Simultaneous Fracturing Research Articles

The viscous-brittle transition in flowing crystal-bearing volcanic dome lavas

The ascent and advance of volcanic dome lava is non-linear and viscoelastic. There exists a mismatch between current theoretical approaches to dome lava rheology, which are based on rheological laws for viscous suspensions, and empirical experimental approaches to convolved viscous-brittle deformation, which show mixed evidence for simultaneous lava flow and fracturing. The missing requirement is a unified framework for understanding the transition between micro-mechanical flow mechanisms that are dominantly viscous, and those that include micro-cracking in multiphase suspensions such as magmas. Here, we use high-temperature compression rheology with sample-scale acoustic emission analysis to constrain the conditions under which crystal-rich volcanic dome lava can flow by mixed viscous and brittle fracturing processes at small scales, leading to ‘crackling’ acoustic signals, even at moderate shear stresses extant in nature. Using multi-directional permeability measurements on large 60 mm diameter quenched samples of natural magmas, we show that this micro-cracking flow mechanism leads to permeability anisotropy, localizing outgassing into pathways that are off-axis relative to the direction of flow. Finally, we use a scaling approach and a database of published observations from real eruptions to upscale our findings, and show that bulk, apparently ductile flow of low-porosity dome magma is likely to involve a local mixed-mode of micro-cracking and viscous flow during the shallowest portions of ascent and during emplacement on the Earth's surface. The micro-cracking involved in lava advance divorces real crystal-bearing lava emplacement from most current rheology models based on a purely viscous micro-mechanism and shows that a revised solution for the rheology of mixed brittle-viscous flow is required. By re-examining published numerical models for dome emplacement, we demonstrate that the viscous-brittle transition can be intercepted in spatially heterogeneous zones within the dome core.

Safety and high-recovery mechanisms and application research for initial mining of thick-coal-seam with complex structure and thick-hard roof

Thick-coal-seam with complex structures and thick-hard roof in the initial mining phase pose various challenges, including a long weighting interval, strong rock pressure, poor top coal caving performance (TCCP), and significant coal loss. These problems directly affect the safety and efficiency of the mining operations. This study employs the principles of elastic thin plates and ellipsoidal bodies to unravel the formation mechanism of strong rock pressure in thick-hard roof and the influence of parting on the TCCP. In addition, a hydraulic fracturing technique is proposed for safe-efficient recovery during the initial mining phase. The reliability of this technique is verified through numerical simulations and field experiments. The research findings reveal the following. (1) The primary causes of strong rock pressure in the mining face are attributed to a long weighting interval and wide collapse range of the main roof, and the weighting interval is primarily influenced by the thickness and tensile strength of the main roof. (2)The key factor affecting the TCCP is the cantilever beam structure formed by the fracture of the thick-hard parting, as it intersects the ellipsoidal body during coal caving. The simultaneous fracturing of both the top coal and roof can reduce the weighting interval on the working face. This process effectively decreased the strength of the thick-hard parting within the top coal, while simultaneously enhancing its load strength and eliminating the cantilever structure resulting from the parting fracture. It not only reduces the first weighting intensity but also promotes the early and proper coal release, thereby enhancing the TCCP and ensuring safe-efficient mining during the initial mining phase. (3) Aiming at the difference in the strengths of the top coal and roof, a graded hydraulic fracturing technique and system were proposed. Fracturing boreholes with a diameter of 60 mm, spacing of 10 m, and height of 20.85 m, which can economically and effectively ensure the fracturing results. Field applications have demonstrated that fractured coal and rocks in fracturing areas exhibit well-developed fractures. During the initial mining phase, the weighting interval in the working face was reduced by 20 m, resulting in a decrease in the overburden pressure and 26.9% reduction in the lumpiness of the top coal. Additionally, the recovery rate increased by 31.19%.

