Multi-cluster Fracturing Research Articles

Experimental Study on Diverter Transport Through Perforations in Multicluster Fracturing of Horizontal Well

Summary Temporary plugging and diverting fracturing of the horizontal well is the primary option to promote production for tight reservoirs. Successful entry of diverters into the perforation is the basis and prerequisite for effective plugging. However, the transport behavior of the diverter during multicluster fracturing remains unclear. In this paper, we build a large-scale diverter transport experimental system, capable of conducting experiments with large flow rates and high pressures. The concerned factors include the injection rate, perforation flow ratio (PFRO), fluid viscosity, and perforation angle. The results show that the diverter transport effect is significantly different because of different flow distribution among perforations. Also, the diverter can enter the perforation only when the flow rate of the perforation reaches a certain value. In addition, the minimum critical PFRO has an “oblique L-shaped” relationship with the injection rate. Although it is difficult for the diverter to enter the perforation on the high side of the horizontal wellbore, increasing the viscosity of the carrying fluid or using a multidensity mixed diverter can effectively solve this problem. Furthermore, the field case shows that the experimentally obtained diverter transport pattern can be applied to the field to predict the location of the diverter and improve the temporary plugging effect. The findings of this work lay a theoretical foundation for subsequent temporary plugging and diverting fracturing control.

Numerical Investigation of Hydraulic Fracture Deflection in Large-Angle Oblique Horizontal Wells with Staged Multi-Cluster Fracturing

Numerical Investigation of Hydraulic Fracture Deflection in Large-Angle Oblique Horizontal Wells with Staged Multi-Cluster Fracturing

Numerical Simulation of Proppant Transport Coupled with Multi-Planar-3D Hydraulic Fracture Propagation for Multi-Cluster Fracturing

Numerical Simulation of Proppant Transport Coupled with Multi-Planar-3D Hydraulic Fracture Propagation for Multi-Cluster Fracturing

Experimental and numerical investigation on particle diverters transport during hydraulic fracturing

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.

Simulation of multi-cluster fracture propagation by hydraulic fracturing in shale reservoir

China is rich in shale oil resources. With the continuous breakthrough of exploitation technology, shale oil has become an important strategic oil resource in China. As it turns out, Multi-cluster fracturing technology of horizontal well is an efficient method to produce shale oil. However, most of the research on the initiation and propagation of fractures in shale reservoirs is based on numerical simulation. In this paper, through the improved true triaxial hydraulic fracturing physical simulation system, the rock multi-cluster fracturing physical simulation experiment, obtain the fracturing process parameters, and analyze the mechanism of fracture generation. The numerical model of fracture is established to analyze the interfracture interference phenomenon and optimize the segmenting and clustering problem. The results showed that: The larger the fracturing fluid viscosity is within a certain range, the smaller the probability of fracturing fluid leakage is, and the more conducive to the formation of fractures; Cluster spacing has a great influence on fracture propagation. Compared with double clusters, three clusters of fractures are more conducive to forming effective fractures. This paper comprehensively considers the multi-field coupling effect in the fracturing process, deeply understands the mechanical parameters of natural fractures, improves the understanding of the regularity of mechanical parameters of natural fractures under the multi-field coupling effect of fluid-solid-temperature, and provides further guidance for the optimization design of fracturing parameters.

