Search

Hydraulic fracturing technology with horizontal wells is a highly efficient approach to stimulate the unconventional reservoir by creating lots of fractures to increase the oil and gas recovery rate. However, some field monitoring data indicate that multiple fractures could not propagate uniformly due to stress interference, which leads to a lower stimulation volume and production capacity. Therefore, investigating the stress interference mechanism between multiple fractures is important to obtain a higher production capacity for unconventional reservoirs. In this study, a two-dimensional XFEM (expanded finite element method) model was established to investigate the influence of stress interference on the fracture propagation of multiple fractures in horizontal wells. The different injection parameters, completion parameters, and fracturing patterns were investigated by sixteen simulation cases. The results show that first the influencing degree of stress interference from different parameters is fracture spacing, injection flow rate, number of fractures, and fluid viscosity, respectively. Second, compared to the simultaneous fracturing and sequential fracturing, the zip fracturing pattern (sides fractures created first and then mid-fracture) could create more uniform fractures in the reservoir. Third, a small fracture spacing and a higher injection flow rate could lead to a faster stress interference, but the fluid viscosity and the number of fractures have little effect on the occur timing of stress interference. This study could provide some meaningful perspective on the field fracturing design in horizontal wells. [ABSTRACT FROM AUTHOR]

Multi-cluster fracturing technology with horizontal wells is significant for the production enhancement of unconventional reservoirs. However, affected by the natural fracture distribution in the reservoir, stress shadowing between multi-fractures and perforation erosion has non-negligible influence on the multi-fracture uniform propagation, which results in uneven reservoir stimulation and lower production capacity. In this study, a multi-field coupled stress-seepage-fracture model for hydraulic fracturing of fractured reservoirs based on the continuum–discontinuum method was developed, adequately simulating the full scenario of stress disturbances, perforation erosion, and fracture interactions during the fracturing process. The effect of different geological and engineering parameters on the competing propagation of multi-fractures was investigated in detail, and the results show: Different geological and engineering parameters have significant influence on the competitive propagation of multi-fractures; among the geological parameters, the elastic modulus has the highest impact on the uniform fluid intake of multi-fractures, while the horizontal stress difference has the least impact on the uniform fluid intake of multi-fractures. Among the engineering parameters, the effect of natural fracture angle on the standard deviation of the fluid injection volume is gradually reduced with the increase in perforation number, flow rate, and fluid viscosity. For a low number of perforations and high fluid viscosity, both have great influence on promoting uniform fluid entry in multiple fractures. In addition, geological parameters have a significantly greater influence on the merging of multi-fractures than engineering parameters, and the probability of merging of multi-fractures increases significantly under low stress differentials and long natural fractures. [ABSTRACT FROM AUTHOR]

Tight-sandstone oil and gas resources are the key areas of unconventional oil and gas resources exploration and development. Because tight-sandstone reservoirs usually have the characteristics of a low porosity and ultralow permeability, large-scale hydraulic fracturing is often required to form artificial fractures with a high conductivity to achieve efficient development. The brittleness of rock is the key mechanical factor for whether fracturing can form a complex fracture network. Previous scholars have carried out a lot of research on the brittleness characteristics of conglomerate and shale reservoirs, but there are few studies on the brittleness characteristics of sandstone with different types and different coring angles in tight-sandstone reservoirs and the fracture propagation law of sandstone with different brittleness characteristics. Based on this, this paper carried out a systematic triaxial compression and hydraulic fracturing experiment on the tight sandstone of Shan 1 and He 8 in the Longdong area of the Changqing oilfield. Combined with CT scanning cracks, the brittleness characteristics and fracturing crack propagation law of different types and different coring angles of sandstone under formation-confining pressure were clarified. The results show that there are great differences between different types of sandstone in the yield stage and the failure stage. The sandstone with a quartz content of 100% has the highest peak strength and a strong brittleness. Sandstones with a high content of natural fractures and dolomite have a lower peak strength and a weaker brittleness. There are also differences in the peak strength and fracture morphology of sandstone with different coring angles due to geological heterogeneity. The sandstone with a comprehensive brittleness index of 70.30 produces a more complex fracture network during triaxial compression and hydraulic fracturing than the sandstone with a comprehensive brittleness index of 14.15. The research results have important guiding significance for on-site fracturing construction of tight-sandstone reservoirs. [ABSTRACT FROM AUTHOR]

