulting from fluid injection. However, this interpreted rotation coincided with a change in the stations used to make the SWS measurements. We investigate the temporal variability of fast azimuths in southern Kansas by making SWS measurements on earthquake families with similar source–receiver paths recorded on a stable local seismic network. We select high-quality SWS measurements by investigating the stability of results across 65 different frequency bands between 0.5 and 15 Hz. We find that the fast polarization direction in southern Kansas is relatively constant with an average east-northeast (∼N79°E) orientation between 2014 and 2017. Our fast polarization measurements are primarily a reflection of the maximum principal horizontal stress direction (SHmax). We observe a slight spatial change in SHmax to the northeast (∼N55°E) near the Nemaha ridge in Oklahoma. However, we do not observe any significant temporal rotation of SHmax or variation in delay time (i.e., crack density) in southern Kansas, contrary to the earlier study. The previously interpreted ∼90° rotation may either be a reflection of a very local stress change or a misinterpretation of SWS results potentially due to the use of inconsistent source–receiver paths. Our SWS measurements cover the period of peak wastewater disposal and seismicity rates and suggest an absence of significant temporal rotations in the local anisotropy and stress orientations associated with wastewater disposal.

Formation of fractures in carbonate rocks by pad acid fracturing with different states of carbon dioxide

Carbonate outcrops were taken from Ma 5 1 sub-member in the Lower Paleozoic in the Yan’an gas field to conduct true tri-axial hydraulic fracturing experiments with water, liquid CO 2 and supercritical CO 2 . CT scan was applied to analyze initiation and propagation laws of hydraulic fractures in carbonate rocks. The experiments show that supercritical CO 2 has low viscosity, strong diffusivity and large filtration during fracturing, which is more liable to increase pore pressure of rocks around wellbore and decrease breakdown pressure of carbonate rocks. However, it would cost much more volume of supercritical CO 2 than water to fracture rocks since the former increases the wellbore pressure more slowly during fracturing. For carbonate rocks with few natural fractures, tensional fractures are generated by fracturing with water and liquid CO 2 , and these fractures propagate along the maximum horizontal principal stress direction; while fracturing with supercritical CO 2 can form shear fractures, whose morphology is rarely influenced by horizontal stress difference. Besides, the angle between propagation direction of these shear fractures near the wellbore and the maximum horizontal principal stress is 45°, and the fractures would gradually turn to propagate along the maximum horizontal principal stress when they extend to a certain distance from the wellbore, leading to an increase of fracture tortuosity compared with the former. For carbonate rocks with well-developed natural fractures, fracturing with fresh water is conducive to connect natural fractures with low approaching angle and form stepped fractures with simple morphology. The key to forming complex fractures after fracturing carbonate rocks is to connect the natural fractures with high approaching angle. It is easier for liquid CO 2 with low viscosity to realize such connection. Multi-directional fractures with relatively complex morphology would be formed after fracturing with liquid CO 2 .

Rock mechanics and wellbore stability of deep shale during drilling and completion processes

The seepage coupling effect between the borehole rock mass and borehole fluid and the effective stress field distribution around the wellbore are obviously different under different working conditions of drilling, fracturing and completion, which affect the stability of horizontal borehole of shale gas reservoir. The physical, chemical, and mechanical properties of the shale in Longmaxi formations in a block of China immersed with different working fluids were tested experimentally. Combined with the experimental data, a theoretical model was established to evaluate and analyze the wellbore stability of Longmaxi formations shale under different working conditions of drilling, fracturing and completion. The results show that the shale of the Longmaxi Formation is a typical hard and brittle shale, the bedrock is dense, with high mechanical strength and weak hydration and expansion ability. The immersion effect of different working fluids has little influence on the expansion and mechanical properties of shale bedrock. However, the mechanical strength of shale with relatively developed bedding fractures is low, and the immersion effect of different drilling fluid further weakens the mechanical strength, resulting in obvious anisotropy of mechanical strength of underground rock. The coupling effect of seepage between wellbore and formation has obvious influence on the dynamic distribution of borehole pore pressure and wellbore stability under different working conditions. The effective fluid column pressure at the bottom of the hole is the highest during the fracturing process, which leads to the increase of pore pressure near the wellbore, and then to the increase of collapse pressure around the borehole wall. The equivalent density of collapse pressure rises to the maximum of 1.93 g/cm3, and the wellbore stability is the worst in the fracturing process. The wellbore stability is secondary in the drilling process, and the wellbore stability is good when drilling along the direction of the maximum horizontal principal stress, and the equivalent density of collapse pressure is 1.69 g/cm3. During the completion process, the formation fluid flows into the wellbore, the pore pressure in the borehole is relieved, and the equivalent density of the collapse pressure is reduced to approximately 1.35 g/cm3, the wellbore stability is the best and open hole completion can be attempted in the horizontal well section. The results provide a certain theoretical reference for the selection of the drilling and completion schemes in shale formation.

Quantitative evaluation indexes of the spatial steering effect of directional perforation hydraulic fractures

To better evaluate the spatial steering effect of directional perforation hydraulic fractures, evaluation indexes for the spatial steering effect are first proposed in this paper. Then, these indexes are used to quantitatively evaluate existing physical experimental results. Finally, with the help of RFPA2D-Flow software, the influence of perforation length and azimuth on the spatial steering process of hydraulic fracture are quantitatively analysed using four evaluation indexes. It is shown by the results that the spatial deflection trajectory, deflection distance, deflection angle and initiation pressure of hydraulic fractures can be used as quantitative evaluation indexes for the spatial steering effect of hydraulic fractures. The deflection paths of directional perforation hydraulic fractures are basically the same. They all gradually deflect to the maximum horizontal principal stress direction from the perforation hole and finally represent a double-wing bending fracture. The deflection distance, deflection angle and initiation pressure of hydraulic fractures increase gradually with increasing perforation azimuth, and the sensitivity of the deflection angle to the perforation azimuth of hydraulic fractures also increases. With increasing perforation length, the deflection distance of hydraulic fractures increases gradually. However, the deflection angle and initiation pressure decrease gradually, as does the sensitivity.