Multiple Perforation Clusters Research Articles

Experimental study of hydraulic fracture propagation with multi-cluster in-plane perforations in a horizontal well

Tri-axial fracturing studies were carried out to understand the impact of lateral mechanical parameters on fracture propagation from multiple in-plane perforations in horizontal wells. Additionally, the discussion covered the effects of geology, treatment, and perforation characteristics on the non-planar propagation behavior. According to experimental findings, two parallel transverse fractures can be successfully initiated from in-plane perforation clusters in the horizontal well because of the in-plane perforation, the guide nonuniform fishbone structure fracture propagation still can be exhibited. The emergence of transverse fractures and axial fractures combined as complex fractures under low horizontal principal stress difference and large pump rate conditions. The injection pressure was also investigated, and the largest breakdown pressure can be also found for samples under these conditions. The increase in perforation number or decrease in the cluster spacing could provide more chances to increase the complexity of the target stimulated zone, thus affecting the pressure fluctuation. In a contrast, the increase in fracturing fluid viscosity can reduce the multiple fracture complexity. The fracture propagation is significantly affected by the change in the rock mechanical properties. The fracture geometry in the high brittle zone seems to be complicated and tends to induce fracture reorientation from the weak-brittle zone. The stress shadow effect can be used to explain the fracture attraction, branch, connection, and repulsion in the multiple perforation clusters for the horizontal well. The increase in the rock heterogeneity can enhance the stress shadow effect, resulting in more complex fracture geometry. In addition, the variable density perforation and temporary plugging fracturing were also conducted, demonstrating higher likelihood for non-uniform multiple fracture propagation. Thus, to increase the perforation efficiency along the horizontal well, it is necessary to consider the lateral fracability of the horizontal well on target formation.

Mechanisms of fracture propagation from multi-cluster using a phase field based HMD coupling model in fractured reservoir

Natural fractures (NFs) are common in shale and tight reservoirs, where staged multi-cluster fracturing of horizontal wells is a prevalent technique for reservoir stimulation. While NFs and stress interference are recognized as significant factors affecting hydraulic fracture (HF) propagation, the combined influence of these factors remains poorly understood. To address this knowledge gap, a novel coupled hydro-mechanical-damage (HMD) model based on the phase field method is developed to investigate the propagation of multi-cluster HFs in fractured reservoirs. The comprehensive energy functional and control functions are established, while incorporating dynamic fluid distribution between multiple perforation clusters and refined changes in rock mechanical parameters during hydraulic fracturing. The HMD coupled multi-cluster HF propagation model investigates various scenarios, including single HF and single NF, reservoir heterogeneity, single HF and NF clusters, and multi-cluster HFs with NF clusters. The results show that the HMD coupling model can accurately capture the impact of approach angle (θ), stress difference and cementation strength on the interaction of HF and NF. The criterion of the open and cross zones is not fixed. The NF angle (α) is not a decisive parameter to discriminate the interaction. According to the relationship between approach angle (θ) and NF angle (α), the contact relationship of HF can be divided into three categories (θ=α, θ<α, and θ>α). The connected NF can increase the complexity of HF by inducing it to form branch fracture, resulting in a fractal dimension of HF as high as 2.1280 at angles of ±45°. Inter-fracture interference from the heel to the toe of HF shows the phenomenon of no, strong and weak interference. Interestingly, under the influence of NFs, distant HFs from the injection can become dominant fractures. However, as α gradually increases, inter-fracture stress interference becomes the primary factor influencing HF propagation, gradually superseding the dominance of NF induced fractures.