Hot dry rock (HDR) geothermal resource is a renewable green energy source with great exploitation potential. The burial depth of HDR generally exceeds 2500 m and is typically under high in situ stress conditions, resulting in an ultralow permeability of the rock formations. To enhance the permeability of these formations, hydraulic fracturing is widely used as a reservoir stimulation technique in HDR geothermal resource exploitation. The differences in burial depth, in situ stress, and geological environment require different engineering designs when implementing hydraulic fracturing. Therefore, the confining pressure and injection rate play significant roles in determining the effectiveness of hydraulic fracturing, as they affect the propagation and distribution of fractures in the rock formation. To quantify the impact of these factors on the effectiveness of hydraulic fracturing, simulation experiments, and permeability tests were conducted using granite specimens under various confining pressure and injection rate conditions. The results of these experiments revealed the relationships among the confining pressure, injection rate, breakdown pressure, and permeability enhancement of the granite. The breakdown pressure of granite increased with the confining pressure, while the injection rate had little effect on the breakdown pressure. The hydraulic fractured sample produced new penetrating fractures, which increased the reservoir permeability, and owing to the higher complexity of hydraulic fractures under low confining pressure, the increase of permeability is correspondingly higher. The research results can provide an important reference for efficient stimulation development technology of deep HDR geothermal resources. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing and water/nanoemulsion flooding technology are effective stimulation measures for tight oil production. The complex pore structure of ultra-low permeability sandstone greatly affects oil recovery. At present, scholars have done a lot of research on the characteristics of oil-water flow in micro-nano pores of ultra-low permeability sandstone reservoirs. However, few scholars have comprehensively studied the whole process of ultra-low permeability sandstone reservoirs from hydraulic fracturing to enhanced oil recovery. In this paper, the whole process from fracturing to production of ultra-low permeability sandstone reservoirs is comprehensively studied at the laboratory scale by combining triaxial fracturing, CT scanning, online LF-NMR and other technologies, aiming to promote the understanding of tight oil development. The results show that the high confining pressure (45 MPa) inhibits the propagation of fractures in the rock samples more than the low confining pressure (15 MPa) during the triaxial fracturing process. The oil recovery of the rock sample with high fracture volume (1213.42 mm3) is 16.32 % higher than that of the rock sample with low fracture volume (674.22 mm3). Whether it is fractured rock or non-fractured rock, nanoemulsion can further improve the oil recovery of micropores on the basis of water flooding, with a maximum increase of 28.21 %. The oil recovery of the nanoemulsion with a concentration of 0.3 wt% was 25.34 % higher than that of the nanoemulsion with a concentration of 0.05 wt%. Confining pressure conditions have an important impact on oil recovery. The oil recovery of non-fractured rock samples with initial confining pressure of 5 MPa is 8.15 % lower than that of non-fractured rock samples with initial confining pressure of 30 MPa. At the same time, we propose a new T 1 -T 2 map region division scheme based on sandstone to better characterize oil displacement behavior at the pore scale. The research results can guide the exploitation of ultra-low permeability sandstone reservoirs. • A systematic experimental process from hydraulic fracturing to enhancing oil recovery was presented. • The fluid dynamics and flooding behavior at the pore scale were revealed. • MRI and 2D NMR techniques can easily distinguish oil signals from different pores. • A T 1 -T 2 map region division scheme based on sandstone is proposed. [ABSTRACT FROM AUTHOR]

