Horizontal Principal Stress Difference Research Articles

Discrete element-based multi-cluster hydraulic fracture extension study with temporary plugging and fracturing in horizontal wells

In this paper, a two dimensional-discrete element method complex fracture extension model was developed to simulate the fracture extension behavior in reservoirs during multi-cluster perforation fracturing of horizontal wells by particulate flow and considering temporary plugging conditions. The effect of particle inhomogeneity on the extension of multi-cluster fractures, the change of fracture extending characteristics before and after temporary plugging, and the effect of geological and construction parameters on the extension of multi-cluster fractures under the consideration of temporary plugging conditions are investigated in this model. The results show that: (1) the middle cluster is more difficult to extend than the outermost cluster due to interstitial stress interference; the increased nonuniformity of particles aggravates interstitial stress interference and favors shear fracture extension. (2) Fractures expand substantially when the fracturing fluid reaches 3/4 of the total fluid volume; after temporary plugging, it can promote the uniform extension of fractures and increase the total length of fractures, but the fractures formed in the middle are easy to be captured by the fractures at the two ends and expand along it, and it is not easy to form complex fractures; the timing of plugging is based on the percentage of perforation clusters with fractures already formed, with fracture extension being preferred. (3) The horizontal principal stress difference mainly affects the direction of fracture extension; the number of clusters mainly affects the uniformity of fracture extension; and the fluid flow mainly affects the length of fracture extension.

Experimental study of hydraulic fracture propagation with multi-cluster in-plane perforations in a horizontal well

Tri-axial fracturing studies were carried out to understand the impact of lateral mechanical parameters on fracture propagation from multiple in-plane perforations in horizontal wells. Additionally, the discussion covered the effects of geology, treatment, and perforation characteristics on the non-planar propagation behavior. According to experimental findings, two parallel transverse fractures can be successfully initiated from in-plane perforation clusters in the horizontal well because of the in-plane perforation, the guide nonuniform fishbone structure fracture propagation still can be exhibited. The emergence of transverse fractures and axial fractures combined as complex fractures under low horizontal principal stress difference and large pump rate conditions. The injection pressure was also investigated, and the largest breakdown pressure can be also found for samples under these conditions. The increase in perforation number or decrease in the cluster spacing could provide more chances to increase the complexity of the target stimulated zone, thus affecting the pressure fluctuation. In a contrast, the increase in fracturing fluid viscosity can reduce the multiple fracture complexity. The fracture propagation is significantly affected by the change in the rock mechanical properties. The fracture geometry in the high brittle zone seems to be complicated and tends to induce fracture reorientation from the weak-brittle zone. The stress shadow effect can be used to explain the fracture attraction, branch, connection, and repulsion in the multiple perforation clusters for the horizontal well. The increase in the rock heterogeneity can enhance the stress shadow effect, resulting in more complex fracture geometry. In addition, the variable density perforation and temporary plugging fracturing were also conducted, demonstrating higher likelihood for non-uniform multiple fracture propagation. Thus, to increase the perforation efficiency along the horizontal well, it is necessary to consider the lateral fracability of the horizontal well on target formation.

Fracture propagation and evolution law of indirect fracturing in the roof of broken soft coal seams

Indirect fracturing in the roof of broken soft coal seams has been demonstrated to be a feasible technology. In this work, the No. 5 coal seam in the Hancheng block was taken as the research object. Based on the findings of true triaxial hydraulic fracturing experiments and field pilot under this technology and the cohesive element method, a 3D numerical model of indirect fracturing in the roof of broken soft coal seams was established, the fracture morphology propagation and evolution law under different conditions was investigated, and analysis of main controlling factors of fracture parameters was conducted with the combination weight method, which was based on grey incidence, analytic hierarchy process and entropy weight method. The results show that “士”-shaped fractures, T-shaped fractures, cross fractures, H-shaped fractures, and “干”-shaped fractures dominated by horizontal fractures were formed. Different parameter combinations can form different fracture morphologies. When the coal seam permeability is lower and the minimum horizontal principal stress difference between layers and fracturing fluid injection rate are both larger, it tends to form “士”-shaped fractures. When the coal seam permeability and minimum horizontal principal stress between layers and perforation position are moderate, cross fractures are easily generated. Different fracture parameters have different main controlling factors. Engineering factors of perforation location, fracturing fluid injection rate and viscosity are the dominant factors of hydraulic fracture shape parameters. This study can provide a reference for the design of indirect fracturing in the roof of broken soft coal seams.