Hydraulic fracturing fracture propagation in tight sandstone reservoir

The tight sandstone reservoir of Shaximiao Formation of Middle Jurassic in Qiulin block of central Sichuan Basin, with a burial depth about 2200 to 2300 m, is characterized by undeveloped natural fractures. It is difficult to form complex fracture network during the implementation of traditional volume fracturing technology, affecting the benefit of tight sandstone gas development. A series of true triaxial hydraulic fracturing tests were conducted on the sandstone outcrop (dimensions: 300 mm × 300 mm × 300 mm), collected from this block, to investigate the initiation and propagation law of hydraulic fracture in tight sandstone reservoir under different conditions. Experimental results indicate that the peak value disturbance and increasing stress shadow in the back section of the pump pressure curve of staged multi-cluster fracturing in horizontal wells of sandstone reservoir will dominate the fracture morphology. The small interval leads to the increase in dynamic hydrostatic pressure on the fracture wall and increase in the deflection angle and degree of multi-cluster fractures, and it is easy to form crosscut fractures. The symmetrical middle segment perforation cluster inhibits the initiation and expansion of fractures, and the fractures will extend perpendicular to the maximum horizontal in situ stress direction and gradually approach the fractures on both sides and finally stop expanding. High-viscosity fracturing fluid is easy to form high hydrostatic pressure in the perforated section, reducing the difficulty of fracture initiation. The angle between the borehole and the minimum horizontal in situ stress ( σ h) will affect the propagation direction of hydraulic fractures. When the angle is large, the propagation direction of fractures is easily affected by the natural weak surface and induced stress field, resulting in fracture diversion. This study has mastered the initiation and propagation mechanism of artificial hydraulic fractures in tight sandstone reservoir, established the optimization chart of fracturing parameters in sandstone reservoir, and has certain guiding significance for the establishment of on-site fine optimization fracturing scheme design.

Numerical Study of the Effect of Perforation Friction and Engineering Parameters on Multicluster Fracturing in Horizontal Wells

Simultaneous multiple-fracture treatments in horizontal wellbores have become one of the key methods for economically and efficiently developing oil and gas resources in unconventional reservoirs. However, field data show that some perforation clusters have difficulty propagating fractures due to the internal mechanism of competing initiation and propagation among the fractures. In this paper, the physical mechanisms that influence simultaneous multiple-fracture initiation and propagation are investigated, and the effects of engineering parameters and in situ conditions on the nonuniform development of multiple fractures are discussed. A 3D fracture propagation model was established with ABAQUS to show the influence of the stress shadow effects and dynamic partitioning of the flow rate by simulating the propagation of multiple competing fractures generated in the perforation clusters. Based on the results of these simulations, simultaneous flow in multiple fractures can propagate evenly. Through adjusting the number of perforations in each cluster or the perforation diameter, the effect of the stress shadow can be significantly reduced by increasing the perforation friction, and the factors that affect the development of multiple fractures are changed, from the stress shadow effect to the dynamic partitioning of the flow rate. When the stress shadow effect is dominant, increasing the fracturing fluid viscosity promotes the uniform development of multiple fractures and increases the fracture width. When the dynamic partitioning of the flow rate is dominant, increasing the injection rate greatly affects the uniform development of multiple fractures.

Large-Section and Multi-Cluster Fracturing Technology for Horizontal Wells in the Jimsar Shale Oil Reservoir

Reservoirs in the Lucaogou Formation of the Jimsar Sag have poor physical properties, strong heterogeneity, and poor crude oil fluidity. Accordingly, it is difficult to accurately stimulate favorable reservoirs with horizontal wells, and staged fracturing has only a limited positive effect on production. Contradiction is highlighted by the fact that a high investment yields only a low level of output. In order to solve these problems, non-uniform limited-entry perforation was conducted to improve the distribution of fracturing fluid in the fracture clusters in the sections. Temporary plugging at the fracture entrance and in the fracture were adopted to increase the net pressure, so as to generate complex fracture networks. In addition, fracturing parameters were optimized to facilitate the balanced initiation of multiple clusters of fractures in the sections. Finally, the large-section and multi-cluster fracturing technology suitable for the horizontal wells in the Jimsar shale oil reservoir was developed. Field applications showed that the technology can enhance the stimulation and production of shale oil reservoirs, and provide strong support for the economic and effective development of the shale oil reservoir in the Jimsar Sag.

Research on optimization of perforation parameters for formation fractures based on response surface optimization method.