Abstract Hydraulic fracturing to enhance permeability of tight reservoirs is meaningful but difficult in constructing of complex fracture network. This study aimed to examine the effectiveness of tight porous reservoir stimulation in the context of hydraulic fracturing to increase the fracture complexity as much as possible. The simulation experiments were performed to reasonably establish the relation between the laboratory results and the field fracturing operations. Large size stratified and intact samples were casted to resist the size and boundary effects by mixing river sand and/or pulverized coal with concrete cement and freshwater in specific proportions. The samples with artificial slot orienting were casted to understand the fracture initiation, orientation, and deflection mechanisms considering the influence of caprock structure and re-fracturing without temporary plugging. The samples were tested under the scaled geostress conditions of corresponding in-situ stress with a true triaxial loading system, and the reasonable dimensionless groups derived from the literature. Consequently, this study proposed an asymmetric fracturing method based on the experimental results considering the scaling structure of overlying-reservoir-underlying in porous reservoirs. The local asymmetric fracturing is a type of reservoir stimulation method with small scale high-pressure fluids such as carbon dioxide，liquid nitrogen, water, etc., jet cutting to create slot and perforation for the aid of fracture initiation and orientation. Hydraulic fractures were influenced by the fracture-induced stress shadow and ratio of the maximum to minimum horizontal stress. The validation and comparative results showed that, compared with the re-fracturing, the asymmetric fracturing significantly increased the complexity of fracture network. Although the applicability of the asymmetric fracturing depending on the corresponding engineering pilots, the derived method could hopefully address the properties of the strata structure of the reservoirs efficiently. Therefore, it has a high potential for application in the fracturing of local underground coal seam and special-trap type oil gas reservoirs to construct fracture network. Highlights • Fracturing to enhance permeability of tight reservoirs is meaningful but difficult. • Asymmetric fracturing converts the disadvantage of stress shadow into the advantage. • Non-planar or new fracture generates a relatively high fracture extending pressure. • Complex fracture network tends to be created in the context of asymmetric fracturing. • Coupling of stress shadow and fracturing fluids remains to be captured in visualization. [ABSTRACT FROM AUTHOR]

Fluid-solid coupling investigations of the geological storage of CO, efficient unconventional oil and natural gas exploitations are mostly conducted under conventional triaxial stress conditions ( σ = σ ), ignoring the effects of σ on the geomechanical properties and permeability of rocks (shale, coal and sandstone). A novel multi-functional true triaxial geophysical (TTG) apparatus was designed, fabricated, calibrated and tested to simulate true triaxial stress ( σ > σ > σ ) conditions and to reveal geomechanical properties and permeability evolutions of rocks. The apparatus was developed with the capacity to carry out geomechanical and fluid flow experiments at high three-dimensional loading forces and injection pressures under true triaxial stress conditions. The control and measurement of the fluid flow with effective sealing of rock specimen corners were achieved using a specially designed internally sealed fluid flow system. To validate that the apparatus works properly and to recognize the effects of each principal stress on rock deformation and permeability, stress-strain and permeability experiments and a hydraulic fracturing simulation experiment on shale specimens were conducted under true triaxial stress conditions using the TTG apparatus. Results show that the apparatus has advantages in recognizing the effects of σ on the geomechanical properties and permeability of rocks. Results also demonstrate the effectiveness and reliability of the novel TTG apparatus. The apparatus provides a new method of studying the geomechanical properties and permeability evolutions of rocks under true triaxial stress conditions, promoting further investigations of the geological storage of CO, efficient unconventional oil and gas exploitations. [ABSTRACT FROM AUTHOR]