Fracture Propagation of Multi-Stage Radial Wellbore Fracturing in Tight Sandstone Reservoir

Radial wellbore fracturing is a promising technology for stimulating tight sandstone reservoirs. However, simultaneous fracturing of multiple radial wellbores often leads to unsuccessful treatments. This paper proposes a novel technology called multi-stage radial wellbore fracturing (MRWF) to address this challenge. A numerical model based on the finite element/meshfree method is established to investigate the effects of various parameters on the fracture propagation of MRWF, including the azimuth of the radial wellbore, the horizontal stress difference, and the rock matrix permeability. The results show that previously created fractures have an attraction for subsequently created fractures, significantly influencing fracture propagation. A conceptual model is proposed to explain the variations in the fracture propagation of MRWF, highlighting three critical effect factors: the attraction effect, the orientation effect of the radial wellbore, and the deflection effect of the maximum horizontal principal stress. Fracture geometry is quantitatively assessed through the deviation distance, which indicates the radial wellbore’s ability to guide fracture propagation along its axis. As the azimuth increases, the deviation distances can either increase or decrease, depending on the specific radial wellbore layouts. Decreasing the horizontal stress difference and increasing the rock matrix permeability both increase the deviation distance.

Reservoir stress analysis during simultaneous pulsating hydraulic fracturing based on the poroelastodynamics model

Reservoir stress analysis during simultaneous pulsating hydraulic fracturing based on the poroelastodynamics model

Normal fault reactivation induced by hydraulic fracturing: Poroelastic effects

Numerous surface-felt earthquakes have been spatiotemporally correlated with hydraulic-fracturing operations. Because large deformations occur close to hydraulic fractures (HFs), any associated fault reactivation and resulting seismicity must be evaluated within the length scale of the fracture stages and based on the precise fault location relative to the simulated rock volumes. To evaluate the changes in Coulomb failure stress (CFS) with injection, we conduct fully coupled poroelastic finite-element simulations using a pore-pressure cohesive zone model for the fracture and fault core in combination with a fault-fracture intersection model. The simulations quantify the dependence of CFS and the fault reactivation potential on the host rock and fault properties, spacing between the fault and the HF, and the fracturing sequence. We find that fracturing in an anisotropic in-situ stress state does not lead to fault tensile opening but rather dominant shear reactivation through a poroelastic stress disturbance over the fault core ahead of the compressed central stabilized zone. In our simulations, poroelastic stress changes significantly affect fault reactivation in all the simulated scenarios of fracturing 50–200 m away from an optimally oriented normal fault. Asymmetric HF growth due to the stress shadowing effect of the adjacent HFs leads to (1) a larger reactivated fault zone following the simultaneous and sequential fracturing of multiple clusters compared with single-cluster fracturing and (2) a larger unstable area ([Formula: see text].1) over the fault core or a higher potential of the fault slip following sequential fracturing compared with simultaneous fracturing. The fault reactivation area is further increased for a fault with lower conductivity and a higher opening-mode fracture toughness of the overlying layer. To reduce the risk of fault reactivation by hydraulic fracturing under the reservoir characteristics of the Barnett Shale, Fort Worth Basin, it is recommended to (1) conduct simultaneous fracturing instead of sequential and (2) maintain a minimum distance of approximately 200 m for HF operations from known faults.

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

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

Numerical analysis of hydraulic fracture propagation in deep shale reservoir with different injection strategies

Deep shale reservoirs are characterized by elevated breakdown pressures, diminished fracture complexity, and reduced modified volumes compared to medium and shallow reservoirs. Therefore, it is urgent to investigate particular injection strategies that can optimize breakdown pressure and fracturing efficiency to address the increasing demands for deep shale reservoir stimulation. In this study, the efficiency of various stimulation strategies, including multi-cluster simultaneous fracturing, modified alternating fracturing, alternating shut-in fracturing, and cyclic alternating fracturing, was evaluated. Subsequently, the sensitivity of factors such as the cycle index, shut-in time, cluster spacing, and horizontal permeability was investigated. Additionally, the flow distribution effect within the wellbore was discussed. The results indicate that relative to multi-cluster simultaneous fracturing, modified alternating fracturing exhibits reduced susceptibility to the stress shadow effect, which results in earlier breakdown, extended hydraulic fracture lengths, and more consistent propagation despite an increase in breakdown pressure. The alternating shut-in fracturing benefits the increase of fracture length, which is closely related to the shut-in time. Furthermore, cyclic alternating fracturing markedly lowers breakdown pressure and contributes to uniform fracture propagation, in which the cycle count plays an important role. Modified alternating fracturing demonstrates insensitivity to variations in cluster spacing, whereas horizontal permeability is a critical factor affecting fracture length. The wellbore effect restrains the accumulation of pressure and flow near the perforation, delaying the initiation of hydraulic fractures. The simulation results can provide valuable numerical insights for optimizing injection strategies for deep shale hydraulic fracturing.