For staged multi-cluster fracturing, methods for controlling perforation friction to adjust the flow distribution of each cluster can effectively promote the uniform extension of multiple fractures but lacks a fast and quantitative optimization method for different perforation parameters of each cluster. By establishing a numerical model of single-stage three-cluster flow-limited fracturing under stress-seepage coupling, and based on the response surface optimization method, fully considering the impact of perforation parameters interaction among three perforation clusters, according to the regression equation fitted under the global response, the rapid optimization of perforation parameters of segmented multi-cluster fracturing model is realized. The results show that: in determining the three factors of the study, it is found that there is an obvious interaction between the number of intermediate cluster perforations and the number of cluster perforations on both sides, the number of cluster perforations on both sides and the diameter of intermediate cluster perforations, the response surface optimization method gives the optimal perforation parameter combination of three clusters of fractures under global response; When the perforation parameters were combined before optimization, the fracture length difference was 32.550m, and the intermediate perforation cluster evolved into invalid perforation cluster, when the perforation parameters were combined after optimization, the fracture length difference was 0.528m, the three perforation clusters spread uniformly, and there are no invalid clusters. At the same time, the regression equation under the response is optimized before and after the comparison between the predicted value of the equation and the actual simulation value. It is found that the estimated deviation rate of the equation before optimization is 1.2%, and the estimated deviation rate after optimization is 0.4%. The estimated deviation rates are all less, and the response regression equation based on the response surface optimization method can quickly optimize the perforation parameters. The response surface optimization method is suitable for the multi parameter optimization research of formation fracturing which is often affected by many geological and engineering factors. Combining with the engineering practice and integrating more factors to optimize the hydraulic fracturing parameters, it is of great significance to improve the success rate of hydraulic fracturing application.

Practice of high-intensity volume fracturing in the Shaximiao Formation tight sandstone gas reservoirs of the Qiulin Block, central Sichuan Basin

In order to solve the difficulties in the volume fracturing stimulation of Middle Jurassic Shaximiao Formation tight sandstone reservoirs in the Qiulin Block of Central Sichuan Basin and explore the adaptability of high-intensity volume fracturing technology, we selected the outcrop samples of Shaximiao Formation tight sandstone in the Qiulin Block to carry out the physical simulation experiment of true triaxial hydraulic fracturing. On this basis, horizontal well cluster perforation was optimally designed by using the production prediction model of staged multi-cluster fracturing horizontal wells. Then, based on the liquid control and proppant increase mode, three rounds of pilot tests were carried out on the tight sandstone reservoirs in this area. And the following research results were obtained. First, natural fractures in the Shaximiao Formation tight sandstone reservoir of the Qiulin Block are undeveloped, and hydraulic fractures are morphologically dominated by symmetric double-wing fractures, so complex fracture networks can be hardly formed. In addition, the reservoir is of medium to strong water sensitivity, so conventional volume fracturing is not adaptive to the reservoir stimulation in this block. Second, the connotation of high-intensity volume fracturing technology is to carry out multi-cluster perforation in each section to form multiple independent double-wing fractures and to implement the proppant injection mode of liquid control and proppant increase to reduce the inflow fluid while ensuring the high-intensity proppant injection, so as to reduce the damage of inflow fluid to the formation. Third, there are 10 fracturing sections in Well Qiulin 207-5-H2, with 7–12 clusters in each section, and the displacement is in the range of 16–18 m3/min. According to the fluid control and proppant increase mode, 12146 m3 slick water and 4170 t proppant are injected in total. The tested production rate and absolute open flow of natural gas after the fracturing are up to 83.88 × 104 m3/d and 214.05 × 104 m3/d, respectively. Fourth, with the decrease of cluster spacing, the cumulative gas production increases gradually, but when the cluster spacing is less than 15 m, the increase amplitude of cumulative gas production decreases. Fifth, when the proppant injection intensity is lower than 6 t/m, the tested gas production per kilometer of stimulated section in a horizontal well overall presents an increasing trend with the increase of proppant injection intensity. When the proppant injection intensity is higher than 6 t/m, however, the tested gas production per kilometer of stimulated section does not increase significantly with the increase of proppant injection intensity. Sixth, as the included angle between the borehole trajectory and the direction of maximum horizontal principal stress increases, the tested gas production per kilometer of stimulated section overall presents an increasing trend. When the hydraulic fracture is nearly perpendicular to the borehole, the effective drainage area is the largest and the tested gas production per kilometer of stimulated section is also the highest. In conclusion, the fracturing mode of high production well has a borehole trajectory of large included angle, perforation cluster spacing of 10 m or so, proppant injection intensity of 5 t/m and large-displacement slick water + continuous injection of combined particle size proppant.

