Simultaneous Propagation Of Multiple Fractures Research Articles

Numerical Study on Simultaneous Propagation of Multiple Fractures: A Method to Design Nonuniform Perforation and In-Stage Diversion

Summary The uneven propagation of multifractures is a key factor restricting production growth due to stress shadow and heterogeneity. To date, limited-entry fracturing techniques, nonuniform perforation, and in-stage diversion have been commonly used to promote even multifracture growth. In this study, a fully coupled multiple pseudo-3D (P3D) fracture simulator has been developed to examine the competitive propagation of multifractures during multicluster fracturing in a horizontal well. The present model considers stress interaction among multiple fractures, perforation erosion, fluid distribution among clusters, and in-stage diversion. The results of the model are validated against the reference data. Using the model, a series of numerical simulations are performed to investigate multifracture propagation with nonuniform perforation and in-stage diversion fracturing. We estimate the value of stress interaction for different fractures and time based on the approximate solution of Perkins-Kern-Nordgren (PKN) fracture in the viscosity-dominated regime and improve the dimensionless parameter that characterizes the competition between stress interaction and perforation friction. The fluid distributes evenly when the dimensionless parameter is less than unity (perforation friction is larger than stress interference). Based on this dimensionless parameter, a method to design nonuniform perforation and in-stage diversion is proposed. Results show that in the case of homogeneous in-stage stress, the perforation parameters should be selected under the condition that the dimensionless parameter is less than unity. In the case of heterogeneous in-stage stress and based on the perforation parameters selected under homogeneous stress conditions, the perforation holes in the high-stress cluster should be increased, making the reduction of perforation friction equal to the value of the in-stage stress heterogeneity. The stress heterogeneity can be balanced by decreasing the perforation friction of the high-stress clusters. In this way, nonuniform perforation under heterogeneous in-stage stress conditions can be designed quantitatively without numerical simulation. For in-stage diversion treatment, a method to design the number of ball sealers is proposed based on the results of nonuniform perforation, and only several or even zero groups of simulation are necessary to find the optimal number of ball sealers. A series of numerical simulations shows that the proposed design method is reliable and achieves a satisfactory result in an actual field case. The results can be helpful for nonuniform perforation and in-stage diversion design for multicluster fracturing in a horizontal well.

Study on multi-fracture propagation based on the uniform coefficient of fracture propagation

Multi-cluster fracturing is one of the most critical technologies for the efficient exploitation of unconventional reservoirs, and fracture uniform propagation during synchronous multi-fracture propagation is the goal of hydraulic fracturing. Most of the current studies on the simultaneous propagation of multiple fractures characterize the degree of uniform fracture propagation by the difference in fracture length or width, while the uniformity of multiple fractures is affected by multiple fracture parameters simultaneously, which makes such a characterization method biased. In this article, the stress interference of multiple fractures, fluid flow in fractures, flow distribution, and fracturing fluid filtration are comprehensively considered to establish the synchronous propagation model of multiple fractures in horizontal wells, and the fracture propagation patterns are simulated for different parameters. Then, the propagation uniformity coefficient of multiple fractures is proposed to represent the propagation uniformity of multiple fractures. The fracture uniformity coefficient is the result of the combination of fracture length, fracture width, inlet volume and fracture deflection angle, which can more comprehensively represent the uniformity of fracture propagation. Based on the uniformity coefficient of multi-fracture propagation, the law of multi-fracture interference is studied, and the parameters of multi-fracture propagation are optimized combining with an engineering background.

Field-Scale Experimental Study on the Perforation Erosion in Horizontal Wellbore under Real Fracturing Conditions

