Zipper Fracturing Research Articles

The law of fracture propagation and intersection in zipper fracturing of deep shale gas wells

In response to the unclear understanding of fracture propagation and intersection interference in zipper fracturing under the factory development model of deep shale gas wells, a coupled hydro-mechanical model for zipper fracturing considering the influence of natural fracture zones was established based on the finite element – discrete element method. The reliability of the model was verified using experimental data and field monitoring pressure increase data. Taking the deep shale gas reservoir in southern Sichuan as an example, the propagation and interference laws of fracturing fractures under the influence of natural fracture zones with different characteristics were studied. The results show that the large approaching angle fracture zone has a blocking effect on the forward propagation of fracturing fractures and the intersection of inter well fractures. During pump shutdown, hydraulic fractures exhibit continued expansion behavior under net pressure driving. Under high stress difference, as the approaching angle of the fracture zone increases, the response well pressure increase and the total length of the fractured fracture show a trend of first decreasing and then increasing, and first increasing and then decreasing, respectively. Compared to small approach angle fracture zones, natural fracture zones with large approach angles require longer time and have greater difficulty to intersect. The width of fractures and the length of natural fractures are negatively and positively correlated with the response well pressure increase, respectively, and positively and negatively correlated with the time required for intersection, the total length of hydraulic fractures, and fracturing efficiency, respectively. As the displacement distance of the well increases, the probability of fracture intersection decreases, but the regularity between displacement distance and the response well pressure increase and the total length of fractures is not obvious.

Determination of Fracturing Risk-Units base on seismic and micro-seismic

Abstract During hydraulic fracturing, unconventional oil and gas reservoirs often encounter risks, such as fracturing casing deformation, which affects the efficiency and effectiveness of fracturing. We can accurately identify the location of fracturing risks by using logging data and seismic data. However, we cannot avoid the occurrence of risks, and the location of deformation may be in the un-fractured stage, which is relatively far from the already fractured stage. In severe cases, it is necessary to merge or discard the un-fractured stage of the casing deformation, which will inevitably affect the fracturing effect and production. Therefore, it is particularly important to effectively divide risk units, optimize the sequence of fracturing in the stage, and reduce the impact of risk occurrence. In order to reduce the impact, the author proposes a method to determine risk-units. Firstly, before hydraulic fracturing, seismic data is used to interpret faults and determine risk-units based on the connectivity of faults. Then, during hydraulic fracturing, micro-seismic events are applied to explain artificial faults, optimize the connectivity of faults, and determine risk-units. Finally, based on the risk-units, determine the sequence of fracturing, the stage within the same risk-units should be subjected to hydraulic fracturing as the same batch, after all stages within the same risk-units are fractured, the stages of next risk-units can be carried out. The method proposed by this paper has been applied in zipper fracturing, which can effectively reduce the impact of casing deformation. After the application of this method, the deformation only occurs in the fractured stage or near the un-fractured stage, thus significantly reducing the impact on the un-fractured stage. However, due to the complexity of geological structure and stress transmission, the stages may also have mutual influence, located within different risk-units. Therefore, how to optimize the sequence of fracturing based on risk-units is a topic that needs further-deepening research. This paper innovatively proposes a method for determining fracturing risk-units based on seismic and micro-seismic data to optimize the sequence of fracturing in zipper fracturing, this is of great significance for improving fracturing efficiency, improving fracturing effectiveness, and increasing single well oil and gas production.

Quantitative Study on the Anticollapse Ability of Casing in Shale Gas Wells under Complex Load Conditions.

Implementing novel technologies, including the "well factory" model and zipper fracturing techniques, has become prevalent in shale gas development. During completion operations such as lowering casing and multistage fracturing, the casing is subjected to many complex loads, reducing its strength and increasing the risk of casing deformation. By establishing a casing wear model and conducting multistage cyclic loading experiments and numerical simulations, we analyzed the change rule of casing anticollapse strength under complex loads, developed a calculation method for casing comprehensive anticollapse ability under complex loads, and applied the method to an illustrative calculation. The study shows that the wear effect during completion has a negligible impact on the strength of the casing. The casing anticollapse strength exhibits a linear decline in correlation with the number of cycles. The zipper fracturing operation resulted in a nonuniform distribution of geo-stress around the well, and the casing anticollapse strength demonstrated a nearly linear decline in correlation with the nonuniformity of geo-stress. In the presence of both internal and external effects, the casing anticollapse strength exhibited a decline exceeding 15%, thereby increasing the risk of casing deformation. This research method can provide computational guidance for preventing casing deformation in field fracturing construction.