Promoting the uniform propagation of multi-fracture through adjusting the injection rate and perforation parameter: A numerical study

The strong stress interference and fluid competitive distribution result in the non-uniform propagation of multiple fractures during horizontal well multi-stage hydraulic fracturing (HWMHF). The uniform stimulation effect can be effectively improved by adjusting the injection rate and perforation scheme. In this work, a two-dimensional fluid–solid fully coupled multi-fracture propagation model is established to investigate the pattern of multi-fracture propagation and the flow distribution. The considering factors include the fluid injection rate, the perforation parameters, and the reservoir heterogeneity. The pipe element is developed to realize the free fluid distribution among multiple fractures. The results show that multiple fractures are more likely to initiate simultaneously and propagate uniformly with a higher fluid injection rate. The stress interference among multiple fractures can be balanced and the equilibrium propagation degree of multiple fractures can be greatly improved by reducing the perforation number. In the homogeneous reservoir, when the fluid injection rate is 12 m3/min and the perforation number per cluster is reduced to 10, the difference coefficient of fluid distribution can be reduced to 3.31%. In the heterogeneous reservoir, multiple perforation clusters with different tensile strengths can generate nearly uniform fractures by increasing the fluid injection rate and reducing the perforation number. When the fluid injection rate is 12 m3/min and the perforation number per cluster is 6, the difference coefficient of fluid distribution can be reduced to 3.26%. This work is of great significance in clarifying the propagation pattern and optimizing the fluid injection rate and the perforation number to improve the uniform propagation degree of multiple fractures.

Investigation of Natural Weak Interface Properties and Their Impact on Fracture Propagation in Shale Reservoirs

Horizontal well multi-cluster fracturing technology is crucial for the economic development of fractured shale reservoirs. The abundance of natural fractures in shale reservoirs significantly influences the propagation path of hydraulic fractures and determines the formation of complex fracture networks. To investigate the impact of natural weak planes on the geometric parameters of fractures in shale reservoirs, we first conducted tests on the mechanical characteristics of core samples from outcropping shale in the Weiyuan area using the indoor three-point bending method and digital image correlation (DIC) technology, providing data validation for subsequent numerical models. Secondly, considering the interaction between hydraulic and natural weak planes in three-dimensional space, we established a three-dimensional numerical model for horizontal well fracturing to simulate the synchronous competition and expansion of fractures in multi-cluster fracturing. Based on this foundation, we analyzed the influence of formation parameters and engineering parameters on the formation patterns of complex fracture networks. The results indicate that the difference in in situ stress is a significant factor affecting the selection of fracture propagation paths. As the in situ stress difference increases, it becomes more challenging to open natural fractures, leading to a reduced probability of activation of natural weak interfaces. When the cohesive strength of natural fractures is smaller, they are more likely to open and capture hydraulic fractures, thereby increasing shear slip length and fracture network area. Each fracturing stage has an optimal perforation density combination, where a higher density of perforations leads to reduced perforation pressure drop and weaker ability to mitigate inter-cluster stress interference. To achieve a comprehensive and balanced development of multi-clusters, the inter-cluster stress interference can be alleviated by increasing the perforation pressure drop. For dense perforation clusters, higher injection rates and viscosity can be employed to ensure the uniform development of multiple perforation clusters. This study provides new insights into predicting the formation of complex fracture networks in shale reservoirs and offers valuable guidance for optimizing hydraulic fracturing designs.

Proppant Distribution Prediction between Perforation Clusters Using Fresh Water in a Horizontal Wellbore.

In multistage, multicluster treatments, distributing the proppant evenly among the multiple perforation clusters is considered one of the key challenges. Uneven proppant distribution in multiple clusters can significantly affect fracture conductivity, which in turn severely affects the well's productivity. Dimensional analysis using the Buckingham π-theorem was utilized to develop the experimental correlations using the experimental data. The laboratory tests data of 100 mesh brown and 40/70 mesh white sands (SG of 2.65) at various proppant concentrations, injection rates, and with various perforation configurations were combined and used to develop the correlation. This paper offers a newly developed experimental correlation for the proppant distribution that may be utilized to forecast the distribution of proppant between various clusters in the multistage, multicluster stimulation. Such correlations are essential to be considered in future hydraulic fracturing treatment designs, as they offer insightful information and aid in optimizing the projected proppant distribution across several perforation clusters within a single hydraulic fracturing stage. Thus, the multistage, multicluster horizontal wellbore achieves uniform proppant distribution between the various perforation clusters.

