Horizontal Stress Difference Research Articles

Numerical Simulation of Artificial Fracture Propagation in Shale Gas Reservoirs Based on FPS-Cohesive Finite Element Method

The simulation of hydraulic fracturing by the conventional ABAQUS cohesive finite element method requires a preset fracture propagation path, which restricts its application to the hydraulic fracturing simulation of a naturally fractured reservoir under full coupling. Based on the further development of a cohesive finite element, a new dual-attribute element of pore fluid/stress element and cohesive element (PFS-Cohesive) method for a rock matrix is put forward to realize the simulation of an artificial fracture propagating along the arbitrary path. The effect of a single spontaneous fracture, two intersected natural fractures, and multiple intersected spontaneous fractures on the expansion of an artificial fracture is analyzed by this method. Numerical simulation results show that the in situ stress, approaching angle between the artificial fracture and natural fracture, and natural fracture cementation strength have a significant influence on the propagation morphology of the fracture. When two intersected natural fractures exist, the second one will inhibit the propagation of artificial fractures along the small angle of the first natural fractures. Under different in situ stress differences, the length as well as aperture of the hydraulic fracture in a rock matrix increases with the development of cementation superiority of natural fractures. And with the increasing of in situ horizontal stress differences, the length of the artificial fracture in a rock matrix decreases, while the aperture increases. The numerical simulation result of the influence of a single natural fracture on the propagation of an artificial fracture is in agreement with that of the experiment, which proves the accuracy of the PFS-Cohesive FEM for simulating hydraulic fracturing in shale gas reservoirs.

Research on Shallow Stress Variation before and after the 2011Japan Earthquake in Bohai Strait and Its Adjacent Area

To identify the geostress adjustment process of Japanese earthquake in Bohai Strait and its surrounding area, based on the data of 10 shallow hydraulic fracturing drills in the axis of the channel acquired from 2012 to 2014, the dispersed point charts and regression formulas of each parameter with depth were established and calculated. The results showed that between 2010 and 2014, the horizontal differential stress first decreased and then increased, and the direction of stress first rotated clockwise and then counter-clockwise. Therefore, coseismic and postseismic displacement triggered by Japanese earthquake caused shallow extensional effect which lasted from 2011 to 2012 in research area, and shallow stress didn’t return to the original level until 2013. During the period of shallow stress reduction, the fault zone of seismic hazard was reduced as the compressive strain energy was released, while during the period of sustained stability after stress recovery the NWW fault zone had the maximum seismic hazard than NNE. The results were of great significance for research on shallow stress accumulation caused by earthquake, fault activity and the construction of tunnel in Bohai Strait and its surrounding area.

The influence of deviatoric and horizontal differential stress and pore pressure on hydraulic fracture opening by fully coupled 3D cohesive elements method

Estimation of hydraulic fracture opening is one of the most difficult technical challenges in hydraulic fracturing treatment of vertical or horizontal oil wells since proppant transport as well as any possible premature screen out are considerably affected by hydraulic fracture opening (or width). In this study, the influences of deviatoric and horizontal differential stress as well as pore pressure regimes on hydraulic fracture opening in vertical and horizontal oil wells with normal fault regime are investigated. Novel three-dimensional cohesive elements with traction-separation law (XFEM-based cohesive law) are used for simulating the fracture initiation and propagation in a fluid-solid coupling finite element model. The stress intensity factor is verified for both XFEM-based cohesive law and analytical solution to show the validation of cohesive law in fracture modeling, where the compared results are in a very good agreement with less than 1% error. Moreover, the propagation of the hydraulic fracture by cohesive elements approach was also validated by KGD M-vertex solutions with negligible error about 1.6%. The results showed that, generally, by increasing deviatoric and horizontal differential stress, the fracture opening has been strongly affected for both vertical and horizontal oil wells. Additionally, increasing pore pressure from under-pressure regime to over-pressure state of formation pressure has made a considerable rise on fracture opening.