Numerical simulation of the simultaneous propagation of multiple hydraulic fractures based on expanded finite element method

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.

Analysis of competition and interference mechanisms of multiple hydraulic fractures in deep-sea hydrate reservoirs in the South China Sea

Improving the fundamental theory of hydraulic fracturing in deep-sea hydrate reservoirs is of great significance in enhancing the efficiency of gas hydrate mining. In this paper, a multi-cluster fracturing model for a horizontal well was established with a three-dimensional extended finite element method and the measured geo-mechanical data from hydrate reservoirs in the South China Sea. The performance, fracture initiation, and expansion mechanisms of multi-hydraulic fractures are investigated under different fracturing sequences, hydrate saturation and cluster deployment patterns. Some interesting results have been discovered: In the case of three-cluster, the fractures produced by sequential fracturing are notably more affected by pre-existing fractures. Furthermore, the effect of fracture interference between simultaneous and sequential fracturing is reflected in the middle fracture is compressed by the fractures on its left and right sides. The increase in hydrate saturation enhances the cementing ability of the sediments and results in an increased difficulty of fracture initiation for the five-cluster simultaneous fracturing. It also results in a decrease in the geo-stress field, where the interaction between fractures plays a dominant role in the process of fracture expansion, leading to non-uniform expansion.

Unstable propagation of hydraulic fractures under varying well spacing and initiation sequence of multiple horizontal wells considering shear stress shadows

PurposeMulti-well hydrofracturing is an important technology to create new fractures and expand existing fractures to increase reservoir permeability. The propagation morphology of the fracture network is affected by the disturbance between the fractures initiation sequences and spacings between adjacent wells. However, it remains unclear how well spacing and initiation sequences lead to fracture propagation, deflection and connection.Design/methodology/approachIn this study, the thermal-hydro-mechanical coupling effect in the hydrofracturing process was considered, to establish a finite element-discrete element model of multistage hydrofracturing in a horizontal well. Using typical cases, the unstable propagation of hydraulic fractures in multiple horizontal wells was investigated under varying well spacing and initiation sequences. Combined with the shear stress shadow caused by in situ stress disturbed by fracture tip propagation, the quantitative indexes of fracture propagation such as length, volume, displacement vector, deflection and unstable propagation behavior of the hydrofracturing fracture network were analyzed.FindingsThe results show that the shear stress disturbance caused by multiple hydraulic fractures is a significant factor in multi-well hydrofracturing. Reducing the spacing between multiple wells increases the stress shadow area and aggravates the mutual disturbance and deflection between the fractures. The quantitative analysis results show that a decrease of well spacing reduces the total length of hydraulic fractures but increases the total volume of the fracture; compared with sequential and simultaneous fracturing, alternate fracturing can effectively reduce stress shadow area, alleviate fracture disturbance and generate larger fracture propagation length and volume.Originality/valueThe numerical models and results of the unstable propagation and stress evolution of the hydraulic fracture network under thermal-hydro-mechanical coupling obtained in this study can provide useful guidance for the evaluation and design of rock mass fracture networks in deep unconventional oil and gas reservoirs.