Correlating Surface and Downhole Perforation Entry Hole Measurements Leads to Development of Improved Perforating Systems

Summary Studies have shown that achieving a consistent perforation hole size in casing (i.e., entry hole) and zero-phase perforation gun orientation led to improved treatment distribution among multiple perforation clusters in plug-and-perf limited entry treatments. In addition to reducing variation in the perforation entry hole by establishing uniformity in gun clearance and the angle of incidence of the perforation jet at the wall of the casing, oriented perforating has been shown to minimize the tendency of proppant to separate from the fracturing fluid while traveling across the perforated intervals (inertial effect) and mitigate nonuniform entry hole erosion due to gravity-induced proppant stratification. The primary goal of this study was to determine the controllable perforating gun elements and accessories that effect the accuracy of gun orientation and entry hole dimensions. Surface tests were conducted at manufacturing facilities for determining the characteristics of the entry holes in pipe produced by shaped explosive charges using various system configurations and the robustness of various gun orientation devices. Promising perforating systems were then used in wellbores to create calibration entry holes (downhole tests) that were measured for equivalent diameter and orientation accuracy using high-resolution acoustic imaging before conducting treatments. This process enabled components of the perforating system influencing entry hole size and gun orientation to be evaluated and modified, as necessary. Elements of the perforating system and downhole environment that influenced entry hole size and consistency included casing type, cement sheath characteristics, perforating gun clearance and orientation, perforating charge type and density, packing arrangement of multiple charges, charge tube and charge carrier design, gun detonation system, hydrostatic pressure, and locking devices. Achieving tight control of these elements significantly reduced variation in entry hole size. Deviations from surface and downhole testing results were commonly attributed to using perforating system elements in the field that differed from those used by the manufacturer in surface testing. Factors affecting gun orientation accuracy and consistency included weight bar type, gun string length, weight, and stiffness, the presence of modified standoff bands, progressive gun deformation during firing, wellbore tortuosity, and self-orienting devices. Several orientation systems were found that achieved orientation within the target 20°-window. To assess the value of this workflow process, the paper includes information on the results of diagnostic tests for evaluating the accuracy of the ultrasonic measuring device, the derivation process used for determining coefficients for a two-component perforation erosion model, and the use of the derived erosion rate coefficients for computing the mass of proppant that enters each perforation and perforation cluster during a fracturing treatment.

Investigating the simultaneous fracture propagation from multiple perforation clusters in horizontal wells using 3D block discrete element method

Multi-cluster horizontal well fracturing is one of the key technologies to develop the unconventional reservoirs such as shales. However, the field data shows that some perforation clusters have little production contribution. In this study, a three-dimensional (3D) numerical model for simulating the multiple fracture propagation based on 3D block discrete element method was established, and this model considers the stress interference, perforation friction and fluid-mechanical coupling effect. In order to determine the most appropriate measures to improve the uniformity of multiple fracture propagation, the effect of the geologic and engineering parameters on the multiple fracture propagation in shale reservoirs is investigated. The modeling results show that the geometry of each fracture within a stage is different, and the outer fractures generally receive more fracturing fluid than the interior fractures. The vertical stress almost has no effect on the geometries of multiple fractures. However, higher horizontal stress difference, larger cluster spacing, smaller perforation number, higher injection rate, and smaller fracturing fluid viscosity are conducive to promote the uniform propagation of multiple fractures. The existence of bedding planes will increase the fluid filtration, resulting in a reduction in fracture length. The middle two fractures receive less fluid and the width of them is smaller. Through analyzing the numerical results, a large amount of fracturing fluid should be injected and the proppant with smaller size is suggested to be used to effectively prop the bedding planes. Cluster spacing and perforation number should be controlled in an appropriate range according to reservoir properties. Increasing the injection rate and reducing the viscosity of fracturing fluid are important means to improve the geometry of each fracture.