Study of hydraulic fracture growth behavior in heterogeneous tight sandstone formations using CT scanning and acoustic emission monitoring

Tortuous hydraulic fractures (HFs) are likely to be created in heterogeneous formations such as conglomerates, which may cause sand plugging, ultimately resulting in poor stimulation efficiency. This study aims to explore HF growth behavior in conglomerate through laboratory fracturing experiments under true tri-axial stresses combined with computed tomography scanning and acoustic emission (AE) monitoring. The effects of gravel size, horizontal differential stress, and AE focal mechanisms were examined. Especially, the injection pressure and the AE response features during HF initiation and propagation in conglomerate were analyzed. Simple HFs with narrow microfractures are created in conglomerate when the gravels are considerably smaller than the specimen, whereas complex fractures are created when the gravels are similar in size to the specimen, even under high horizontal differential stresses. Breakdown pressure and AE rates are high when a HF is initiated from the high-strength gravel. A large pressure decline after the breakdown may indicate the creation of a planar and wide HF. Analyzing the focal mechanism indicates that the shear mechanism generally dominates with an increase in the HF complexity. Tensile events are likely to occur during HF initiation and are located around the wellbore. Shear events occur mainly around the nonplanar and complex matrix/gravel interfaces.

Multi-fractured stimulation technique of hydraulic fracturing assisted by radial slim holes

The SRV fracturing is the core of commercial exploitation of shale gas, and the design philosophy of the SRV fracturing is gradually introduced into the stimulation of conventional low permeability reservoir and unconventional reservoir. However, in reservoirs with poorly developed natural fractures or high horizontal principal stress difference, it is difficult to achieve the multi-fracture or fracture network by conventional hydraulic fracturing, and effectively stimulating reservoir needs other assisting methods. This paper aims to present a method of guiding the multi-directional hydraulic fractures by multiple radial slim holes drilled in spatial positions, and creating multi-fractures and even complex fracture network. In this paper, a 3D hydraulic fracturing extended finite element numerical model in the vertical well assisted by planar multi-radial slim holes was established, and the effect of radial slim hole number, diameter, length, vertical density, azimuth and phase angle, Poisson's ratio, horizontal principal stress difference and rock Young's modulus, fracturing fluid viscosity and injection rate, reservoir permeability, etc. on guidance of radial slim holes were investigated. Then, we conducted the true tri-axial hydraulic fracturing test with the artificial core, and the feasibility of the technology was verified. The results show that decreasing azimuth and phase angle and increasing hole diameter, length and vertical density strengthen the guiding effect of planar multi-radial slim holes, and increasing Poisson's ratio, reservoir permeability, and fracturing fluid injection rate and viscosity enhance the guidance effect of radial slim holes. Increasing horizontal principal stress difference and reservoir Young's modulus weaken the radial slim holes guiding effect. The physical modeling experiment proves that controlling generation of complex multi-fractures is feasible by rationally arranging the spatial positions of radial slim holes. The spatial arrangement of four planar radial slim holes with phase angle of 90°, azimuth of 45° and vertical density not less than 2 holes/m in the vertical well is a scientific scheme. With the reasonable treatment scheme, creating 6 main fractures is possible. The research results provide theoretical support for multi-fractured stimulation assisted by planar multi-radial slim holes, and help the design of well completion parameters and fracturing treatment parameters of the technology.

Settlement Prediction and Differential Settlement Criterion for Heightening and Thickening Levee

Owing to the pavement cracking of heightening and thickening levees (HTL) caused by differential settlement, it is necessary to study the settlement prediction and differential settlement criterion for HTLs. In this study, an HTL was taken as the study object. First, leveling observation and layered settlement observation were adopted, and a generalized settlement prediction model (GSPM) that can describe both S-shaped and hyperbolic settlement–time curves was proposed. The physical meanings of GSPM parameters were analyzed. Moreover, the effectiveness of the GSPM was verified using field monitoring data. Second, finite element analysis based on a viscoelastic constitutive model was used to reveal the quantitative relationship between the differential settlement and horizontal stress of the pavement. On this basis, the differential settlement criterion was obtained. The results showed that the GSPM can unify the logistic model and hyperbolic model by introducing two morphological change parameters. In addition, the GSPM can be more widely used, more accurate, and more stable than the traditional logistic model and hyperbolic model. When an asphalt concrete pavement (whose ultimate tensile stress is 0.12 MPa) is used, the corresponding differential settlement criterion is approximately 4.3 cm. The research results can provide a reference for reasonable differential settlement control for HTLs in order to prevent pavement cracking.