Mechanism of inter-well hydraulic fracture interference in the primary horizontal well pattern

Shale reservoirs are characterized by low porosity and low permeability and are difficult to be developed, single horizontal well fracturing has the shortcomings of low modification and insufficient fracture coverage, so multi-well fracturing is often used in the field. Primary horizontal well pattern is one of the main modes for commercialized shale oil and gas extraction, but the interwell interference existing in multi-well fracturing will affect extraction. In this paper, a 3D hydraulic fracturing model was established with the geology‒engineering integration method, and the effects of cluster spacing, fracturing sequence, fracture placement, well spacing, horizontal stress difference and natural fractures (NFs) on hydraulic fracture (HF) propagation and cumulative gas production in multi-well fracturing were investigated. The results are described as follows. Appropriately reducing the cluster spacing to 10 m can effectively reduce interwell fracture hits while obtaining higher initial production; Zipper fracturing has a significant effect on reducing interwell fracture hits. Compared with sequential fracturing, zipper fracturing increases the HF area and cumulative gas production by 4.7% and 4.2% respectively; In order to ensure production while reducing the risk of fracture hits, the well spacing was chosen to be 400m; Compared to symmetric fracture placement, the cumulative production increased by 7.0% and 9.8% for the 1/4 staggered fracture placement and 1/2 staggered fracture placement, respectively; Although more NFs can be communicated and a larger fracture area can be obtained under small stress difference conditions, but the risk of interwell and intercluster fracture hits increases; When the NF inclination is less than 80°, the fracture hits is weakened with increase of NF inclination, when the NF inclination exceeds 80°, the HF tend to pass through the NFs and the fracture hits increases; As the NF density increases, the fracture complexity near the wellbore increases and the fracture hits decreases.

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.

Simulation Optimization on Multi-Well Fracturing Sequence of Chang 7 Formation

In order to study the engineering problems of shale oil horizontal well platform fracturing mode, a three-dimensional geological model and a three-dimensional geomechanical model were constructed according to the characteristics of Chang 7 section in Ordos Basin. The fracture propagation under different well spacing, cluster spacing, and fracturing sequence was simulated. The PEBI grid was used to refine the local grid of the fractures and predict the performance of the multi-well pad. The finite element model was used to analyze the change of induced stress field. The numerical simulation analysis shows that: 1) Comparing the simulation results of sequence fracturing and zipper fracturing under the condition of 300 m well spacing, the induced stress generated by zipper fracturing is more conducive to the formation of complex fractures, improving the effect of transformation and higher production after fracturing. 2) Under the condition of 200 meters or 300 meters well spacing, zipper fracturing is carried out, and the cluster spacing is reduced from 5 meters to 3 meters. The stress shadow between wells and sections leads to adverse effects, and the production will decrease after fracturing. Therefore, 5 meters is recommended as the cluster spacing.

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, proppant transport and parameter optimization of multi-well pad fracturing treatment

This paper establishes a 3D multi-well pad fracturing numerical model coupled with fracture propagation and proppant migration based on the displacement discontinuity method and Eulerian-Eulerian frameworks, and the fracture propagation and proppant distribution during multi-well fracturing are investigated by taking the actual multi-well pad parameters as an example. Fracture initiation and propagation during multi-well pad fracturing are jointly affected by a variety of stress interference mechanisms such as inter-cluster, inter-stage, and inter-well, and the fracture extension is unbalanced among clusters, asymmetric on both wings, and dipping at heels. Due to the significant influence of fracture morphology and width on the migration capacity of proppant in the fracture, proppant is mainly placed in the area near the wellbore with large fracture width, while a high-concentration sandwash may easily occur in the area with narrow fracture width as a result of quick bridging. On the whole, the proppant placement range is limited. Increasing the well-spacing can reduce the stress interference of adjacent wells and promote the uniform distribution of fractures and proppant on both wings. The maximum stimulated reservoir volume or multi-fracture uniform propagation can be achieved by optimizing the well spacing. Although reducing the perforation-cluster spacing also can improve the stimulated reservoir area, a too low cluster spacing is not conducive to effectively increasing the propped fracture area. Since increasing the stage time lag is beneficial to reduce inter-stage stress interference, zipper fracturing produces more uniform fracture propagation and proppant distribution.