Experimental investigation of hydraulic fracture propagation behavior in layered continental shale

In this study, numerous triaxial hydraulic fracturing experiments are performed in continental shale outcrop samples and artificial samples with similar properties to investigate the HF propagation behavior under single-cluster and multi-cluster fracturing treatments. The single-cluster fracturing experimental results show that the interlayer interface (IF) has a significant effect on the HF height containment, which tends to shear slip under conditions of low IF bonded strength, low vertical stress difference, low wellbore traversing layer strength and low q⋅μ value (the product of injection rate and fracturing fluid viscosity), resulting in the arrest of HF by the IF, and the vertical propagation of HF is hindered. Seven types of HF geometries were observed under the interference of multiple IFs, which could be classified into three categories in the vertical plane based on degree of connection with IFs, namely, simple fracture, small-scale fishbone-like fracture, and large-scale fishbone-like fracture. The multi-cluster fracturing experimental results indicate that the vertical propagation law of HF under multi-cluster fracturing is similar to single-cluster fracturing. Under the conditions of small cluster spacing and large number of perforation clusters, it is difficult for each perforation cluster to initiate HF evenly, and the middle perforation cluster is more difficult than outers. In addition, injection-pressure curves in theory can provide some useful information for HF geometries identification. But it is difficult to judge whether the HFs initiate evenly and develop uniformly at multiple perforation clusters by observing the injection fracturing curves because the evolution law of the injection-pressure curves under the simultaneous propagation of multiple HFs and the propagation of a single HF are similar.

Optimization of Non-Uniform Perforation Parameters for Multi-Cluster Fracturing

Stress shadowing affects the simultaneous propagation of fractures from multiple perforation clusters. Employing uniform perforation parameters for all clusters cause the unbalanced growth of fractures, which arouses the demand of optimizing non-uniform perforation parameters. An optimization workflow combining a fracture propagation model and the particle swarm optimization method (PSO) is proposed for multi-cluster fracturing in this study. The fracture model considers the coupling of rock deformation and fluid flow along the wellbore and fractures, and it is solved by using the Newton iteration method. The optimization is performed by taking the variance of multiple fracture lengths as fitness value function in the frame of the PSO method. Numerical results show that using the same spacings and perforation parameters for all clusters is detrimental to the balanced growth of multiple fractures. The variance of fracture lengths drops greatly through optimization of cluster spacings and perforation number/diameter. Properly increasing the spacing and perforation number/diameter for the middle clusters promotes the balanced growth of multiple fractures. This study provides an efficient optimization workflow for multi-cluster fracturing treatment in horizontal wells.

Simultaneous propagation of hydraulic fractures from multiple perforation clusters in layered tight reservoirs: Non-planar three-dimensional modelling

In this paper, a hydromechanical coupled finite-discrete element method, which considers the non-planar three-dimensional growth, pressure continuity along the horizontal well, dynamic flow rate distributions among clusters, perforation friction, and fracturing fluid leakage, is employed to simulate the simultaneous growth of hydraulic fractures from an array of five perforation clusters in tight reservoirs interbedded with alternating stiff and soft layers. The simulation results highlight that the stress shadow induced by the non-planar propagation of the outmost hydraulic fractures stops the planar growth of the interior and middle hydraulic fractures and causes uneven fracturing fluid distribution among perforation clusters. The results demonstrate that the generated fracture pattern in the stage becomes more symmetric overall with the decreasing modulus of the soft layers. As the soft layer's modulus decreases, the total fracture height decreases significantly, but the local fracture aperture distribution increases, which leads to the reduction of total fracture area and leak-off volume of fracturing fluid as well as the increase of total fracture volume. The total fracture area decreases with the increasing leak-off coefficient and perforation number, but the total leak-off volume and total fracture volume increase. The violation of the fluid pressure continuity by without considering the dynamic flow rate distributions overestimates the growth of the interior and middle hydraulic fractures and produces a smaller total fracture area. It is also found that the adjustment of pumping rate is more effective than using nonuniform cluster spacing in promoting the simultaneous hydraulic-fracture growth in layered tight reservoirs.