Earth stress and reservoir quality evaluation in high and steep structure: The Lower Cretaceous in the Kuqa Depression, Tarim Basin, China

Knowledge of the state of the in-situ stress field provides important insights into the petroleum engineering and geology. The in-situ stress orientation and magnitudes in the high and steep structure of Lower Cretaceous Bashijiqike Formation in Kuqa Depression were analyzed based on well log calculations, and the impacts of in-situ stress field on reservoir quality (macroscopic reservoir properties and microscopic pore structure as well as fracture aperture) were investigated. The maximum horizontal stress (SHmax) orientation was determined as nearly north-south according to the image log derived drilling induced fractures and borehole breakouts. The vertical stress (Sv), SHmax, and minimum horizontal stress (Shmin) magnitudes were calculated via well logs. The result reveals a dominant strike-slip stress regime (SHmax > Sv > Shmin) in the Bashijiqike Formation in the Kuqa Depression. The Sv at the depth ranging from 5000 to 7100 m is 150–180 MPa, and horizontal stresses (SHmax and Shmin) vary significantly from 130 to 210 MPa. The horizontal stress difference (Δσ) is in the range from 40 to 70 MPa, and fluid overpressures are commonly encountered. The effective stress (σe) has a positive relationship with the Δσ when the Δσ is less than 50 MPa. Intergranular pores can be preserved in layers with low Δσ values. The microscopic pore structure will be complex, and macroscopic reservoir quality will be poor with the increasing Δσ values. In addition, fractures are mainly corresponding with the layers where the Δσ value is low, and the fracture aperture and porosity decreased significantly with the effective stress σe. By determining the magnitude and orientation of in-situ stresses around the wellbore, insights can be provided into the wellbore instability, optimum drilling trajectories and reservoir quality evaluation.

Study on the mechanisms of refracturing technology featuring temporary plug for fracturing fluid diversion in tight sandstone reservoirs

AbstractWell production rates in unconventional plays usually decline dramatically in the first year. Refracturing, which is a remedial production operation, is often done because original hydraulic fracturing failed to contribute any significant amount of flow or significant unfractured pay exists in the well. In order to maximize the fracturing fluid contact with the intact rock and to stimulate more reservoir volume in previously stimulated wells, a refracturing technology featuring a novel temporary plugging for fluid diversion is developed to enable the fracturing fluid to reach the untouched areas and to create reoriented fractures. In this paper, laboratory physical simulation tests of refracturing using fiber for effective temporary plugging is carried out to study the refracture morphology and the influencing factors of refractures. Results show that the refracture morphology is affected by the horizontal stress difference, the injection rate of initial fracturing fluid, and the natural fractures. Under condition of the different horizontal stress differences, the fracture initiation and orientation angle are different. When the horizontal stress difference is small, it is easy to form large angle fractures. The injection rate of initial fracturing fluid affects the length of initial fractures and refractures. The smaller the initial fracturing fluid injection rate is, the better the effect of temporary plugging in refracturing. The presence of natural fractures will lead to reorientation of refractures to form a complex fracture network. This study provides a theoretical guidance and technology support for refracturing operations.

A Numerical Study on the Diversion Mechanisms of Fracture Networks in Tight Reservoirs with Frictional Natural Fractures

An opened natural fracture (NF) intercepted by a pressurized hydro-fracture (HF) will be diverted in a new direction at the tips of the original NF and subsequently form a complex fracture network. However, a clear understanding of the diversion behavior of fracture networks in tight reservoirs with frictional NFs is lacking. By means of the extended finite element method(XFEM), this study investigates the diversion mechanisms of an opened NF intersected by an HF in naturally fractured reservoirs. The factors affecting the diversion behavior are intensively analyzed, such as the location of the NF, the horizontal principal stress difference, the intersection angle between HF and NF, and the viscosity of the fracturing fluid. The results show that for a constant length of NF (7 m): (1) the upper length of the diverted fracture (DF) decreases by about 2 m with a 2 m increment of the upper length of NF ( L u p p e r ), while the length of DF increases 9.06 m with the fluid viscosity increased by 99 mPa · s; (2) the deflection angle in the upper parts increases by 30.8° with the stress difference increased by 5 MPa, while the deflection angle increases by 61.2° with the intersection angle decreased by 30°. It is easier for the opened NF in lower parts than that in upper parts to be diverted away from its original direction. It finally diverts back to the preferred fracture plane (PFP) direction. The diversion mechanisms of the fracture network are the results of the combined action of all factors. This will provide new insight into the mechanisms of fracture network generation in tight reservoirs with NFs.