Temporary plugging and diverting fracturing (TDPF) is an effective stimulation method to achieve multiple uniform fracture propagation in unconventional reservoirs. The plugging mechanism of diverters and the feasibility of diverted fractures have been investigated well in previous studies. However, few studies aim at the relationship between the multi-fracture morphology and injection pressure curve during TDPF because multiple fractures can not be directly observed during the fracturing process. This study established a visual true tri-axial fracturing physical experiment based on transparent polymethyl methacrylate (PMMA) to investigate the multi-fracture initiation sequence and breakdown pressure in the horizontal wellbore during TDPF. The critical fracture parameters, including completion method, injection flow rate, injection fluid type, fracture density, perforation depth and stress difference coefficient, were analyzed in detail based on twelve PMMA samples. The results showed that all experiments just produced one hydraulic fracture in the initial fracturing stage, but sequentially created multiple fractures after multiple diversion stages. Meanwhile, due to the effect of stress shadow, the break-down pressure of the first and second diverted fracture was 1.36 times and 1.67 times of the initial fracture respectively. In addition to the transverse fractures produced in the diversion stages, longitudinal fractures were also created along the horizontal wellbore, which decreased the multiple fracture initiation effectiveness. Optimized parameters, i.e. perforation completion method, high injection flowrate, moderate fracture density, uniform perforation depth and high-viscosity carrier fluid with diverters, were recommended to obtain more transverse fractures during TPDF. In addition, our results also revealed three fracture initiation sequences of multiple fractures in the horizontal well during TPDF. This study described the relationship between the visual multi-fracture geometries and breakdown pressure in horizontal wells during TPDF, which provides new insights into the propagation mechanism of multiple fractures and guidance for fracturing design in the oilfield. • A new visualization true tri-axial fracturing physical experiment was established to investigate the multi-fracture initiation sequence and breakdown pressure during TDPF. • Three fracture initiation sequences of multiple fractures during TDPF could be found. • Only one hydraulic fracture was created in the initial stage, and multiple fractures can be sequentially created after multiple diversion stages. [ABSTRACT FROM AUTHOR]

Staged multi-cluster fracturing of horizontal wells is a commonly used stimulation method in unconventional reservoirs. The ultimate goal is to increase capital efficiency by placing a dense fracture network more contained within the producing formation. However, the increase in the number of perforation clusters usually leads to a large number of clusters becoming inefficient. To improve perforation cluster efficiency, the method of using diverters to plug the main fracture to open new fractures is employed, but the key lies in how to control the diverter's entry into the target fracture. In this paper, a visual multi-perforation device is established to investigate the diverter transport behavior in the horizontal wellbore. And the coupled computational fluid dynamics and discrete element method (CFD-DEM) is adopted to capture the key transport features. Results show that there is a backflow region in the wellbore downstream of the perforation, which facilitates diverters' transport into the perforation. Increasing the fluid viscosity or perforation flow ratio could help to improve transport efficiency. When enhances the injection rate, the transport efficiency rises first and then decreases. The optimal injection rate is about 0.10 m3/min/perf, but for the larger casing, the rate should be increased further. The purpose of this study is to improve diverter transport efficiency in addition to guiding temporary plugging fracturing design. • There are three types of particle transport behaviors at the perforation. • Particle transport was simulated using the CFD-DEM method. • The impact of injection rate on particle transport was compared. • The fluid viscosity on diverter transport efficiency was investigated. [ABSTRACT FROM AUTHOR]

Energy conversion is an essential driving force of shale anisotropic deformation, damage initiation and evolution. In this study, theoretical calculations of five energy indexes (total input energy U t , releasable energy U e , dissipated energy U d , dilatational energy U V and distortional energy U D ) during the shale failure process considering the inherent anisotropy of shale are presented in detail for the first time. The anisotropic features of the five energy indexes are systematically discussed. The results indicate that bedding structure considerably influences these energy indexes. U e , U V and U D all decrease to a minimum as the bedding orientation reaches 60° and then increase with the bedding degree. The relationships between U e , U V and U D and the confining pressure can be described by linear functions, similar to the variation in the compressive strength with either bedding inclination angle or confining pressure. As the axial strain increases, the rates of increase in U e , U V and U D first increase, then remain constant, and then decrease before failure; however, the rates of increase in U t and U d continue to increase until failure. U e , U D and U V have strong relationships with the peak compressive strength but weak relationships with the peak axial strain, whereas U t and U d exhibit the opposite relationships with the peak strength and strain. The anisotropic characteristics of the energy indexes can help to further elucidate the failure mechanisms of shale and are of great significance to hydraulic fracturing design. • Five energy indexes considering the inherent anisotropy of shale are calculated for the first time. • Bedding plane dip angle has significant effect on the variations of energy indexes. • U e , U V and U D present similar anisotropic characteristics, while U t and U d have similar variation behaviors. • 3D nonlinear fitting method of energy index, bedding direction and confining pressure is proposed. [ABSTRACT FROM AUTHOR]