Fracturing and production analysis of the efficacy of hydraulic fracture stage reduction in the improvement of cost‐effectiveness in shale oil development: A case study of Jimsar shale oil, China

AbstractThe effective production of shale oil is important for the cost‐effectiveness in the oil and gas industry. Oil price volatility and uncertainty require suitable hydraulic fracturing schemes to maximize production and to minimize fracturing costs. Since the reduction of fracture stage saves bridge plugs and fracturing costs, it is considered as a possible method to improve the cost‐effectiveness. To maintain the stimulated reservoir volume, cluster numbers per stage are consequently increased, which may induce nonuniform fracture propagation and interference which affects the stimulation efficiency. This study conducts a comparative study for single‐stage (reduced) and two‐stage (not reduced) fracturing schemes with the same fracturing fluid volume. Reservoir geomechanics and rock mechanics analyses are conducted for Jimsar shale oil in north‐western China with a single‐stage multicluster fracturing model established. Fracture geometries and key factors are investigated followed by the evaluation of the fractured well production. The relationship between multicluster parameters and production is quantified. Meanwhile, two‐stage fracturing modeling is also set up to compare with single‐stage fracturing modeling. Fracturing results show that although single‐stage fracturing results in nonuniform fracture propagation, the overall stimulation is satisfactory. Production results show that the nonuniform growth of fracture clusters leads to different contributions to the production from individual clusters and the production from inner fractures is significantly inhibited. Compared with two‐stage fracturing with the same pumping volume, it is found out that single‐stage fracturing can lead to cost‐effective horizontal well production in Jimsar shale oil given certain values of hydraulic fracturing costs, and the reduced stage scheme is preferable in low oil price and high uncertainty scenarios.

Effects of Fracturing Parameters on Fracture Unevenness During Large-Stage Multi-Cluster Fracturing in Horizontal Wells

Horizontal wellswith multi-cluster fracturing technology is an effective approach to exploit unconventional hydrocarbon reservoirs. The on-site diagnosis results indicate that multi-cluster fractures always tend to propagate unevenly due to stressinterference, therefore it is very essential to study the effect of fracturing parameters on fracture propagation unevenness. In this paper, the unconventional fracturing model (UFM, Unconventional Fracturing Model) is used to study the effect of multi-cluster fracturing parameters on fracture unevenness in a large stage. This model has been validated with the actual fracturing case on-site in the Longmaxi shale. The investigated parameters include completion parameters (cluster spacing, number of perforations per cluster), pumping parameters (fluid injection intensity and proppant injection intensity). Our simulation results show that firstly reducing fracture spacing will increase stress interference, andhydraulic fractures exhibit a “radial” pattern. Secondly, reducing the perforation number of a single cluster can promote the more uniform propagation of multi-cluster fractures. Thirdly, increasing the fluid injection intensity will increase the fracture length, but will also increase the fracture unevenness. Besides, the injection strength of the proppant has a little effect on the average fracture length and the unevenness of the fracture length. Finally, setting a reasonable cluster spacing and injection fluid strength can obtain a more uniform fracture propagation. Meanwhile reducing the number of perforations per cluster can also reach the goal of propagating evenly. This paper provides a certain reference for the optimization of multi-cluster fracturing parameters in large-stage and multi-cluster wells.

Progress and development directions of stimulation techniques for ultra-deep oil and gas reservoirs