Influences on unstable hydraulic fractures: propagation of adjacent multiple perforations and bedded interfaces in multilayered reservoir via FE-DE model

PurposeSimultaneous hydrofracturing of multiple perforation clusters in vertical wells has been applied in the stimulation of hydrocarbon resources reservoirs. This technology is significantly impeded due to the challenges in its application to the multilayered reservoirs that comprise multiple interlayers. One of the challenges is the accurate understanding and characterization of propagation and deflection of the multiple hydraulic fractures between reservoirs and embedded interlayers.Design/methodology/approachNumerical models of the tight multilayered reservoirs containing multiple interlayers were established to study hydrofracturing of multiple perforation clusters and its influencing factors on unstable propagation and deflection of hydraulic fractures. Brittle and plastic multilayered reservoirs fully considering the influences of different in situ stress ratio and physical attributes for reservoir and interlayer strata on propagations of hydraulic fractures were investigated. The combined finite element–discrete element method and mesh refinement strategy were adopted to guarantee the accuracy of stress solutions and reliability of fracture path in computation.FindingsResults show that the shear stress fields between adjacent multiple hydraulic fractures are superposed to cause fractures deflection. Stress shadows induce the shielding effects of hydraulic fractures and inhibit fractures growth to emerge unstable propagation behaviors, and a main single fracture and several minor fractures develop. As the in situ stress ratio increases, hydraulic fractures more easily deflect toward the direction of maximum in situ stress, and stress shadow and mutual interaction effects between them are intensified. Compared to brittle reservoir, plastic-enhanced reservoir may limit fracture growth and cannot form long fracture length; nevertheless, plastic properties of reservoir are prone to induce more microseismic events with larger magnitude.Originality/valueThe obtained fracturing behaviors and mechanisms based on engineering-scale multilayered reservoir may provide effective schemes for controlling and estimating the unstable propagation of multiple hydraulic fractures.

Experimental Study on the Initiation and Propagation of Multi-Cluster Hydraulic Fractures within One Stage in Horizontal Wells

Competitive propagation of fractures initiated from multiple perforation clusters is universal in hydraulic fracturing of unconventional reservoirs, which largely influences stimulation. However, the propagation mechanism of multi-fractures has not been fully revealed for the lack of a targeted laboratory observation. In this study, a physical simulation experiment system was developed for investigating the initiation and propagation of multi-cluster hydraulic fractures. Different from the traditional hydro-fracking test system, the new one was equipped with a multi-channel shunting module and a strain monitoring system, which could guarantee the full fracture extension at each perforation clusters and measure the internal deformation of specimens, respectively. Several groups of true tri-axial fracturing tests were performed, considering the factors of in situ stress, cluster spacing, pumping rate, and bedding structures. The results showed that initiation of multi-cluster hydraulic fractures within one stage could be simultaneous or successive according to the difference of the breakdown pressure and fracturing fluid injection. For simultaneous initiation, the breakdown pressure of the subsequent fracture was lower than or equal to the value of the previous fracture. Multiple fractures tended to attract and merge. For successive initiation, the breakdown pressures of fractures were gradually increasing. The subsequent fracture tended to intersect with or deviated from the previous fracture. Multiple fractures interaction was aggravated by the decrease of horizontal stress difference, bedding number and cluster spacing, and weakened by the increase of pump rate. The propagation area of multiple fractures increased with the pump rate, decreased with the cluster spacing. The strain response characteristics corresponded with the initiation and propagation of fracture, which was conducive to understanding the process of the fracturing. The test results provide a basis for optimum design of hydraulic fracturing.

Optimization of Plug-and-Perforate Completions for Balanced Treatment Distribution and Improved Reservoir Contact