Limited-entry fracturing (LEF) technology is a widely used method to realize the simultaneous propagation of multiple fractures in horizontal wells. The key of this technology is to create high perforation friction to maintain the high treatment pressure in the wellbore and realize the uniform fluid entry of multi-fractures; however, high perforation friction cannot be effectively maintained due to the serious perforation erosion effect. Considering that the current laboratory studies mostly used small fluid injection flowrate, low injection pressure, and small proppant dosage, this study has developed a field-scale flow system to investigate the effect of various factors on perforation erosion under real field conditions. The filed-scale flow system uses the real fracturing trucks, proppant, and perforated wellbore, the fluid flow rate through perforation could reach 200 m/s and the injection pressure could reach 105 MPa. The effects of different parameters, such as injection flow rates, proppant concentration, proppant type, proppant size, and carrying fluid viscosity, on the perforation erosion were investigated. The experimental results show that: (1) The perforation friction during erosion goes through two stages, i.e., the roundness erosion stage and the diameter erosion stage. The reduction of perforating friction mainly occurred in the first stage, which was completed after injecting 1 m3 proppant. (2) After erosion, the perforation changes from the original circular shape to a trumpet shape, the inner diameter is much larger than the outer diameter. (3) The more serious perforation erosion is caused by the conditions of high injection flow rate, large proppant size, using ceramic proppant, and low viscosity fluid. The findings of this study can help for a better understanding of perforation erosion during the limited-entry fracturing in the horizontal wells, and also could promote the establishment of a theoretical model of perforation erosion under the field-scale conditions.

Optimization method of fracture parameters in multistage fracturing of horizontal wells

ABSTRACT Analyzing the simultaneous propagation of multiple fractures in horizontal wells is a challenging task. Although many fracture propagation models exist, the primary controlling factors are unclear. A fast and relatively accurate optimization method is urgently needed. In this study, a semi-analytical model of fracture propagation is established that couples stress interaction with fracture propagation using the energy balance principle, which can run a complete simulation process within 10 seconds. The simulation results show that the pumping rate, perforation cluster number, and fracturing fluid viscosity are the primary factors controlling fracture propagation. For two areas with different elastic modulus, the process of parameter optimization is analyzed. The reservoir with an elastic modulus of 10 MPa has an optimal perforation cluster number of 5, while the reservoir with an elastic modulus of 35 MPa has an optimal perforation cluster number of 8. The results show that the area with larger elastic modulus need to set a larger number of clusters, while the area with smaller elastic modulus have a smaller number of perforation clusters. This work aims to provide a practical and simple workflow for optimizing the fracture design of fractured horizontal wells.

Quantitative analysis of the sequential initiation and simultaneous propagation of multiple fractures in shale gas/oil formations

The excessive reach of new fractures during hydraulic fracturing can interact with the pre-existing fracture system during reservoir stimulation. This phenomenon is known as Frac Driven Interaction (FDI), which can greatly compromise the production performance of the wells. The key to minimizing the impact of FDI is to effectively control the length of fractures through a proper fracturing design. This study provides a computer program that outputs the length and initiation time of each fracture to quantitively analyze the sequential initiation and propagation of multiple fractures. Parametric analysis results show that increasing the injection rate, apparent viscosity, and the dilatancy of the fluid can help shorten fracture length and increase fracture count, which ultimately mitigate fracture interaction. Higher dilatancy mainly contributes to the resultant populated length of fractures, while a higher injection rate mainly contributes to the resultant number of fractures. The results also show that a better estimation of critical fracture width is critical to improving the accuracy of the simulation. The effect of various parameters on the FDI index is analyzed i.e. decreasing the injection time from 2h to 1h has led to a 46% decrease of fracture length while the number of fractures is only reduced from 10 to 8. It is found that factors that lead to a higher energy dissipation rate can result in shorter fractures and higher fracture counts. The methodology and results of this paper can deepen our understanding of the fracture interaction mechanism, which may also be used as a guideline and tool to control fracture length and count to mitigate FDI issues.

Use of Mathematical Model to Predict the Maximum Permissible Stage Injection Time for Mitigating Frac-Driven Interactions in Hydraulic-Fracturing Shale Gas/Oil Wells

Abstract Frac-driven interactions (FDIs) often lead to sharp decline in gas and oil production rates of wells in shale gas/oil reservoirs. How to minimize the FDI is an open problem in the oil and gas industry. Xiao et al.’s (2019, “An Analytical Model for Describing Sequential Initiation and Simultaneous Propagation of Multiple Fractures in Hydraulic Fracturing Shale Oil/Gas Formations,” Energy Sci Eng., 7(5), pp. 1514–1526.) analytical model for two-fracture systems was extended in this study to obtain a general model for handling multiple fractures. The general model was used to identify engineering factors affecting the maximum permissible stage fluid injection time for minimizing FDI. On the basis of model results obtained, we found that increasing fluid injection rate can create more short fractures and thus increase the maximum permissible stage injection time before FDI occurs. Use of dilatant type of fracturing fluid (n > 1) can reduce the growth of long fractures, promote the creation of more short fractures, and thus increase the maximum permissible stage injection time before FDI occurs. It is also expected that injecting dilatant type of fracturing fluid at high rate will allow for longer injection time and thus larger injection volume, resulting in larger stimulated reservoir volume (SRV) with higher fracture intensity and thus higher well productivity and hydrocarbon recovery factor.