Hydraulic fracturing in ice boreholes: Theory and tests

Hydraulic fracturing is the well-known phenomenon that can occur intentionally, as it is used in the petroleum industry to stimulate the flow of natural gas or oil, or naturally, for example, as a reason for calving of ice shelves. For many years, occurrence of hydro-fracturing in ice boreholes was much disputed as there was no clear evidence that it exists. This paper proposes a method of estimating the over-pressure caused by hydraulic fracturing in ice allowing to identify a critical balance between hydraulic fracture pressure and overburden ice pressure at the certain depth of ice borehole. A series of experiments to study hydraulic fracturing behaviour using a set of artificial ice samples and a triaxial fracturing testing system were performed. The tests results were in good agreement with theory with the variation of just 0.9–8.7%. Results from the fracture experiments confirm that in real ice borehole, hydraulic fracturing phenomenon is possible if the horizontal differential stress is large within a fissured ice borehole wall. For fracture morphology, vertical fractures formed and extended nearly perpendicular to the minimum horizontal principal stress direction. The proposed estimation method allows drillers to adjust drilling fluid density for reasons of safety and environmental security.

Numerical and Experimental Investigations of the Interactions between Hydraulic and Natural Fractures in Shale Formations

Natural fractures (NFs) have been recognized as the dominant factors that increase hydraulic fracture complexity and reservoir productivity. However, the interactions between hydraulic and natural fractures are far from being fully understood. In this study, a two-dimensional numerical model based on the displacement discontinuity method (DDM) has been developed and used to investigate the interaction between hydraulic and pre-existing natural fractures. The inelastic deformation, e.g., stick, slip and separation, of the geologic discontinuities is captured by a special friction joint element called Mohr-Coulomb joint element. The dynamic stress transfer mechanisms between the two fracture systems and the possible location of secondary tensile fracture that reinitiates along the opposite sides of the NF are discussed. Furthermore, the model results are validated by a series of large tri-axial hydraulic fracture (HF) tests. Both experimental and numerical results showed that the displacements and stresses along the NFs are all in highly dynamic changes. When the HF is approaching the NF, the HF tip can exert remote compressional and shear stresses on the NF interface, which results in the debonding of the NF. The location and value of the evoked stress is a function of the far-field horizontal differential stress, inclination angle of the NF, and the net pressure used in fracturing. For a small approaching angle, the stress peak is located farther away from the intersection point, so an offset fracture is more likely to be generated. The cemented strength of the NF also has an important influence on the interaction mechanism. Weakly bonded NF surfaces increase the occurrence of a shear slippage, but for a moderate strength NF, the hybrid failure model with both tensile and shear failures, and conversion may appear.

Method for Fracture Network Calculations in Multi-Staged Horizontal Drilling in Shale Reservoirs

In this paper, based on the principle of superposition of acting forces, we propose a mathematical model for injected fluid-induced stress and calculate the stress distribution and SRV width vs. the stress difference in the formation. The results show that the nature of the propagation of the fracture network in multi-staged hydraulic fracturing depends on the stress in the initial cycle. The larger the horizontal stress difference, the larger the width of the fracture network in the rock. If the stress difference is greater than 5 MPa, the width of the fracture network decreases dramatically. When the net pressure is greater than 8 MPa, the width slowly increases. The calculation results are in good agreement with microseismic monitoring data for conditions in the Southwest shale field (China), suggesting that the proposed model is sufficiently reliable and accurate.