An XFEM-based CZM Numerical Strategy for Modeling Hydraulic Fracture Propagation under Different Fracturing Schemes

Segmented and cluster fracturing can improve the efficiency of volume stimulation in shale reservoirs and reduce construction costs. It is important to clarify the propagation characteristics of hydraulic fractures and stress interference under different fracturing techniques to optimize the process of clustered and staged fracturing. For this purpose, we have developed a 2D XFEM-based CZM hydraulic fracturing model. The capability of this model was validated by analytical solutions and then used to study the propagation paths of hydraulic fractures and the characteristics of stress interference under simultaneous fracturing, sequential fracturing, zipper fracturing and multi-cluster fracturing. The results show that in simultaneous fracturing, the middle cluster is compressed by the external position clusters, and the opening width of hydraulic fractures is reduced. In sequential fracturing, the fracture that first initiates create an additional stress field that inhibits the propagation distance of subsequent fractures and the propagation path of hydraulic fractures is also affected in the stress shadow region. Zipper fracturing can effectively alleviate stress interference between multiple fractures, and internal fractures can also propagate a certain distance. In multi-cluster fracturing, the fluid rate into the internal fractures may be limited, and fracture propagation is also limited by stress interference. Therefore, it is necessary to optimize the parameters of clusters to ensure that all clusters can initiate fractures normally. The research results are important for the parameter optimization of clustered and staged fracturing, especially for well factory fracturing mode.

Fracture propagation laws of staged hydraulic fracture in fractured geothermal reservoir based on phase field model

Hydraulic fracturing is widely used in geothermal resource exploitation, and many natural fractures exist in hot dry rock reservoirs due to in-situ stress and faults. However, the influence of natural fractures on hydraulic fracture propagation is not considered in the current study. In this paper, based on the phase field model, a thermo-hydro-mechanical coupled hydraulic fracture propagation model was established to reveal the influence of injection time, fracturing method, injection flow rate, and natural fracture distribution on the fracture propagation mechanism. The results show that fracture complexity increases with an increase in injection time. The stress disturbance causes the fracture initiation pressure of the second cluster significantly higher than that of the first and third clusters. The zipper-type fracturing method can reduce the degree of stress disturbance and increase fracture complexity by 7.2% compared to simultaneous hydraulic fracturing. Both low and high injection flow rate lead to a decrease in fracture propagation time, which is not conducive to an increase in fracture complexity. An increase in the natural fracture angle leads to hydraulic fracture crossing natural fracture, but has a lesser effect on fracture complexity. In this paper, we analyzed the influence of different factors on initiation pressure and fracture complexity, providing valuable guidance for the exploitation of geothermal resources.

Numerical analysis of unstable propagation of three-dimensional parallel hydraulic fractures induced by interferences of adjacent perforation clusters and thermal diffusion

PurposeThe purpose of this study is to investigate the unstable propagation of parallel hydraulic fractures induced by interferences of adjacent perforation clusters and thermal diffusion. Fracture propagation in the process of multistage fracturing of a rock mass is deflected owing to various factors. Hydrofracturing of rock masses in deep tight reservoirs involves thermal diffusion, fluid flow and deformation of rock between the rock matrix and fluid in pores and fractures.Design/methodology/approachTo study the unstable propagation behaviours of three-dimensional (3D) parallel hydraulic fractures induced by the interferences of adjacent perforation clusters and thermal diffusion, a 3D engineering-scale numerical model is established under different fracturing scenarios (sequential, simultaneous and alternate fracturing) and different perforation cluster spacings while considering the thermal-hydro-mechanical coupling effect. Stress disturbance region caused by fracture propagation in a deep tight rock mass is superimposed and overlaid with multiple fractures, resulting in a stress shadow effect and fracture deflection.FindingsThe results show that the size of the stress shadow areas and the interaction between fractures increase with decreasing multiple perforation cluster spacing in horizontal wells. Alternate fracturing can produce more fracture areas and improve the fracturing effect compared with those of sequential and simultaneous fracturing. The larger the temperature gradient between the fracturing fluid and rock matrix, the stronger the thermal diffusion effect, and the effect of thermal diffusion on the fracture propagation is significant.Originality/valueThis study focuses on the behaviours of the unstable dynamic propagation of 3D parallel hydraulic fractures induced by the interferences of adjacent perforation clusters and thermal diffusion. Further, the temperature field affects the fracture deflection requires could be investigated from the mechanisms; this paper is to study the unstable propagation of fractures in single horizontal well, which can provide a basis for fracture propagation and stress field disturbance in multiple horizontal wells.