The extraction of coalbed methane can supply clean energy to society and improve the safety of subsurface coal mining. The low and ultra-low permeability of coal seams has resulted in the significantly low efficiency of methane production in surface and subsurface extraction systems. Fluid stimulation has been extensively applied for the enhancement of formation permeability. However, the response of coal seams to fluid stimulation may be different from other formations due to their unique structure and mechanical properties. Moreover, the presence of shear-fracturing stimulation and its effect on permeability enhancement is more significant in coal seams during fluid stimulation. In this study, a fluid injection-induced hydrofracturing treatment and shear-fracturing stimulation were separately investigated through laboratory experiments. The permeability of both intact and fractured specimens was used to directly evaluate the efficiency of these two types of fluid stimulations in different types of reservoirs. The experimental results indicated that the closure of hydraulic (i.e., tensile) fractures significantly inhibited the permeability enhancement of the hydrofracturing treatment in coal seams, and resulted in the permeability of fractured coal specimens were considerably lower than that of hard rocks (e.g., shale and sandstone). The results also demonstrated that fluid injection-induced shear stimulation in the coal specimens resulted in dilation behavior primarily associated with permeability enhancement. This was attributed to the fact that the exfoliated particles and masses prop shear fractures and increase their stiffness. It was found that, because of the weak mechanical and discontinuous properties of coal seams, an increase in the deviator stress improved the feasibility of shear-fracturing stimulation. Finally, fluid stimulation can be achieved using low viscosity fluids, such as L/Sc-CO 2 , as the fracturing fluid tended to form a shear dilation zone with high permeability in the coal seams. • The closure of hydraulic fractures inhibit the efficiency of hydrofracturing in coal seams. • The shear-dilation zone induced by fluid stimulation significantly enhance coal permeability. • The mechanical properties of coal and low viscosity fluid advance the feasibility of shear stimulation. [ABSTRACT FROM AUTHOR]

Hydraulic fracturing is an effective technology for enhancing the extraction of reservoir methane, as proved by field experience and laboratory experiments. However, unlike conventional reservoirs, coal seams had high stress sensitivity and high anisotropy. Therefore, the efficiency of hydraulic fracturing in coal seams needs to be investigated. In this study, hydraulic fracturing was performed at Nantong mine in the southeast Sichuan basin, China. The field investigation indicated that the hydraulic fracturing could significantly enhance the methane extraction rate of boreholes ten times higher than that of normal boreholes in one of the minable coal seams (named #5 coal seam). The performance of hydraulic fracturing in three districts revealed that compared with south flank, the fluid pressure was higher and the injection rate was lower in north flank. The methane extraction rate of south flank was inferior to that of north flank. It indicated hydraulic fracturing had less effect on #5 coal seam in south flank. Moreover, the injection of high-pressure water in coal seams could also drive methane away from boreholes. The methane extraction rate of the test boreholes demonstrated the existence of methane enrichment circles after hydraulic fracturing. It indicated that hydraulic fracturing did act on #5 coal seam in south flank. However, due to the high stress sensitivity of coal seams and the high geo-stress of south flank, the induced artificial fractures in #5 coal seam might close with the decline of the fluid pressure that led to a sharp decline of the methane extraction rate. [ABSTRACT FROM AUTHOR]

Recently, the use of CO 2 as a treatment fluid in the petroleum and gas industry as well as enhanced geothermal system projects has attracted the attention of many scholars. In this paper, hydrofracturing experiments were performed on shale and coal specimens employing CO 2 and water as fracturing fluids, respectively. The differences of hydraulic and L-CO 2 fracturing in the breakdown pressure and fracture features were investigated under different geological and engineering conditions. The experimental results indicate that preexisting flaws in reservoirs have a significant effect on the stimulation mechanism of the hydrofracturing treatment. The non-reactivation of preexisting flaws leads to tensile stimulation. In contrast, the reactivation of preexisting flaws might lead to shear stimulation under a high horizontal stress difference. The shear stimulation would impact fracture propagation, and can even induce the slip of slant fractures, that generates splay cracks at the fracture tips. Because shear slip requires a high horizontal stress difference, which reduces the conductivity of reservoirs. Comparing with hydraulic fracturing, the low viscosity of CO 2 is beneficial for shear stimulation. [ABSTRACT FROM AUTHOR]