Summary Heel-dominated treatment distribution among multiple perforation clusters is frequently observed in plug-and-perforate (plug-and-perf) stages, causing small propped surface areas, suboptimal production, and unexpected fracture hits. A multifracture simulator with a novel wellbore-fluid and proppant-transport model is applied to quantify treatment distribution among multiple perforation clusters in a plug-and-perf operation. A simulation base case is set up on the basis of a field treatment design with four clusters. Simulation results show that the two toe-side clusters screened out early in the treatment and the two heel-side clusters were dominant. The simulated proppant placement is consistent with distributed-acoustic-sensing observations. The impact of different perforating strategies and pumping schedules on final treatment distribution is investigated. Two criteria are defined that quantify the proppant distribution and fracture area: the weighted average (WA) and standard deviation (SD) of the final fluid and proppant distribution, as well as the hydraulic surface area (HSA) and propped surface area (PSA) of the created fractures. An optimal plug-and-perf design is defined as one that minimizes the SD of the treatment distribution among perforation clusters, and maximizes the PSA. Both perforating strategy and pumping schedule are found to affect the final treatment distribution significantly, and uniform treatment distribution is shown to create more PSA. Having fewer perforations per cluster was found to promote uniform fluid and proppant placement. Other helpful strategies include reducing the number of perforations near the heel and using small, lightweight proppant. The stress shadow effect is accounted for using the displacement discontinuity method (DDM) and was found to play a smaller role than perforation friction and proppant inertia in most cases. An automated process is developed to optimize plug-and-perf completion design with multiple decision variables using a genetic algorithm (GA). Thirteen parameters are optimized simultaneously. The optimal design solution creates an almost even treatment distribution and more than doubles the PSA compared with the base case. The multifracture model presented in this paper provides a way to quantify fluid and proppant distribution for any perforating strategy and pumping schedule, and provides more insight into the physics relevant to plug-and-perf treatment distribution. The perforation and pumping schedule recommendations presented in this paper provide directional guidance for the design of fracturing jobs with balanced treatment distribution and large PSA.

The effect of heterogeneity on multiple fracture interaction and on the effect of a non-uniform perforation cluster distribution

Multistage hydraulic fracturing is essential for the exploitation of unconventional petroleum resources and to realize new energy sources, such as enhanced geothermal systems (EGS). Multistage fracturing is normally done by increasing the pressure in a well with multiple perforation clusters, with the aim of forcing significant fractures from all perforations. Various methods have been proposed to optimize multistage fracturing treatments, including creating a non-uniform distribution of clusters along the well. Many of these optimization methods have, however, been tested in numerical models assuming homogeneous reservoirs and symmetric growth from the wellbore. An important question is then how heterogeneity affects the multistage fracturing optimization methods. This paper presents a numerical investigation using the modified discrete element method showing how the effect of a non-uniform cluster distribution is affected by the heterogeneity. It is shown that a non-uniform cluster distribution can, in some cases, promote more even contributions from all clusters, but it is sensitive to the reservoir heterogeneity. This indicates that the heterogeneity and fracture complexity have to be accounted for when investigating multistage optimization designs. The focus of this work is the general fracture behaviour during multistage fracturing in heterogeneous rock, aimed at optimizing fracturing designs for EGS and shale gas operations.

A Laboratory Study of Multiple Fracture Initiation from Perforation Clusters by Cyclic Pumping

Hydraulic fracturing using cyclic pumping is a potential method for effective stimulation of unconventional oil and gas reservoirs. In our study, the laboratory-scale hydraulic fracturing experiments using cyclic pumping were conducted using cuboid concrete specimens with multiple perforation clusters. The results showed that by cyclic pumping more fractures were created from perforation clusters when compared to non-cyclic monotonic pumping. This trend was shown to increase when using high-viscosity fracturing fluids. In our cyclic pumping experiments, the maximum pressure in the first pumping cycle reached the breakdown pressure and at each subsequent cycle the maximum pressure increased. The increased maximum pressure likely acted to generate more fractures initiating from the given perforation clusters even under greater fracture height to spacing ratios and more acoustic emission (AE) events were generated by cyclic pumping compared with those by monotonic pumping. Well compliance was examined and showed a negligible change in the system compressibility among cycles. This suggests that the fractures formed by previous injection cycles were isolated from the wellbore at the start of this cycle and at least their near-well portions are closed. There are a few possibilities contributing to the difficulty in reopening the existing hydraulic fractures in subsequent injection cycles. Detailed discussion is provided based on the varying features of injection pressure. This cyclic injection method could deliver an alternative for multi-stage fracturing treatments in field applications.