Macro and Meso Characteristics of In-Situ Oil Shale Pyrolysis Using Superheated Steam

The efficiency of oil shale pyrolysis is directly related to the feasibility of in-situ mining technology. Taiyuan University of Technology (China) proposed the technology of in-situ convective heating of oil shale, which uses superheated steam as the heat carrier to heat the oil shale’s ore-body and transport the pyrolysis products. Based on the simulated experiments of in-situ oil shale pyrolysis using superheated steam, the changes in fracture characteristics, pyrolysis characteristics and mesoscopic characteristics of the oil shale during the pyrolysis have been systematically studied in this work. The Xinjiang oil shale’s pyrolysis temperature ranged within 400–510 °C. When the temperature is 447 °C, the rate of pyrolysis of kerogen is the fastest. During the pyrolysis process, the pressure of superheated steam changes within the range of 0.1–11.1 MPa. With the continuous thermal decomposition, the horizontal stress difference shows a tendency to first increase and then, decrease. The rate of weight loss of oil shale residue at various locations after the pyrolysis is found to be within the range of 0.17–2.31%, which is much lower than the original value of 10.8%, indicating that the pyrolysis is more adequate. Finally, the number of microcracks (<50 µm) in the oil shale after pyrolysis is found to be lie within the range of 25–56 and the average length lies within the range of 53.9636–62.3816 µm. The connectivity of the internal pore groups is satisfactory, while the seepage channel is found to be smooth. These results fully reflect the high efficiency and feasibility of in-situ oil shale pyrolysis using superheated steam.

Effects of previously created fracture on the initiation and growth of subsequent fracture during TPMSF

Temporary plugging multi-cluster staged fracturing (TPMSF), using multi-cluster perforations and self-degradable diverting agents, is a novel technique to develop oil and gas reservoirs in deep wells. In this paper, the effects of previously created fractures on the initiation and growth of subsequent fracture were numerically investigated by utilizing XFEM-based CZM (i.e., cohesive zone model based on the extended finite element method). A 2D plane-plain model (coupling fluid flow and solid deformation) was established and truss model (truss elements distributed on the previous fracture surfaces) was used to maintain the different width of previously created fracture. Based on the model, multiple fracture simultaneous propagation was firstly simulated and then parametric studies were conducted for different fracture spacing, the propped width of the previous fracture, fracture created sequence, horizontal differential stress, Young’s modulus, and rock tensile strength. Numerical results show that multiple fractures cannot propagate uniformly because of fracture interaction even with equal flow rate distributions. During TPMSF, the propped width of the previous fracture, fracture spacing, and Young’s modulus primarily control the injection pressure of subsequent fractures; while rock tensile strength has a relatively weaker influence and horizontal stress contrast plays a negligible role. Moreover, the growth of subsequent fractures is dominated by fluid pressure, initial horizontal stresses and induced stresses.

Experimental investigation of propagation mechanisms and fracture morphology for coalbed methane reservoirs

Fracture propagation mechanisms in coalbed methane (CBM) reservoirs are very complex due to the development of the internal cleat system. In this paper, the characteristics of initiation and propagation of hydraulic fractures in coal specimens at different angles between the face cleat and the maximum horizontal principal stress were investigated with hydraulic fracturing tests. The results indicate that the interactions between the hydraulic fractures and the cleat system have a major effect on fracture networks. “Step-like” fractures were formed in most experiments due to the existence of discontinuous butt cleats. The hydraulic fractures were more likely to divert or propagate along the butt cleat with an increase in the angles and a decrease in the horizontal principal stress difference. An increase in the injection rate and a decrease in the fracturing fluid viscosity were more conducive to fracture networks. In addition, the influence on fracture propagation of the residual coal fines in the wellbore was also studied. The existence of coal fines was an obstacle in fracturing, and no effective connection can be formed between fractures. The experimental investigation revealed the fracture propagation mechanisms and can provide guidance for hydraulic fracturing design of CBM reservoirs.

Characteristics of fracture propagation in compact limestone formation by hydraulic fracturing in central Sichuan, China