Challenges and best practices in interpreting crosswell strain signals to monitor multicrew zipper-fracturing operations

Crosswell strain measurements using low-frequency distributed acoustic sensing (LF-DAS) are an emerging technique to monitor hydraulic-fracture propagation. Qualitative interpretations of the strain rate data have been used to evaluate fracturing stimulation efficiency, hydraulic-fracture geometry, and crosswell communication. Limited studies have investigated the crosswell strain signals recorded by offset fibers during a zipper-fracturing treatment, though zipper fracturing becomes a routine method of stimulating horizontal wells in unconventional reservoirs. This gap will be filled in this research by presenting the methods we developed to investigate complicated LF-DAS signals. These approaches were further demonstrated using the field data sets recorded by two temporary sensing fiber cables during the zipper-fracturing operation of seven offset wells with two fracking crews operating simultaneously. By exploring, comparing, and presenting the LF-DAS data recorded in wireline and disposable fiber cables, this research shares the best practices for visualizing and interpreting crosswell strain signals. The LF-DAS data set shown in this study, is, to the best of our knowledge, one of the most complicated LF-DAS data sets ever presented. The approaches proposed here can be extended and applied to visualize and interpret different kinds of complicated LF-DAS signals recorded using permanent, wireline, and disposable fiber cables.

Hydraulic fracture network propagation in a naturally fractured shale reservoir based on the “well factory” model

In different geological and engineering conditions, it is important to study the stress interference of multiple fractures and interaction of natural fractures in “well factory” fracturing to improve the complexity of fracture networks. For this purpose, a 2D, coupled stress-seepage-damage field cohesive model with random propagation of hydraulic fracture was developed, and a random function was used to simulate the natural fractures distribution in a real reservoir. Orthogonal test was designed to investigate the geological and engineering parameters impacting the evolution of zipper fracturing and modified zipper fracture networks. The results show that in naturally fractured reservoirs, hydraulic fracture can open natural fractures and promote the formation of complex fracture networks by combined, induced manner. Higher stress differences lead to simpler fracture networks and straighter hydraulic fracture propagation paths; When pre-existing natural fracture is longer, the propagation direction of hydraulic fracture is more likely to be paralleled with natural fracture; However, the stress shadow effect of multiple fractures may potentially lead to failure of some hydraulic fracture initiation. In addition, the fracture initiation sequence of multiple fractures has a significant effect on the evolution of the fracture networks. The results of the study have important significance for parameter optimization of well factory fracturing in naturally fractured reservoirs.

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.

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.

Hydraulic Fracture Propagation in Denver-Julesburg Basin Constrained by Cross-Well Distributed Strain Measurements

Summary Evaluating hydraulic fracturing completion is critical for low porosity, low permeability unconventional reservoir development. In this study, we use low-frequency distributed acoustic sensing (LF-DAS) measurements to monitor the hydraulic fracturing in the Chalk Bluff field in the Denver-Julesburg (DJ) Basin, Colorado. Interpreting fracture-hits from crosswell LF-DAS data yields insights into the fracture geometry and propagation across and within two targeted formations: Niobrara and Codell. We observe significant differences in hydraulic fracture propagation between the two formations; the half length of hydraulic fractures in the Codell formation is much longer than that in the Niobrara. In addition, hydraulic fracture propagates significantly faster in Codell than in Niobrara under the same pumping rate. The differences could be explained by higher natural fracture density and potentially lower stress anisotropy in the Niobrara formation. We also observed different fracture orientations between the two formations and inconsistent fracture orientations within Niobrara. Hydraulic fractures observed in Codell oriented at 100 degree consistently, while two group of fracture azimuths (110 and 240 degrees) can be observed in Niobrara. The difference in fracture orientations in Niobrara and Codell indicates stress regime changes between the formations. The inconsistency of fracture azimuth in Niobrara may be caused by the zipper fracturing sequence. Strong cross-formation fracture connections between the two formations can also be observed, with different up-going and down-going fracture propagation velocities. These observations help us better understand the complex fracture geometry in the DJ Basin and provide critical constraints on the optimization of the unconventional reservoir development.