Concerns and Clarifications for Drilled Uncompleted (DUC) Wells in the Williston Basin

Summary The recent slump in oil prices has resulted in new terminology: “drilled uncompleted wells,” often referred to as DUC wells by the industry. In 2013 and 2014, when oil prices were more than USD 100/bbl, rate of return (ROR) from most unconventional plays was in the range of 15 to 50%, depending on the quality of rock and the operator's portfolio in the basin. The objective of this paper is to address key challenges associated with DUC completions when they are eventually fractured and brought on line for production. The paper addresses four main concerns that can have significant impacts on productivity of DUC wells: fracture hits (well interference), reservoir quality (hydrocarbon drainage), multiple horizons (zone connectivity), and well spacing (high-density drilling). The paper also showcases case studies in which real-time observations made from wells have been used to validate predictions from forward-looking fracture and production models. First, fracture hits commonly have been observed in all unconventional plays throughout the US, with effects on offset wells being mixed. Some fracture hits result in a positive uptick in production in offset wells, whereas other fracture hits affect production negatively in the form of increased water cut, reduced wellhead pressure, and other responses. Understanding fracture hits and their influence on other wells is very critical to avoid any detrimental impacts or to leverage positive effects on production. Second, reservoir quality decides how much oil in place is available for the DUC wells to drain, which, in turn, depends on length of production history and parent-well-completion geometries in offset wells. Third, in basins where there are multiple producing horizons or formations, fracture-height growth and interference between adjacent formations can result in asymmetric fracture propagation toward depleted zones. The longer these wells completed in the same/adjacent formations have been on production, the greater the extent of asymmetry will be. Addressing this concern requires a good understanding of drainage patterns from offset wells and evaluation of their impact on fracture geometries in DUC wells. Last, in areas with high-density drilling, a combination of longer production and fracturing stages with multiple perforation clusters per stage can leave very little oil available for the DUC well to produce.

A new method to calculate slurry distribution among multiple fractures during fracturing and refracturing

An accurate calculation of slurry distribution among multiple perforation clusters is necessary for all multi-fracture simulators. The slurry distribution was solved by applying the Newton-Raphson Method in the past. This method requires a numerical evaluation of the Jacobian matrix and can be unstable in some cases. In this paper, we propose a new approach to calculate slurry distribution among multiple fractures, the Resistance Method. This new approach is computationally efficient, requires no Jacobian matrix calculation, and has been proved to converge locally at least linearly.The Resistance Method is derived by reforming the mass conservation and pressure continuity equations for multiple fractures into a system of nonlinear fixed-point iteration equations. In the new equation system, the fracture resistance is defined as the pressure drop per unit flow rate. And the injected slurry is distributed among multiple fractures in inverse proportion to the resistance of each fracture. This new slurry distribution method has been applied to several different fracture models. The convergence of this new slurry distribution method has been analyzed with the map contraction theory.In this paper, we show three example applications of the Resistance Method. First, the Resistance Method is implemented with an analytical PKN model, with direct comparison made with the Newton-Raphson method. Simulation results show that both methods solve the slurry distribution accurately, but we show that the Resistance Method can be more computationally efficient than the Newton-Raphson Method.Second, the Resistance Method is used to obtain the proppant distribution in multiple clusters with particle transport efficiency curves implemented implicitly. It is shown that the proppant distribution among multiple clusters can be quite different from the fluid distribution. Due to inertial effects, proppant tends to accumulate towards the toe side clusters and causes pre-mature screen-out, resulting in a heel-biased treatment.Third, the Resistance Method is used in a refracturing example problem. A synthetic refracturing treatment with 50 existing fractures, 49 new clusters and about 15 h' treatment time was simulated. The results show that the refrac treatment is heel biased with the 20 heel side clusters receiving most of the treatment and that both existing fractures and new perforations propagate during refracturing. In such simulations extending over long periods of injection and with a large number of perforation clusters it is important to have a very efficient solver to obtain the fluid and proppant distribution in a reasonable computational time frame.In this work, we developed a new method to calculate slurry distribution among multiple clusters with applications to fracturing and refracturing treatments. This new method can be integrated with any fracture model. The main advantages of this new approach include: 1) simple implementation, 2) no numerical evaluation of the Jacobian matrix, 3) computationally efficiency, and 4) easy integration of slurry distribution, proppant transport, stress shadow effects and multiple fracture propagation.