The present-day in-situ stress field and its effect on shale gas development in Zigong area of southern Sichuan Basin

The Zigong area in southern Sichuan Basin is one of the key shale gas production areas in China, and the Longmaxi and Wufeng Formations within it are currently the key exploration and development layers. The present-day in-situ stress field has a significant impact on well trajectory deployment, horizontal well construction, hydraulic fracturing and other aspects. However, it has not been finely quantified in shale gas reservoirs of Zigong area, making it difficult to effectively guide development practice. Therefore, this study constructed a geomechanical model of the target layer in the Zigong area, quantitatively characterized the distribution of present-day in-situ stress, and explored its shale gas development effects. The results show that: (1) Based on the analysis of drilling induced fractures obtained from rock acoustic emission experiments and imaging logging interpretation, the dominant orientation of the maximum horizontal principal stress in the target layer is mainly NW-SE-trending; (2) The horizontal principal stress difference in the Layer S1l1-1−1 and Wufeng Formation is 5–30 MPa (the majority is 6–15 MPa) and 5–35 MPa (the majority is 6–18 MPa), respectively. Low stress difference values are mainly concentrated in the southeastern and northern parts. The overall prediction accuracy is high, with an error rate of less than 8% for the Layer S1l1-1−1 and less than 10% for the Wufeng Formation; (3) Based on rock acoustic emission experiments and finite element model simulation results, the study area is mainly under strike-slip stress faulting mechanism; (4) Due to the influence of the strike-slip faulting stress mechanism, when deploying horizontal wells in the area, priority should be given to selecting areas with low stress mechanism factor (Aφ) and low stress differences, to achieve better fracturing and transformation effects and reduce the risk of casing deformation and wellbore instability.

The non-plane initiation and propagation mechanism of multiple hydraulic fractures in tight reservoirs considering stress shadow effects

Multi-cluster fracturing in horizontal wells is an important stimulation technology for unconventional tight reservoirs. The interference among multiple simultaneously growing hydraulic fractures results in a non-uniform fluid distribution into fractures and non-planar fracture geometries. In this paper, the competition mechanism between perforations under the stress shadow, the formation mechanism of complex non-plane fractures near the wellbore, and the initiation and propagation mechanism of multiple hydraulic fractures are investigated by using the 3D lattice method. Also, the effects of fracture geometry, fracture spacing and rock mechanics parameters on the stress interaction among fractures are analyzed by the dislocation theory. The results showed the complexity of fracture initiation near the wellbore and the low efficiency of the perforation initiation in the case of spiral perforation, due to a strong interaction between perforations. However, these initial micro-cracks will eventually coalesce to be a macroscopical fracture which perpendicular to the direction of minimum principal stress. The study indicated that stress interaction and fracture propagation is significantly affected by inclined well orientation. The hydraulic fracture is more likely to deflect and propagate non-uniformly with the increasing width of hydraulic fracture, the decreasing spacing between perforation clusters and the increasing rock modulus. When the horizontal principal stress difference is small, it is obvious to cause a large deflection angle and sometimes furcation of fractures. This study can provide a theoretical basis and technical guidance for hydraulic fracturing design in tight reservoirs.

Optimization of integrated geological-engineering design of volume fracturing with fan-shaped well pattern

According to the variable toe-to-heel well spacing, combined with the dislocation theory, discrete lattice method, and finite-element-method (FEM) based fluid-solid coupling, an integrated geological-engineering method of volume fracturing for fan-shaped well pattern is proposed considering the geomechanical modeling, induced stress calculation, hydraulic fracturing simulation, and post-frac productivity evaluation. Besides, we propose the differential fracturing design for the conventional productivity-area and the potential production area for fan-shaped horizontal wells. After the fracturing of the conventional production area for H1 fan-shaped well platform, the research shows that the maximum reduction of the horizontal principal stress difference in the potential productivity-area is 0.2 MPa, which cannot cause the stress reversal, but this reduction is still conducive to the lateral propagation of hydraulic fractures. According to the optimized fracturing design, in zone-I of the potential production area, only Well 2 is fractured, with a cluster spacing of 30 m and an injection rate of 12 m3/min per stage; in zone-II, Well 2 is fractured before Well 3, with a cluster spacing of 30 m and an injection rate of 12 m3/min per stage. The swept area of the pore pressure drop in the potential production area is small, showing that the reservoir is not well developed. The hydraulic fracturing in the toe area can be improved by, for example, properly densifying the fractures and adjusting the fracture distribution, in order to enhance the swept volume and increase the reservoir utilization.