Compact limestone formations are significant and serve as enrichment petroleum sources in the central Sichuan basin. However, they are difficult to stimulate by hydraulic fracturing owing to their low porosity, low permeability, and high density. The fracture geometry is neither a bi-wing fracture found in a homogeneous sandstone formation nor a complex fracture network occurring in a shale formation with well-developed discontinuities and high brittleness. In this study, a series of laboratory stimulated experiments were conducted to investigate the fracture initiation and propagation behavior in compact limestone formation. Concurrently, the effects of the horizontal stress difference, pump rate, fracturing fluid viscosity, and completion method were also studied. The results revealed that a complex fracture network was formed with difficultly in a compact limestone formation. The fractures initiated from open-hole section or perforations to propagate in four forms, resulting in three categories of the main fracture geometry: transverse fracture, longitudinal fracture, and complex fracture. Complex fracture formations occurred under a small horizontal stress difference, whereas a simple fracture was induced by a large horizontal stress difference, similar to a transverse or longitudinal fracture. Moreover, the cement strength, development degree, and distribution position of the discontinuities had a significant effect on the fracture geometry. A low fluid viscosity or low pump rate was demonstrated to be an effective method for activating and opening the discontinuities, forming complex fractures under a small horizontal stress difference rather than under a large horizontal stress difference. However, the variable pump rate fracturing with a low-viscosity fluid could promote the interaction of the induced fracture and discontinuities, leading to a large stimulated reservoir volume under the large horizontal stress difference. In addition, perforated fracturing generated a more complex fracture at a higher fracture pressure than an open-hole fracturing. Furthermore, an acid fluid should be injected into limestone formation before the fracturing operation to reduce the rock strength, resulting in a low fracture pressure of limestone formation.

Analysis of hydraulic fracture initiation and propagation in deep shale formation with high horizontal stress difference

Deeply buried shale formations (vertical depth > 3500 m) that are rich in shale gas are abundant in south China. The primary problems in the exploitation of these formations are the relatively small stimulated reservoir volume (SRV) and low production rates in comparison with their shallower counterparts. These issues are attributed to the high fracturing pressure, limited fracture extension because of sand plugging, significant horizontal stress contrast, and discontinuities produced during hydraulic fracturing. To accurately evaluate and improve the SRV in deep shale formations, the mechanism of fracture propagation must be understood and described. In this regard, a series of large-scale true tri-axial experiments with acoustic emission (AE) monitoring were conducted to characterize the fracture initiation and propagation in a selected deep shale formation. It was found that the difficulty in the complex fracture network formation was because the high stress contrast controls the fracture propagation path to generate large main fractures instead of activating discontinuities. The hydraulic fractures initiated either from open-hole positions or stress concentration locations on a wellbore wall that displayed two types of intersection in terms of crossing and deflection. In general, four types of fracture morphologies were identified: transverse fracture, transverse fracture with bedding planes, natural fracture with bedding planes, and transverse fracture with bedding planes and natural fractures. In addition, the horizontal stress contrast, fluid viscosity, pump rate and fracturing procedure on fracture propagation were evaluated for their effect on the resulting SRV. A low-viscosity fluid can activate discontinuities to form a complex fracture network, whereas a high-viscosity fluid is likely to produce large fractures under a high contrast in the horizontal stresses. On the basis of the above-mentioned analyses, a fracturing procedure applying a periodically varying pump rate and shut-in using a low-viscosity fluid was suggested to be an effective approach to enhance the interaction between the hydraulic fractures and discontinuities as well as to increase the fracture length.

Multi-well synchronous hydraulic conformance fracturing technology used for deep coal beds and its field application in the Southern Qinshui Basin

For deep CBM productivity improvement, a technical concept of conducting multi-well synchronous hydraulic conformance fracturing was proposed based on its geological characteristics. First, a mathematical model for multi-fracture induced stress was established by using the boundary element displacement discontinuity method, to simulate the distribution of induced stress field in deep coal beds and analyze the possibility of the formation of complex fracture networks induced by the hydraulic conformance fracturing. Then, the propagation situation of fracture networks interfered by stress and its influencing factors were studied by using the discrete element method. And finally, the feasibility of synchronous hydraulic conformance fracturing technology was verified through tri-axial fracturing experiment and field application. It is shown that by virtue of synchronous hydraulic conformance fracturing technology, stress interference area and strength are increased, so horizontal major stress difference is decreased and even the direction of earth stress is changed regionally, which is conducive to the connection of developed face cleats and butt cleats in coal rocks, so as to form large, efficient and complex fracture networks. Furthermore, the favorable conditions for the formation of complex fracture networks by hydraulic conformance fracturing include lower initial horizontal major stress difference, low Poisson's ratio, short well spacing and low fracturing fluid viscosity and high net pressure inside the fractures. Finally, it is shown from the 3D true physical simulation experiments that by virtue of this synchronous hydraulic conformance fracturing technology, natural fractures in coal rocks can be connected sufficiently, and consequently complex fracture networks composed of hydraulic fractures, face cleats and butt cleats are created. Based on these research results, a set of optimization design method for the synchronous hydraulic conformance fracturing of deep coal beds was proposed. Five vertical wells located in the deep coal beds of North Shizhuang Block in the Southern Qinshui Basin were chosen for the pilot test. It is indicated from fracture monitoring and drainage/production data that the stimulated reservoir volume (SRV) of synchronous hydraulic fractured wells is large and its fracture network is complex; and that compared with the conventionally fractured wells, the synchronous hydraulic fractured well is earlier in gas breakthrough, and higher and more stable in production rates and casing pressure and its regional pressure drop even spreads to the adjacent wells, so that their production rates are remarkably raised.