An Equivalent Representation of Multiple Hydraulic Fractures with a Fewer Number of Fractures

Summary Multiple hydraulic fractures are often generated simultaneously from a wellbore to increase efficiency of reservoir stimulation. Numerical modeling of such a system of fractures is computationally costly, especially if the goal is to simulate numerous stages, each containing multiple fractures, on different wells, which is the current trend in the petroleum industry. To address the challenge, this study defines a method and a workflow to represent the simultaneous propagation of multiple fractures with a reduced number of equivalent fractures that accurately describes the overall fracture geometry, the created surface area, the propped surface area, the fluid leakoff, and the resulting induced stresses, as resulting from the original configuration. A hybrid approach is used, in which a combination of physical modeling and data science is involved. We first develop a database of numerical solutions using a fully coupled hydraulic fracturing simulator. The equivalent fracture representation is quantified for each set of problem parameters presented in the database. Then, the results of the database solutions are used to tackle more general cases with field pumping schedules and rock properties. Several numerical examples are presented to validate and illustrate the developed concept.

A novel approach of tight oil reservoirs stimulation based on fracture controlling optimization and design

Abstract To deal with the stress interference caused by simultaneous propagation of multiple fractures and the wettability reversal and physical property changes of the reservoir caused by fracturing fluid getting in during large-volume fracturing of tight oil reservoirs through a horizontal well, a non-planar 3D fracture growth model was built, wettability reversal characterizing parameters and change of relative permeability curve were introduced to correct the production prediction model of fractured horizontal well, a fracturing design optimization software (FrSmart) by integrating geological and engineering data was developed, and a fracturing design optimization approach for tight oil reservoirs based on fracture control was worked out. The adaptability of the method was analyzed and the fracture parameters of horizontal wells in tight oil reservoirs were optimized. The simulation results show that fracturing technology based on fracture control is suitable for tight oil reservoirs, and by optimizing fracture parameters, this technology makes it possible to produce the maximum amount of reserves in the well-controlled unit of unconventional reservoirs. The key points of fracturing design optimization based on fracture control include increasing lateral length of and reducing the row spacing between horizontal wells, increasing perforation clusters in one stage to decrease the spacing of neighboring fractures, and also avoiding interference of old and new fracturing wells. Field tests show that this technology can increase single well production and ultimate recovery. Using this technology in developing unconventional resources such as tight oil reservoirs in China will enhance the economics significantly.

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]

An analytical model for describing sequential initiation and simultaneous propagation of multiple fractures in hydraulic fracturing shale oil/gas formations

AbstractFrac‐driven interactions (FDI) cause sharp declines in gas and oil production rates followed by slow recovery or no recovery. How to minimize the FDI is an open problem for researchers and engineers to solve in the energy industry. An analytical model is presented in this paper for simulating simultaneous propagations of multiple fractures to identify factors affecting FDI. Result of sensitivity analyses with the model indicates that increasing fluid injection rate can depress the growth of long fractures and thus reduce FDI intensity. Using dilatant fracturing fluids can slow down the growth of long fractures and thus reduce the chance of FDI. In order to better apply the model to solving the FDI problem, future research should include determination of the critical fracture width in different types of shale gas/oil formations.

Development of a fully implicit 3-D geomechanical fracture simulator

Hydraulic fracturing modelling is a multi-physics problem which must be solved in the reservoir domain, fracture domain, and wellbore domain. The physical phenomena that need to be quantitatively modelled include solid deformation and fluid flow in the poroelastic reservoir, fluid flow in the fractures, and fluid and proppant transport in the wellbore. These problems which are tightly coupled with each other in a highly non-linear manner are solved together implicitly for the first time. This fully implicit, parallelized 3D hydraulic fracturing simulator is capable of simulating simultaneous propagation of multiple fractures in multi-well pads. In this new simulator, four sets of governing equations are coupled implicitly and solved simultaneously. The solid deformation equations and reservoir pressure equations are discretized using the finite volume method while the fracture pressure equation is discretized using the finite area method. Fluid flow in the wellbore is modelled considering wellbore compressibility, perforation pressure drop, and wellbore friction pressure drop. A Barton-Bandis contact model is applied to model fracture closure due to stress shadow effects as well as production. The simulator is parallelized using the distributed memory approach with domain decomposition to improve the computation efficiency. We first validated our simulator with two well-known model problems and the numerical results show good agreement with the existing analytical solutions. We compare the performance of the new simulator with the past explicit methods and show that the new simulator offers significant improvement in numerical stability and computation speed. Finally, the simulator is applied to simulate and analyze one stage of a fracturing treatment with five fractures propagating simultaneously to show the unique capabilities of our simulator.