Fracability evaluation of shale reservoirs considering rock brittleness, fracture toughness, and hydraulic fracturing-induced effects

Fracability is a vital evaluation index for shale reservoir exploration and development and horizontal well fracturing. In this paper, we define the fracability of a shale reservoir as the ability of hydraulic fracturing to generate a complicated large-scale fracture network and large stimulated volume under the same stimulation parameters. Considering factors such as the shale sedimentary environment and confining pressure, we establish a brittleness index calculation method by coupling logging data and in situ rock mechanics test results to obtain parameters such as the reservoir Young's modulus and Poisson's ratio. According to the fractal method, an in situ rock bifurcation/irregular fracture toughness calculation method is established. The hydraulic fracture-induced index is introduced to reflect the influence of stimulation-induced displacement, reservoir seepage, fluid viscosity, horizontal principal stress difference and other parameters on fracability. A fracability evaluation method coupling the reservoir rock brittleness, fracture toughness and hydraulic fracture-induced index is formed. The results of the field application show that the established method can effectively characterize the ability of a reservoir to form complex fracture networks and promote the stimulation effect. This study provides more credible guidance for geological and engineering practices for developing shale reservoirs.

Numerical Simulation on Radial Well Deflagration Fracturing Based on Phase Field Method

A radial well has a unique wellbore configuration. Fracture propagation in radial well deflagration fracturing is studied rarely. The mechanism of interaction between deflagration fractures, natural fractures, and micro-fractures is still unknown. Based on continuum mechanics, damage mechanics, and variational principles, a numerical model of fracture propagation in deflagration fracturing is established with the Hamilton principle and phase-field fracture theory. The effects of horizontal principal stress difference, natural fracture distribution, and micro-fractures around the wellbore on fracture propagation in deflagration fracturing are studied. First, when no natural fractures are developed around the radial well, fractures are initiated at both ends of the radial well. Second, when there are three natural fractures around the radial well, the created fractures have the morphology of shorter fractures in the middle and longer fractures on both sides under stress interference mechanisms. Third, a larger density of natural fractures causes obvious stress superposition, changes the initiation points of radial wells and fracture morphology, and increases fracture width and reservoir stimulation volume. Fourth, as the micro-fractures increase, their interference and induction effects on deflagration fractures are enhanced gradually, and the deflection angle of fractures increases by 38.7%. The study provides a reference for optimizing deflagration fracturing in a radial well.

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.

An unequal fracturing stage spacing optimization model for hydraulic fracturing that considers cementing interface integrity

Determining reasonable fracturing stage spacing is the key to horizontal well fracturing. Different from traditional stage spacing optimization methods based on the principle of maximum stimulated reservoir volume, in this paper, by considering the integrity of the wellbore interface, a fracture propagation model was established based on displacement discontinuity method and the competition mechanism of multifracture joint expansion, leading to the proposal of an unequal stage spacing optimization model. The results show that in the first stage, the interfacial fractures spread symmetrically along the axis of the central point during that stage, while in the second and subsequent stages, the interfacial fractures of each cluster extend asymmetrically along the left and right sides. There are two kinds of interface connectivity behaviour: in one, the existing fractures first extend and connect within the stage, and in the other, the fractures first extend in the direction close to the previous stage, with the specific behaviour depending on the combined effect of stress shadow and flow competition during hydraulic fracture expansion. The stage spacing is positively correlated with the number of fractures and Young’s modulus of the cement and formation and is negatively correlated with the cluster spacing and horizontal principal stress difference. The sensitivity is the strongest when the Young’s modulus of the cement sheath is 10–20 GPa, and the sensitivity of the horizontal principal stress difference is the weakest.