Abstract By reviewing the development history of stimulation techniques for deep/ultra-deep oil and gas reservoirs, the new progress in this field in China and abroad has been summed up, including deeper understanding on formation mechanisms of fracture network in deep/ultra-deep oil and gas reservoir, performance improvement of fracturing fluid materials, fine stratification of ultra-deep vertical wells, and mature staged multi-cluster fracturing technique for ultra-deep and highly deviated wells/horizontal wells. In light of the exploration and development trend of ultra-deep oil and gas reservoirs in China, the requirements and technical difficulties in ultra-deep oil and gas reservoir stimulation are discussed: (1) The research and application of integrated geological engineering technology is difficult. (2) The requirements on fracturing materials for stimulation are high. (3) It is difficult to further improve the production in vertical profile of the ultra-deep and hugely thick reservoirs. (4) The requirements on tools and supporting high-pressure equipment on the ground for stimulation are high. (5) It is difficult to achieve efficient stimulation of ultra-deep, high-temperature and high-pressure wells. (6) It is difficult to monitor directly the reservoir stimulation and evaluate the stimulation effect accurately after stimulation. In line with the complex geological characteristics of ultra-deep oil and gas reservoirs in China, seven technical development directions are proposed: (1) To establish systematic new techniques for basic research and evaluation experiments; (2) to strengthen geological research and improve the operational mechanism of integrating geological research and engineering operation; (3) to develop high-efficiency fracturing materials for ultra-deep reservoirs; (4) to research separated layer fracturing technology for ultra-deep and hugely thick reservoirs; (5) to explore fracture-control stimulation technology for ultra-deep horizontal well; (6) to develop direct monitoring technology for hydraulic fractures in ultra-deep oil and gas reservoirs; (7) to develop downhole fracturing tools with high temperature and high pressure tolerance and supporting wellhead equipment able to withstand high pressure.

Optimization of Multi-stage and Multi-cluster Fracturing Technology for Horizontal Wells in Jilantai Buried Hill

The Jilantai oil field is rich in reserves of metamorphic rock potential reservoirs, Some wells drilled in the early stages, recorded wells see better oil and gas display, but the oil test yield is low, so it needs fracturing to put into production. Conventional fracturing has a small range, low output and short stable production period. In order to improve the effect of reservoir reconstruction, multi-segment and multi-cluster fracturing technology for Jilantai metamorphic rock horizontal wells has been developed.. According to the well trajectory and logging interpretation results of well s1-1, we have formed a volume fracturing mode of large-scale +large displacement + low sand ratio plug-in plus sand by optimizing the staged cluster perforation scheme, fracture pattern scale and operation parameters, thus communicating more natural fractures and forming a complex fracture network system. The well is divided into 10 sections and 28 clusters of fracturing, after pressure of 49.2t daily oil, 31.49t of daily oil, the current production is stable, the effect of fracturing increase yield is remarkable. Through the successful application of multi-stage and multi-cluster fracturing technology in this well, a new technology of volume fracturing for metamorphic reservoir in Jilantai formed, which lays a foundation for the efficient development of this area.

Experimental study on the thermal damage characteristics of cement stone

Aiming at the problem of cement ring sealing failure during deep high temperature shale gas exploitation, comprehensively considering the influence of the characteristics of multi-cluster fracturing of multiple horizontal wells and formation temperature, the cementing cement the southwest region is taken as the research object. After exposure to different temperatures (95?C and 135?C) and for different times (5, 10, and 20 times), axial and triaxial tests with different confining pressures (0, 5 MPa, 15 MPa, and 30 MPa) were carried out. The research shows that: the stress-strain curve of cement stone after heat treatment can be divided into four stages: compaction, elastic, yield, and post-peak stage; as the confining pressure increases, the compaction phase disappears, the yield phase increases, and we see the transition from brittle to ideal plasticity after the peak and as the temperature and number of thermal cycles increase, the cohesive force decreases significantly, and the internal friction angle shows a slight increase. The elastic modulus and the peak strength decreased.

Optimized completion design for triggering a fracture network to enhance horizontal shale well production

Horizontal well drilling accompanied by multicluster fracturing is unlocking shale gas resources. It is essential to create a large-scale fracture network connecting the reservoir to the wellbore by activating the preexisting natural fractures (NFs) through well completion optimization. Coupling the geomechanical characteristics of a shale formation to achieve the maximum stimulated reservoir volume (SRV), we develop a new completion optimization pattern for a single horizontal wellbore called modified alternate fracturing. The new model is a combination of conventional simultaneous and alternate fracturing. Unlike the previous studies that mainly concentrated on predicting the quasistatic dilation of NF failure, the presented paper assesses the dynamic evolution progression of NF growth under different failure criteria. Additionally, an analysis of how the modified alternate fracturing influences the fracture network is performed. The results demonstrate that a NF may be crossed, opened or slipped by an approaching hydraulic fracture (HF), depending on the tensile or shear stresses exerted on the NF. The combination of perforation parameter optimization and injection rate real-time control is able to utilize induced stresses to form a complex fracture network. A field application reveals that the modified well completion pattern performed better than conventional simultaneous fracturing due to increasing the near-wellbore and far-field fracture complexity. In addition, no additional equipment such as bridge plugs or coiled tubingis needed during operation, reducing potential issues.