Because of the low or ultra-low permeability of coal seams, hydraulic fracturing technology has been widely used to improve the permeability of coal seams in both ground and underground extraction. Recently, liquid carbon dioxide (L-CO 2 ) as an environment-friendly fracturing fluid has drawn attention from researchers and engineers. In this study, a true tri-axial system was adopted to perform hydraulic and L-CO 2 fracturing experiments on coal specimens from the South Sichuan Basin in China. The effects of geological and engineering factors (horizontal stress ratio, injection rate, and fracturing fluid) on induced fracture propagation were investigated in this study. The various geological and engineering factors can synthetically, rather than unilaterally, impact the fractures propagation to generate three types of stimulation mechanism. Based on our experimental results, we succeeded in approximately predicting the stimulation mechanism for induced fracturing under different conditions. Generally, the effect of pre-existing natural fractures on the fracture propagation was enhanced, whereas the effect of the geo-stress was reduced, when L-CO 2 was adopted as the fracturing fluid. [ABSTRACT FROM AUTHOR]

• Hydraulic fracture propagation is affected by different DFNs and their grids. • Initial in-situ stress field is affected by rock elastic modulus distribution. • Rough fractures need to be considered in fracturing simulation. Conventional discrete fracture network (DFN) modeling is used to simulate hydraulic fracture propagation in fractured low permeability reservoirs. However, natural fractures have a certain degree of roughness, and reservoir rocks have significant heterogeneity characteristics. Therefore, it is necessary to compare the differences in hydraulic fracturing simulation of homogeneous and heterogeneous reservoirs caused by conventional and rough DFN modeling methods. Herein, new-generation algorithms for different DFNs are proposed. Combined the combined finite-discrete element method and different DFNs, four different simulation models are established, covering hydraulic fracturing models from relatively homogeneous to heterogeneous reservoirs. Then, the differences from different DFN modeling methods are compared and discussed. The results show that the relatively homogeneous and heterogeneous models established by conventional DFN may underestimate the bending propagation characteristics and fracturing time and overestimate the proportion of tensile failure. Meanwhile, when the nonuniform distribution of the mechanical parameters of rock blocks is considered, the bending propagation becomes more marked; thus, the fracturing time and the proportion of shear failure are further increased. The above results imply that to accurately predict the hydraulic fracture propagation in a real reservoir, it is vital to consider the roughness of real natural fractures and the nonuniform distribution of mechanical parameters in reservoir hydraulic fracturing modeling. [ABSTRACT FROM AUTHOR]

In the field construction of shale oil (gas) hydraulic fracturing, the veins present in the shale may deflect the propagation path of hydraulic fractures. The metal chalcopyrite veins are widely contained in Jimusar Shale, but the effect of the metal veins on the fracture propagation at different temperatures is still unclear. Based on the extended finite element simulation and semicircular bending test method, the effect of temperature on the interaction law of the metal veins and hydraulic fractures is studied. The results show that the fracture propagation path in the rock sample is more tortuous with the increasing temperature; Compared with the 23 °C rock sample, the fracture toughness of rock samples without the vein and with the vein at 200 °C increase by 7.79% and 22.78%, respectively. The higher the temperature, the more difficult it is for the fracture to offset at the interface between the rock and the metal veins. But the temperature does not affect the distance of fractures offset. The results of this study can provide a reference for the design of hydraulic fractures of shale with the metal veins. • The shale samples with the metal vein are heated under the different temperature condition. • The interaction between the fracture and the metal vein is quantitatively investigated by the semicircular bending test, the acoustic emission test and wavelet analysis. • The results of the interaction between the metal vein and the fracture are investigated by the extended finite element method. • The effects of temperature on the interaction between the metal vein and the fracture are discussed. [ABSTRACT FROM AUTHOR]