Numerical Study of Simultaneous Multiple Fracture Propagation in Changning Shale Gas Field

Recently, the Changning shale gas field has been one of the most outstanding shale plays in China for unconventional gas exploitation. Based on the more practical experience of hydraulic fracturing, the economic gas production from this field can be optimized and gradually improved. However, further optimization of the fracture design requires a deeper understanding of the effects of engineering parameters on simultaneous multiple fracture propagation. It can increase the effective fracture number and the well performance. In this paper, based on the Changning field data, a complex fracture propagation model was established. A series of case studies were investigated to analyze the effects of engineering parameters on simultaneous multiple fracture propagation. The fracture spacing, perforating number, injection rate, fluid viscosity and number of fractures within one stage were considered. The simulation results show that smaller fracture spacing implies stronger stress shadow effects, which significantly reduces the perforating efficiency. The perforating number is a critical parameter that has a big impact on the cluster efficiency. In addition, one cluster with a smaller perforating number can more easily generate a uniform fracture geometry. A higher injection rate is better for promoting uniform fluid volume distribution, with each cluster growing more evenly. An increasing fluid viscosity increases the variation of fluid distribution between perforation clusters, resulting in the increasing gap between the interior fracture and outer fractures. An increasing number of fractures within the stage increases the stress shadow among fractures, resulting in a larger total fracture length and a smaller average fracture width. This work provides key guidelines for improving the effectiveness of hydraulic fracture treatments.

Numerical investigations of the fracture geometry and fluid distribution of multistage consecutive and alternative fracturing in a horizontal well

Hydraulic fracturing is an important technique used for developing shale gas reservoirs. Compared with multistage consecutive fracturing, multistage alternative fracturing is a more challenging technique due to the current limitations of downhole tools. This paper simulates multistage fracturing and compares the fracture geometries and fluid distributions of consecutive and alternative fracturing. It is found that simultaneous propagation of multiple fractures results in their unequal stimulations. Some perforations do not contribute to well production because their fractures are restrained. Alternative fracturing tends to create more uniform length fractures and thus achieves a greater stimulated reservoir volume than that of consecutive fracturing.

Simultaneous hydraulic fracturing of ultra-low permeability sandstone reservoirs in China: Mechanism and its field test

Based on the impact of the stress perturbation effect created by simultaneous propagation of multiple fractures in the process of simultaneous hydraulic fracturing, a thorough research on the mechanism and adaptation of simultaneous fracturing of double horizontal wells in ultra-low permeability sandstone reservoirs was conducted by taking two adjacent horizontal wells (well Yangping-1 and well Yangping-2 located in Longdong area of China Changqing Oilfield) as field test wells. And simultaneous fracturing optimal design of two adjacent horizontal wells was finished and employed in field test. Micro-seismic monitoring analysis of fracture propagation during the stimulation treatment shows that hydraulic fractures present a pattern of complicated network expansion, and the well test data after fracturing show that the daily production of well Yangping-1 and well Yangping-2 reach 105.8 t/d and 87.6 t/d, which are approximately 9.4 times and 7.8 times the daily production of a fractured vertical well in the same area, respectively. Field test reflects that simultaneous hydraulic fracturing of two adjacent horizontal wells can enlarge the expansion area of hydraulic fractures to obtain a lager drainage area and realize the full stimulation of ultra-low permeability sandstone reservoirs in China Changqing oilfield. Therefore, simultaneous fracturing of two adjacent horizontal wells provides a good opportunity in stimulation techniques for the efficient development of ultra-low permeability reservoirs in China Changqing oilfield, and it has great popularization value and can provide a new avenue for the application of stimulation techniques in ultra-low permeability reservoirs in China.