A numerical method for simulating planar 3D multi-fracture propagation in multi-stage fracturing of horizontal wells

To resolve the issue of design for multi-stage and multi-cluster fracturing in multi-zone reservoirs, a new efficient algorithm for the planar 3D multi-fracture propagation model was proposed. The model considers fluid flow in the wellbore-perforation-fracture system and fluid leak-off into the rock matrix, and uses a 3D boundary integral equation to describe the solid deformation. The solid-fluid coupling equation is solved by an explicit integration algorithm, and the fracture front is determined by the uniform tip asymptotic solutions and shortest path algorithm. The accuracy of the algorithm is verified by the analytical solution of radial fracture, results of the implicit level set algorithm, and results of organic glass fracturing experiment. Compared with the implicit level set algorithm (ILSA), the new algorithm is much higher in computation speed. The numerical case study is conducted based on a horizontal well in shale gas formation of Zhejiang oilfield. The impact of stress heterogeneity among multiple clusters and perforation number distribution on multi-fracture growth and fluid distribution among multiple fractures are analyzed by numerical simulation. The results show that reducing perforation number in each cluster can counteract the effect of stress contrast among perforation clusters. Adjusting perforation number in each cluster can promote uniform flux among clusters, and the perforation number difference should better be 1-2 among clusters. Increasing perforation number in the cluster with high in situ stress is conducive to uniform fluid partitioning. However, uniform fluid partitioning is not equivalent to uniform fracture geometry. The fracture geometry is controlled by the stress interference and horizontal principal stress profile jointly.

Production calculation of multi-cluster fractured horizontal well accounting for stress shadow effect

The objective of this study is to investigate the effect of stress shadow in the process of multi-cluster fracturing on the production of horizontal well. First, the model of simultaneous propagation of multiple fractures is established with coupling rock deformation, fluid flow in fracture and wellbore. This model is solved by using iterative procedure. Based on the solution of hydraulic fracturing, a modified method is presented to calculate transient production of horizontal well by using source function and superposition principle. Two scenarios in regards to fracture conductivity have been considered. In the situation that fracture conductivity is varying, the variation of fracture width is captured by joint element method. Thus, the relation between model of hydraulic fracturing and of production calculation is well established. Both models have been verified, and the effect of stress shadow on well production has been analysed. Results show that accounting for stress shadow helps to increase well production. As fluid is extracted out from wellbore, the fracture conductivity would decrease rapidly, and then it leads to decrease well production. Moreover, the model is extended to calculate production of simultaneously fractured wells. [Received: November 30, 2017; Accepted: April 15, 2018]

Numerical simulation of non-planar fracture propagation in multi-cluster fracturing with natural fractures based on Lattice methods