Stress Relief and Permeability Enhancement with Hydraulic Fracturing in Overlying Key Strata of Deep and Soft Coal Seams.

The technical problems of high in situ stress, high gas pressure, high gas content, and low coal seam permeability are widespread in deep soft coal seam excavation, which leads to frequent occurrences of dynamic phenomena, such as coal cannons and blowout holes. Based on the high-pressure hydraulic fracturing technology and process, this study puts forward a new technology of gas drainage in deep and soft coal seams by fracturing the overlying key strata to cut off the stress transmission path among coal and rock strata. According to the theories of key layer and masonry beams, the distribution locations of the main and subkey strata are determined, and based on uniaxial compression and Brazilian splitting experiments, the mechanical parameters of key stratum were tested. Combined with the results of numerical simulation and field test, initial pressure and fracturing radius of hydraulic fracturing technology for overlying key strata were determined, the stress relief effect and permeability variation law of coal seam after hydraulic fracturing in the main and subkey strata were analyzed, and then technical schemes for simultaneous fracturing of the key layer were designed. Field application results showed that the stress concentration phenomenon in soft coal seam excavation had been alleviated, and the stress relief effect of coal seam and the permeability were increased obviously. The volume and concentration of gas drainage were increased by 10 and 11%, respectively, the gas amount by 1.22 times; the frequency of dynamic phenomena such as coal cannons decreased by 95%, the gas concentration in return air flow by 20% during mining processes. This paper provided an innovative technical idea and process for gas control in deep and soft coal seam excavation, which could effectively solve the common and difficult problems about frequent occurrences of excessive gas concentration and dynamic phenomena.

Competition growth of biwing hydraulic fractures in naturally fractured reservoirs

Hydraulic fracturing, combined with horizontal wells, is one of the key technologies that enables production from shale gas reservoirs. However, data from several fields in the United States verified that the total production is only contributed by a small portion of designed fractures (Miller et al., 2011). In this paper, we aim to provide an explanation about this phenomenon and propose corresponding suggestions to improve stimulation efficiency. Here, a multi-fracture propagation model in reservoirs containing cemented natural fractures (NFs) is presented based on phase field method. The fracture growth resistance of each wing for varying properties of NFs is quantitatively investigated. We found that NFs toughness, roughness and intersection angle play a vital role on the fracture growth resistance. It was demonstrated that the discrepancy of fracture growth resistance of each wing can suppress fracture propagation in heterogeneous reservoirs. Fracture propagation in mixed-regime was found to favor creation of biwing fractures, which can be used to improve stimulation performance. The simulations of simultaneous fracturing in naturally fractured formations demonstrated that the existence of NFs alters the extending paths of hydraulic fractures (HFs), which may result in fracture coalescence. In this case, the presented model can be utilized to improve stimulation efficiency through optimizing design parameters.

Fly-in-fly-out work: A review of the impact of an extreme form of work-related travel on mental health

Large distances between work and home require many workers to stay away from home for work over extended periods. An extreme case of such work is fly-in-fly-out (FIFO) work. FIFO work requires workers to stay, over a fixed number of days or weeks, in remote employer-arranged accommodation. Given the disruptive nature of this work arrangement, it is important to develop an understanding of its implications for worker mental health. Based on a systematic review, we identify mostly negative mental health implications of FIFO work and propose an integrative model that brings together FIFO work's key features: the simultaneous fracturing and blending of personal and work lives. The model can guide future research. For example, we suggest that researchers investigate how FIFO workers and other work-related travelers experience fracturing and blending, and how they manage the frequent fluctuation between these two extremes.