Study on the Law of Hydraulic Fracture Propagation in Low Permeability Thin Interbed Reservoir

The third member of Weixinnanliu in the west of the South China Sea develops thin interbeds, and the vertical extension of fracturing fractures is excessive. Once the fractures extend vertically to the upper and lower aquifers, it is easy to cause water flooding of oil wells, and the effect after fracturing is not obvious. The present work aims to explore the longitudinal extension law of fractures in Low Permeability Thin Interbed Reservoir based on the finite element calculation platform. A three-dimensional expansion model of hydraulic fractures in the target reservoir was established, and the displacement, fracturing fluid viscosity, minimum horizontal principal stress difference, vertical stress, interlayer thickness, perforation point separation were studied. The interlayer distance and other factors affect the crack propagation law. The research results show that the thin interbed fractures have three forms: T-shaped fractures, through-layer fractures, and I-shaped fractures; for the target layer, the overlying stress is relatively large, and the minimum principal stress is along the horizontal direction. Vertical cracks; the farther the perforation point is or the greater the stress difference, the smaller the thickness of the interlayer required to control the fracture height; the stress difference is 3 MPa, and the distance between the perforation points exceeds 10 m, the thickness of the interlayer is required to be ≥4 m; In order to ensure that the width of the fracture in the middle spacer does not affect the placement of the proppant, it is recommended that the displacement be controlled within 3 m3/min and the viscosity of the fracturing fluid is 150 mPa·s; in addition, the thickness of the spacer required to control the fracture height is different due to different geological parameters. Different, different wells need targeted analysis.

Study on H-shaped fracture propagation and optimization in shallow dull-type coal seams in the Hancheng block, China

The initiation and propagation of hydraulic fractures dominate the fracture morphology and conductivity, thereby determining the fracturing stimulation effect. This work took the shallow dull-type coal seam in the Hancheng block as the research object, and focused on the propagation and optimization of the H-shaped fracture. By using a combination of mechanical theoretical analysis, laboratory experiment, numerical simulation, field test, and optimization evaluation, an integrated study on the formation conditions, morphology evolution, propagation laws, main controlling factors, and optimal design of H-shaped fractures was conducted. The mechanical criterion for identifying H-shaped fracture propagation was derived through the analysis of the stress fields on the fracture surface. A 3D numerical model of H-shaped fractures was established with cohesive element method, and the propagation law and influencing factors of H-shaped fractures were investigated. Sensitivity analysis of the influencing factors on the H-shaped fractures was performed with grey incidence analysis method. The optimization of the H-shaped fracture parameters was performed by fracture propagation numerical simulation. The results show that H-shaped fractures dominated by horizontal fractures are generally formed in shallow dull-type coal seams. H-shaped fractures can improve the CBM development effect by parameter optimization of indirect fracturing technology. The judging criterion for H-shaped fracture formation is that the tensile strength of the upper and lower weakly cemented horizontal interfaces is less than that of the coal seam and the vertical principal stress is lower than the minimal horizontal principal stress. The incidence degree of fracturing fluid injection rate, viscosity, perforation location, minimum horizontal stress difference, and coal seam permeability on the H-shaped fracture propagation decreases in turn. The engineering factors (fracturing fluid injection rate and viscosity and perforation location) are the major impact factors. With the increase in coal seam permeability, the horizontal fracture develops from stubby-type to slender-type. The minimum horizontal principal stress difference mainly affects the horizontal fracture. As the fracturing fluid viscosity increases, the horizontal fracture length increases first and then decreases. The fracturing fluid injection rate has a great impact on the height-length profile of horizontal fractures. The optimal injection rate seems to be 6.5–7 m3/min, the optimal fracturing fluid viscosity is between 10 mPa·s and 15 mPa·s, and the optimal perforation location is selected near the formation interface. These research results could provide a reference for the design and optimization of efficient fracturing of coalbed methane reservoirs.

Numerical Simulations of Radial Well Assisted Deflagration Fracturing Based on the Smoothed Particle Hydrodynamics Method

The technology of radial-well-assisted hydraulic fracturing is applied in the stimulation of low-permeability hydrocarbon reservoirs where commercial production cannot be achieved by the conventional fracturing method. Here, a study on the reservoir stimulation effect and the fracture propagation pattern of radial-well-assisted deflagration fracturing was carried out. Based on smooth particle hydrodynamics (SPH), rock mechanics theory, and finite element theory, a numerical model of radial-well-assisted deflagration fracturing was established by integrating the JWL state equation. Research on the effects of the deflagration position, radial well azimuth and horizontal principal stress difference on the fracture propagation was carried out. The results show that the deflagration position, radial well azimuth and horizontal principal stress difference have significant effects on the fracture area in deflagration fracturing. The closer distance from the deflagration position is, the larger the radial well azimuth and the smaller the horizontal stress difference are, leading to a larger fracture area, which is conducive to reservoir stimulation. During fracturing, both shear fractures and tensile fractures are formed. The formation and conversion of shear fractures and tensile fractures are related to the deflagration position, radial well azimuth, horizontal principal stress difference, etc.