Multi-cluster fracturing in horizontal wells is a key technology for successful development of ultra-low permeability reservoir. The propagation of hydraulic fracture during multi-cluster fracturing is complicated, especially in shale reservoir with multiple natural fractures. The design and operation of multi-cluster fracturing requires adequate understanding of influence of different factors on hydraulic fracture propagation. Up to now, many scholars have studied the hydraulic fracture morphology in multi-cluster fracturing, but few have analyzed the effect of natural fractures on hydraulic fracture propagation during multi-cluster fracturing. In this paper, a new computationally version of the particle-based model is established by Xsite to study the fracture propagation in multi-cluster fracturing with natural fractures. The tensile strength and rock toughness are calculated, and tri-axial experiments are performed to verify the accuracy of model. Simulation results show that cluster spacing and in-situ stress difference have a significant influence on the length of the hydraulic fracture and the morphology of fracture. The length of middle fracture increases with the increase of the cluster spacing, but decreases with the increase of the in-situ stress difference during multi-cluster fracturing with three natural fractures. The enhancing of cluster spacing can reduce the deflection of left and right fractures, and the increase of the in-situ stress difference can improve the ability of middle fracturing penetrating the natural fracture. Three fracturing sequence of synchronous fracturing, two-step fracturing and sequential fracturing is simulated. The left and right fractures can always penetrate the natural fracture with different fracturing sequence. But the middle fracture shows different morphology of arresting by natural fracture (during synchronous fracturing), partially penetrating the natural fracture (during two-step fracturing) and directly penetrating the natural fracture (during sequential fracturing). Different cement strengths of natural fracture are analyzed. The increase of strength of natural fracture can enhance the ability of hydraulic fracture penetrating the natural fracture. Hydraulic fracture is arrested by weak natural fracture (shear strength of 0.5 MPa), resulting in natural fracture opening. As hydraulic fracture intersecting with strong natural fracture (shear strength of 20 MPa), the hydraulic fracture can penetrate the natural fracture directly without natural fracture opening.

Chemical-Diversion Agents Can Aid Acid Fracturing in Openhole Horizontal Wells

This article, written by JPT Technology Editor Judy Feder, contains highlights of paper SPE-191415-18IHFT-MS, “Experimental Investigation on Tool-Free Multistage Acid-Fracturing of Openhole Horizontal Wells Using Diversion Agents,” by Jianye Mou, SPE, and Shaodan Tao, China University of Petroleum-Beijing; Xuezhi Hui, CNPC; and Lei Wang, Shicheng Zhang, and Xinfang Ma, China University of Petroleum-Beijing and State Key Laboratory of Petroleum Resources and Prospecting, prepared for presentation at the 2018 SPE International Hydraulic Fracturing Technology Conference and Exhibition, Muscat, Oman, 16–18 October. The paper has not been peer reviewed. In openhole horizontal wells in high-temperature, naturally fractured, deep formations, mechanical segmentation between stages for multistage fracturing operations is high-risk, costly, and sometimes ineffective. Acid fracturing using chemical diversion agents as an alternative to mechanical segmentation has been pilot-tested in some fields with satisfactory results, but the technique has not been studied experimentally or numerically extensively before now. This paper presents a newly designed triaxial fracturing system and describes a series of experiments that verified the validity of tool-free chemical diversion for multistage fracturing of openhole horizontal wells. Introduction Traditional staged fracturing technologies include double-pack single-slip multistage fracturing, packer/sliding-sleeve fracturing, and hydraulic-jetting multi-stage fracturing. These technologies use tools to achieve segmentation between stages. The tools are neither effective nor reliable under some circumstances such as multicluster fracturing inside a stage; refracturing of horizontal wells; and in deep, high-temperature, vuggy, naturally fractured formations. Diverted fracturing with chemical diversion agents has become an option. Diversion has been applied in matrix acidizing and, more recently, in fracturing. Diverted fracturing can be used for both far-field and near-wellbore diversion. Far-field diversion shifts the fracture to unstimulated areas to create complex fractures that increase reservoir contact volume. This paper focuses on near-wellbore diversion, which can be accomplished using mechanical and chemical methods. Recently developed chemical diversion materials include ball sealers, particulates of various sizes, and fiber. These diverters need to be degradable or dissolvable in formation fluids over time. Particulate diverters are available in sizes ranging from several millimeters to a micrometer, allowing them to plug orifices of various sizes and to bridge perforations, fractures, and vugs. Degradable fiber is another commonly used diverter. It can bridge the fracture or fill the gap between particulate diverters to form a tight barrier. Combinations of particulate diverter and fiber have been used often. Diverted fracturing has been used in horizontal wells for refracturing, fracturing multiple clusters in one stage, and even for segmentation in openhole horizontal wells. Refracturing targets regions that have adequate reserves for recovery but are underdeveloped because of non-uniform cluster stimulation. Chemical diversion can restimulate these reservoirs.