Simulation of Fracture Morphology during Sequential Fracturing

During hydraulic fracturing, the aperture of hydraulic fractures will shrink by the in-situ stress, but will not fully close because of the existence of proppant inside the fracture. In previous studies, few people noticed the existence of proppant, which has resulted in the inaccuracy of simulation results. In this study, based on the boundary element method, a numerical simulation model for sequential fracturing was established, which respectively considered the influence of proppant in staged fracturing and zipper fracturing. In addition, the influence mechanism of proppant on fracture morphology is then revealed. Simulation results show that the residual aperture of the previous hydraulic fracture, which was produced by proppant, may increase with the increase of proppant stiffness and fracture spacing and may also be shrunk by the dynamic propagation of subsequent hydraulic fracture. However, the residual aperture will rebound after hydraulic fracturing construction is finished. The shrinkage and rebound values of residual aperture of hydraulic fracture are usually less than 1 mm. In addition, at the same time, the residual aperture of previous hydraulic fracture may also influence the propagation of subsequent hydraulic fracture. These influences are represented by the bend of fractures in multistage fracturing and the intersection in zipper fracturing. With the increase of well spacing, the influence degree of residual aperture on subsequent fracture propagation is reduced. The previous hydraulic fracture cannot have a significant effect on the deflection of subsequent hydraulic fracture when fracture spacing is between 10 and 30 m. The above research has important guiding significance for controlling fracture morphology in hydraulic fracturing.

Numerical Analysis of Zipper Fracturing Using a Non-Planar 3D Fracture Model

Zipper fracturing has become one of the routine reservoir stimulation methods for developing unconventional resources such as shale oil and gas. A non-planar 3D fracture propagation model is used to study the fracture propagation behavior of zipper fracturing. The effects of stage time lag, staggered perforation cluster spacing, and horizontal principal stress contrast on the fracture geometry are analyzed through numerical simulation. The simulation results show that increasing the stage time lag can increase the fracture length and improve inter-well stimulation. For the formation with higher horizontal principal stress contrast, the staggered perforation cluster spacing has a minimum effect on the fracture configuration of zipper fracturing. For the formation with lower horizontal principal stress contrast, the fractures follow the curved paths and intersect with each other, which potentially has adverse effects on the stimulation of the subsequent stages.

Multiple Fracture Growth in Modified Zipper Fracturing

Operational parameters, such as multiple-cluster and well spacing, flow rates, and injection time, play a vital role in the deviation and growth of multiple-cluster hydraulic fractures during the implementation of modified zipper fracturing technology. Fundamental analysis of the dynamic zipper fracture growth because of changing flow rates, well spacing, and injection time does not appear to have been studied in the literature, although it is of critical importance for fracturing optimization. In this study, a fully coupled two-dimensional (2D) extended finite-element method (XFEM)–cohesive zone model (CZM) in combination with a phantom-node method (CPNM) was used in ABAQUS to investigate the initiation and propagation of multiple hydraulic fractures with 10 injection clusters that are distributed in two horizontal boreholes. This study aims to assist fracturing design to achieve minimal deviation in multiple fracture propagation and create deeper and much wider fractures between two horizontal boreholes of a modified zipper fracture (MZF) completion pattern. The growth of a hydraulic fracture in an XFEM region were verified by Khristianovic–Geertsma–de Klerk (KGD) zero toughness solution and good agreement with a negligible error of <2.5% was obtained. A series of simulation scenarios was carried out with various flow rates and three well spacings of 40, 60, and 80 m, over a long injection period to identify the optimum injection time in which a fracture grows straight with minimal deviation. The results show that to create smooth and deeper multiple hydraulic fractures in zipper fracturing, an increase in the flow rate is preferred, but the duration of the injection period should be optimized. The results of this study could be used to better address operational difficulties, such as multiple-fracture crossing and closure that is associated with unfavorable injection times, well spacing, and flow rates in MZF operations.

Analysis of influencing factors of stimulated reservoir volume (SRV) and zipper fracturing simulation in shale oil reservoirs of Hu326 Well area

Horizontal Well SRV of intensive stage and high–intensity for shale oil reservoirs of Hu326, which are different beddings and develop natural fractures locally, is conducive to the formation of complex fracture network with the intersection of main fractures, natural fractures, bedding fractures and other secondary fractures. At present, although there are systematic theories and methods for the study on the extension of hydraulic fracture after intersecting natural fractures and bedding planes, there is no quantitative research on specific parameters, and the research results are not applicable to actual exploitation. Based on the characteristics of shale oil reservoirs and combined with the theory of formation strength with weak surface, this article discusses the formation mechanism of fracture network of jimusar shale oil reservoir and improves the extension law of hydraulic fractures after intersecting with natural fractures and bedding planes. And taking the jimusar shale oil reservoir as the object, the extension law of hydraulic fracture is analysed and the geomechanical influencing factors and engineering control parameters of SRV are determined. The research results provide theoretical basis and parameter guidance for SRV of jimusar shale oil reservoir.