Distribution of In Situ Stress in Northwest Jiaodong Peninsula

In order to study the present state of the tectonic stress field in northwest Jiaodong Peninsula, the characteristics of the ground stress distribution along depth were analyzed with the method of regression analysis. A total of 164 items of in situ stress measurement data were collected. There are mainly two types of in situ stress states, one is σH>σh> σ v and the other is σ> σ v >σh, and the type of in situ stress state is related to the depth. The principal stresses σH, σh, and σ v increase approximately linearly with depth, and the stress gradients are 0.0556, 0.0248, and 0.0346, respectively. The lateral pressure coefficients KH, Kh, and Kav vary approximately hyperbolically with the increase in depth and approach 1.99, 1.12, and 1.56, respectively. The ratio of half of the maximum horizontal differential stress to the average horizontal principal stress μd varies linearly with the increase of depth and approaches the value of 0.33. The maximum shear stress τm increases approximately linearly with depth, and the stress gradients are 0.0146. The ratio of horizontal principal stress difference with vertical stress on 1000 m–2000 m is 0.5– 0.7. In addition, the maximum level of principal stress advantage direction in the northwest Jiaodong Peninsula is nearly E–W.

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.

Proactive stress interference mechanism and its application in the Mahu oil area, Junggar basin

There exists a large horizontal principal stress difference (11–38 MPa) in the tight conglomerates in the Mahu oil area, China. It is difficult to form a complex fracture network via hydraulic fracturing under these conditions. To improve reservoir stimulation, the fracture formation mechanism of the complex conglomerate fracture networks was explored. Based on the geomechanics theory of fracture formation, the mechanism of the “stress wall” formed by fracturing in horizontal wells was analyzed in this paper. The inhibitory effect of the stress wall on the formation of tensile and shear fractures was studied. The reason for the decrease in the stress difference coefficient caused by fracturing fluid was analyzed through numerical simulation, which suggested that the complexity of a fracturing network is mainly controlled by the interference of externally applied stress and the reduction in the coefficient of internal stress difference. In this paper, innovative technologies were developed by proactively introducing stress interference in the application of the Ma131 small-well-spacing pilot area. The core technologies include optimization of the 3-D staggered small-well-spacing pattern, and synergetic optimization of multiple elements and zipper fracturing. The positive effects of proactive stress interference on improving fracturing volume, reserve utilization rate and recovery were discussed. Based on the concept of proactive stress interference, the “serial fracturing mode” of horizontal wells was proposed to reduce drilling and fracturing interference and improve the development effect.

Stability of Natural Fractures under Disturbance of Hydraulic Fractures in Shale Reservoirs

Activating the natural fracture (NF) during hydraulic fracture (HF) propagation is a dominant factor for forming the complex fracture network in shale reservoirs. On basis of Renshaw and Pollard’s criterion for fracture intersection, we simulate the NF stability under dynamic disturbance of HF propagation in shale reservoirs with the cohesive zone model in ABAQUS and analyze the effects of various factors on the NF stability. The results show that activation of the NFs in shale hydraulic fracturing is controlled by NF length, NF density, intersection angle between the NF and the HF, horizontal principal stress difference, frac fluid viscosity, and injection rate. During HF propagation, the NF shorter than 0.2 m is less likely to be activated, and those longer than 0.5 m are more likely to be activated. As the NF density increases, not all NFs are activated. The NF at an approach angle larger than 10° is more likely to be activated. The NF is more likely to be activated under the low stress difference, and the NF cannot be activated when the stress difference is higher than 10 MPa. Both frac fluid viscosity and injection rate have effects on activation of NFs, and the NF is more likely to be activated by higher viscosity frac fluid and under higher injection rate. The results provide reference for fracture network stimulation in shale reservoirs.