Investigation on the hydraulic fracture propagation of multilayers-commingled fracturing in coal measures

Multilayers-commingled fracturing is the vital step to achieve gas co-exploration in coal measures. Hydraulic fracture is required to penetrate vertically through multilayers in Multilayers-commingled fracturing. However, investigation on the propagation of commingled crack in coal measures which is soft-hard interlaced strata is less studied and not well understood. Therefore, based on the rock seepage-stress coupling effect, a three-dimensional (3D) hydraulic fracture model of multilayers-commingled fracturing in coal strata was established in combination with ABAQUS finite element software. The influence of geological factors and construction factors on the hydraulic cracks expansion in sandstone-coal interbedded reservoirs of X well in Linxing area in China was investigated. We found that the minimum horizontal stress difference is an essential geological factor to contain the vertical extension of commingled fractures; The high elastic modulus of neighboring reservoirs promotes the longitudinal penetration of fractures in the multilayers; The low tensile strength contrast between multiple layers increases the possibility of vertical expansion of combined fractures, while the high permeability of adjacent layers restrains the longitudinal propagation of fractures. The critical displacement of fracturing fluid exists in the combined fracturing process. The commingled cracks breakthrough the stratigraphic interface and penetrate into the adjoining layers when reaching the critical displacement; The high-viscosity of fracturing fluid facilitates the extending behaviors of hydraulic cracks in the vertical direction. The research results provide theoretical guidance for optimization of commingled fracturing layers and design of construction parameters in coal strata.

Study on Fracture Initiation Mechanisms of Hydraulic Refracturing Guided by Directional Boreholes

In order to generate a new fracture far away from the original fracture in refracturing and effectively enhancing productivity, the technology of hydraulic refracturing guided by directional boreholes was presented. The effects of induced stress generated by the original hydraulic fracture, fracturing fluid percolation effect, wellbore internal pressure, and in situ stress on stress field distribution around wellbore were considered to obtain a fracture initiation model of hydraulic refracturing guided by two directional boreholes. The variation of maximum principal stress (σmax) under different conditions was investigated. The researches show that the directional boreholes result in a “sudden change region” of maximum principal stress around wellbore, reflecting dual stresses effects from vertical wellbore and directional boreholes on the rock. The width of sudden change region decreases as the distance from wellbore increases. Due to sudden change region, the refracturing fracture tends to initiate around directional boreholes. Whether the new fracture initiates and propagates along directional boreholes depends on comprehensive effect of borehole azimuth, borehole diameter, borehole spacing, horizontal stress difference, height, and net pressure of original fracture. The specific initiation position can be calculated using the theoretical model proposed in this paper. Affected by induced stress of the original fracturing, the rock tends to be compressed during refracturing, i.e., increased fracturing pressure. Sensitivity analysis with “extended Fourier amplitude sensitivity test (EFAST)” method shows the initiation of new fracture is mainly controlled by directional boreholes parameters and has little relation with in situ stress and parameters of original fracture. The influence rank of each parameter is as follows: borehole diameter > borehole spacing > original fracture net stress > borehole azimuth > horizontal stress difference > original fracture height. During design of refracturing, in order to better play the role of directional boreholes, and create a new fracture far away from original fracture, the optimal design is conducted with measures of optimizing boreholes azimuth, increasing borehole diameter and reducing borehole spacing if conditions permit. The research provides the theoretical basis for hydraulic refracturing guided by directional boreholes, which is helpful for the design of fracturing construction programs.