Theoretical understandings, key technologies and practices of tight conglomerate oilfield efficient development: A case study of the Mahu oilfield, Junggar Basin, NW China

A series of theoretical explorations and field tests have been carried out to efficiently develop the Mahu tight conglomerate oilfield in the Junggar Basin. Concepts of steered-by-edge fracturing and proactive fracturing interference were proposed. A series of innovative technologies were developed and implemented including optimization of 3-D staggered well pattern, proactive control and utilization of spatial stress field, and synergetic integration of multiple elements. Different from shale, the unique rock fabric and strong heterogeneities of tight conglomerate formation are favorable factors for forming complex fractures, small space well pattern can proactively control and make use of interwell interference to increase the complexity of fracture network, and the “optimum-size and distribution” hydraulic fracturing can be achieved through synergetic optimization. During pilot phase of this field, both depletion with hydraulically fractured vertical wells and volume fracturing in horizontal wells were tested after water injection through vertical wells, then the multi-stage fracturing with horizontal well was taken as the primary development technology. A series of engineering methods were tested, and key development parameters were evaluated such as well spacing, lateral length, fractures spacing, fracturing size, and fracturing operation process. According to geoengineering approach, the 100 m/150 m tridimensional tight-spacing staggered development method was established with systematic integration of big well clusters, multiple stacked pay zones, small well spacing, long lateral length, fine perforation clustering, zipper fracturing and factory operation. According to half-year production performance, 100 m/150 m small spacing wells outperformed 500 m/400 m/300 m spacing wells. Its average estimated ultimate recovery (EUR) of wells was identical with those best wells from large-spacing area. Compared with the overall performance of Mahu oilfield, the drainage efficiency and estimated recovery factor of this pilot were significantly boosted with improved economics.

Numerical simulation of stress shadow in multiple cluster hydraulic fracturing in horizontal wells based on lattice modelling

Multiple cluster hydraulic fracturing is widely used in the development of shale reservoir. The morphology of multiple fractures propagating from multiple cluster is complex due to the stress shadow effect. The design and operation of multiple cluster hydraulic fracturing in horizontal wells require adequate knowledge of the effect of different factors, including rock properties, in-situ stresses and fluid properties on the fracture morphology. In this paper, lattice simulation, a new particle based computational method was used to investigate the multiple cluster hydraulic fracturing in shale. The results showed that stress anisotropy and cluster spacing play an important role on geometry of the propagating fracture. The middle hydraulic fracture is restricted to propagate when cluster spacing is decreased. Simultaneous, two-step and sequential fracturing scenarios in a single horizontal well were simulated. The simultaneous and sequential fracturing showed to mainly affect the fracture propagation morphology with little effect on fracture length, while the middle fracture is shorter than its two side fractures in two-step fracturing. In two horizontal models, the simultaneous and sequential fractures showed similar morphology during multiple cluster hydraulic fracturing. In term of stimulated reservoir volume (SRV), zipper fracturing showed the largest SRV during multiple cluster hydraulic fracturing in two horizontal wells.

Modified zipper fracturing in enhanced geothermal system reservoir and heat extraction optimization via orthogonal design

Modified zipper fracturing in horizontal wells is an effective tool for promoting the energy productivity of hot dry rock based enhanced geothermal system. However, the following two questions are not fully understood: 1) what extent of heat extraction from geothermal reservoir can be improved; 2) how to obtain the optimized fracture morphology meeting heat extraction requirements. We established a two-dimensional horizontal wells hydraulic fracturing model to study the fracture propagation and effects of modified zipper fracturing, and developed a length index to evaluate the impacts of fracture morphology on reservoir heat transfer behavior. The critical length, which is half of the interval between two horizontal wells, determines whether fracture propagation has effects on heat extraction or not. The orthogonal design is used to study the sensitivity of factors affecting hydraulic fracturing and heat extraction. Fracture spacing has insignificant influence on morphology, and therefore we set it as 30 m which is the optimal spacing. According to different formation parameters (in-situ stress difference and elastic modulus), controlling fracturing fluid injection rate is the best choice to optimize fracture morphology. These research results would supply a guidance for designing hydraulic fractures to meet the maximum heat extraction in enhanced geothermal system. • We studied fracture propagation and thermal effects of modified zipper fracturing. • Crack length has significant impacts on heat extraction rather than crack width. • Critical length determines if crack propagation distinctly improve heat extraction. • Thirty-meter is the best fracture spacing for heat transfer enhancement in reservoir. • Injection flow rate controlling is an effective way to optimize fracture morphology.