Experimental study on hydraulic fracture propagation behavior on dolomite rocks with open-hole completion

To determine to discuss fracture mechanisms and the possible encounter problems in open-hole fracturing in tight carbonation, tri-axial hydraulic fracturing experiments were performed on dolomites with acoustic emission monitoring. The results demonstrate that a complex fracture network is difficult to create in most cases even if the dolomite has high brittleness. In general, fractures likely have three types of fracture geometry, namely, bi-wing fractures, local multiple fractures and complex multiple fractures. However, even if multiple fractures can be initiated with open-hole completion, they still can finally merge into one dominant hydraulic fracture (HF). Branch fractures can be observed with low horizontal stress difference and low fluid viscosity. The breakdown pressure on dolomites has a positive relationship with the high fluid viscosity and fluid injection rate, but a negative relationship with the high horizontal principal stress difference. The existence of natural fractures and critical engineering parameter differences should simultaneously contribute to the ratio of branch fracture and diverge fracture. The microscopic observation results reveal that natural fractures that are infilled with mixed calcite minerals are more likely activated due to the lower strength of these minerals, and the main HF is arrested with a natural fracture that is infilled with high-strength calcite minerals. The separation of calcite clusters may exert competing effects on the fracture conductivity. On one hand, the debonded calcite clusters can seal and plug open HFs and reduce the fracture conductivity if the fracture width is not substantial. On the other hand, a calcite cluster can be a type of natural self-proppant to sustain long-term conductivity. The selection of open-hole completion in dolomite selection increases the risks of wellbore deformation but no serious wellbore collapse can be observed. Thus, hydraulic fracturing using open-hole completion can be recommended in dolomite reservoir development if the fracture height containment problem is not a critical issue.

Parameter Sensitivity Analysis of the Hydraulic Fracture Growth Geometry in a Deep Shale Oil Formation: An Experimental Study

The depth of shale oil of Fengcheng Formation in Mahu of Junggar Basin, China, is 4500-5000 m. The horizontal principal stress difference of deep shale reservoir is high, which makes it difficult to form complex fractures during fracturing reconstruction. In order to fully understand the law of hydraulic fracture propagation in the formation during fracturing construction, the anisotropy characteristics and basic reservoir physical parameters (mineral composition and rock strength parameters) of rock were obtained through mineral composition test and indoor rock mechanics test (Brazil splitting test), and it was found that the heterogeneity was strong. The true triaxial fracturing simulation experimental system is used to carry out experimental research on full-diameter core rock samples, and the propagation patterns of hydraulic fractures under the influence of different geological factors (in situ stress difference and natural fractures) and engineering factors (pumping rate and fracturing fluid viscosity) are compared and analyzed. The results show that the in situ stress is the most important factor affecting fracture propagation, which determines the direction and shape of fracture propagation. The natural weak surface (lamina/bedding and natural fractures, etc.) in shale reservoir is an important reason for complex fractures. The nature of the weak plane, occurrence, and in situ stress jointly determine whether the fracture can extend through the weak plane. With the increase of pumping rate (18 mL/min to 30 mL/min), the ability of hydraulic fractures to penetrate layers is continuously enhanced. The horizontal principal stress difference of deep shale reservoir is high, and the low viscosity fracturing fluid (10 mPa·s) tends to activate the horizontal bedding, while the high viscosity fracturing fluid (80 mPa·s) tends to directly penetrate the bedding to form the vertical main fracture. Therefore, the fracturing technology of alternating injection of prehigh viscosity fracturing fluid and postlow viscosity fracturing fluid can be adopted to maximize the complexity of fracturing fractures in deep shale reservoirs. The research results are designed to provide theoretical guidance for prediction of hydraulic fracturing fracture propagation in shale reservoir and have certain reference significance for field construction.

A New Method of Quantitatively Evaluating Fracability of Tight Sandstone Reservoirs Using Geomechanics Characteristics and In Situ Stress Field

This paper studied the fracability of tight sandstone reservoirs by means of incorporating geomechanics properties and surrounding in situ stresses into a new model. The new fracability evaluation model consists of variables such as brittleness index, critical strain energy release rate index, horizontal stress difference, and minimum horizontal principal stress gradient. The probability of interconnection of a complex fracture network was quantitatively studied by the brittleness index and horizontal principal stress difference index. The probability of obtaining a large stimulated reservoir volume was evaluated by the critical strain energy release rate index and minimum horizontal principal stress gradient which also quantifies conductivity. This model is more capable of evaluating fracability, i.e., it agrees better with the history of production with a high precision and had correlation coefficients (R2) of 0.970 and 0.910 with liquid production of post-fracturing well testing and the average production of six months of post-fracturing, respectively. It is convenient that all model inputs were obtained by means of loggings. Using this model, tight sandstone reservoirs were classified into three groups according to fracability: Frac ≥ 0.3 MPa−1·m for Type-I, 0.22 MPa−1·m ≤ Frac < 0.3 MPa−1·m for Type-II, and Frac < 0.22 MPa−1·m for Type-III.