Balanced stress fracturing theory and its application in platform well fracturing during unconventional oil and gas development

Platform well fracturing is essential for the efficient development of unconventional oil and gas (UOG), and it has been widely applied in oilfields. Field practices have revealed that various fracturing methods result in different stimulation performances, but the mechanisms responsible for these differences are not completely understood. To explore the characteristics of the inter-well stress interference of various platform well fracturing methods, a numerical model was established for the systematic assessment of the stress field of simultaneous fracturing, zipper fracturing (ZF), modified zipper fracturing, and synergic fracturing (SF). Based on these analyses, balanced stress fracturing (BSF) has been proposed to significantly reduce the horizontal stress difference. This study indicates that the fracturing methods significantly impact the evolution of induced stress, and that ZF and SF can achieve a balanced stress field and the utilization of an inter-well reservoir through a reasonable fracture distribution mode and fracturing operation. Combined with the interlaced fracture distribution mode, interactive fracturing, and overall front parallel fracturing, BSF can improve the fracture network complexity (FNC) and reduce the risk of casing deformation. Moreover, BSF helps achieve higher production and a more uniform stress field in platform well fracturing. This study presents an effective measure to increase FNC and reduce the occurrence of casing deformation during UOG development.

Analysis of seepage mechanism and productivity model of multi-stage fractured horizontal well in tight sandstone gas reservoir

During synchronous fracturing of horizontal well clusters, the induced stresses generated by multiple wells affect the formation of complex fractures. By using linear superposition and vector representation, the productivity model for the horizontal stress field between wells was constructed. The results demonstrated simultaneous fracturing can produce stronger and a wider range of stress interference variable impact on induced stress field, the most influential factor is the row spacing, accounting for 32.09% and the engineering factors with the least impact is fracture conductivity, with weight of 0.08%. The research results were used to YA gas field. After the preliminary implementation in the field, the initial production of a well in the reservoir increased by 20.48%, the decline rate slowed down by 13.16% and the output of well increased by 43.75% in the stable production period. In addition, these results might also provide an acceptable explanation for the high productivity of synchronous fracturing operations in horizontal well clusters. Generally speaking, a clear understanding of changes in induced stress fields in horizontal clusters might provide are liable foundation for the determination of optimal well spacing and a fracturing program for horizontal well clusters to develop unconventional formations.

Numerical simulation of single-cluster and multi-cluster fracturing of hydrate reservoir based on cohesive element

Horizontal well multistage fracturing technology has potential as a technical method to improve the permeability of clayey silt natural gas hydrate (NGH) reservoirs as it can increase the contact area between a horizontal well and an NGH reservoir. Although this technology has been successfully applied to the exploitation of shale gas and coalbed methane, the relevant technical experience cannot be directly applied to clayey silt NGH reservoirs. Based on the properties of the clayey silt NGH reservoir at the SH2 site in the Shenhu area of the South China Sea, herein, we established a three-dimensional hydraulic fracturing model based on cohesive elements to analyze the influence of the fracturing fluid injection rate on the initiation and propagation of single-cluster fractures and the influence of fracture spacing on simultaneous fracturing and sequential fracturing of multi-cluster fractures. We comprehensively analyzed the distribution characteristics of fracture morphology and the phenomenon of stress interference between fractures under different conditions. The results showed that without fracture interference, fractures tended to propagate to the middle and upper parts of the reservoir owing to the low fracture propagation resistance. Simultaneous fracturing and sequential fracturing produced different staggered fracture distribution patterns, which were caused by the geostress field changing owing to the generation of fractures. Our findings broaden the understanding of hydraulic fracturing in clayey silt NGH reservoirs.

Fracture spacing in horizontal well multi-perforation fracturing optimized by heat extraction

Multi-perforation fracturing in horizontal wells is the key procedure in the operation of enhanced geothermal system (EGS). However, two questions remain unclear: 1) what are the similarities and differences between three fracturing methods; and 2) how to optimize fracture spacing (FS)? Here we established a three-dimensional hydraulic fracturing model to unveil fracture initiation and propagation mechanisms. The fractures obtained by hydraulic fracturing were subsequently embedded in a geothermal mining model. Heat extraction was used as an evaluation indicator to optimize FS and geothermal mining. We found that the initiation pressure in multi-perforation fracturing is higher than that in single-perforation fracturing. The stress shadowing increases with increasing time and decreasing FS. FS not only changes the complex fracture network, but also affects the low-temperature area of hot dry rock. Quantitative analysis of heat extraction shows that the optimal FS in simultaneous, sequential,and alternate fracturing are 30 m, 40 m, and 50 m, respectively. Our study benefits fracturing method selection and construction parameter optimization in EGS operation.