A Perforation-Erosion Model for Hydraulic-Fracturing Applications

Summary Perforation pressure drop and its decrease caused by perforation erosion during a hydraulic-fracturing treatment are critical factors that need to be considered in treatment design, particularly when the limited-entry technique is implemented along multiple perforation clusters to ensure more-uniform fluid distribution. The simultaneous increases in the discharge coefficient Cd and perforation diameter D during perforation erosion require consideration of the temporal changes of these two variables to characterize the perforation-erosion behavior. In this paper, we present a perforation-erosion model dependent on abrasion mechanisms and the procedure to determine the specific erosion parameters that can be corroborated from laboratory data. Our modeling results demonstrate that it is inappropriate to assume an alternate increase in Cd and D, as considered in some conventional correlations. Once the erosion parameters are empirically inferred, we incorporate our model into a nonplanar hydraulic-fracturing simulator to determine appropriate perforation-number distributions at different clusters to ensure a successful limited-entry treatment that generates relatively even fluid distribution and uniform fractures.

Successful Recovery and Stimulation in a Long HP/HT Horizontal Well in One Intervention

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 188567, “Successful Recovery and Stimulation of a Long HP/HT Horizontal Well in One Intervention: A Case History of Acid Fracturing and New Generation of Diversion Systems Combination, Middle Marrat Carbonate Formation,” by Leopoldo Sierra, Alaa Eldine Alboueshi, Mohamed Elmofti, Walid Eid, Salma Sadeddin, and Ahmed Allam, Halliburton, and Mohamed Al Othman, Zamzam Ahmed, Erkan Fidan, Ibrahim Al-Zaidani, Neoq Nilotpaul, Meshari Ashkanani, Ali Buhamad, Mohammed Abdullah Al-Dousari, Abdul-Samad Mohammed Ahmed, and Yousef Al-Matrouk, Kuwait Oil Company, prepared for the 2017 Abu Dhabi International Petroleum Exhibition and Conference, Abu Dhabi, 13–16 November. The paper has not been peer reviewed. The case history presented in the complete paper describes the performance of an acid-fracturing intervention in a high-pressure/high-temperature (HP/HT) well in which this intervention was the last procedure considered to evaluate the productivity of a Marrat Formation well. The post-acid-fractured well productivity index (PI) showed the high quality of the stimulation performed in a challenging environment, demonstrating the effectiveness of the new diversion system for creating selective fractures in a horizontal wellbore with multiple perforation clusters. Introduction Completion of deep HP/HT carbonate formations in northern Kuwait in overpressured environments comprising interbedded carbonate bodies, complex tectonics, and stress variations resulting in strike/slip or reserve fault regimes is challenging for operator and service companies. Because of the low-permeability conditions, it is necessary to generate long and conductive fractures to produce the reservoir effectively. Additionally, this aggressive fracture- stimulation requirement needs to overcome the high-working-pressure limitations of the completion, in addition to the nature and corrosiveness of the produced hydrocarbon, which makes se-lection of the proper stimulation reactive-fluid system important. These challenges are even more crucial in cases having drilling and completions issues. The alternative implemented for the well candidate presented in the complete paper considers the completion and stimulation of a well having an existing long perforated interval, where the implementation of a selective fracture stimulation of a highly deviated wellbore (88° or almost horizontal) was challenging. Geology, Reservoir, and Geomechanics of Middle Marrat Formation The Marrat reservoir is subdivided into three lithostratigraphic units: Lower, Middle, and Upper. The well candidate was drilled and completed to produce hydrocarbon reserves from the Middle Marrat Formation. The Middle Marrat unit represents a wide range of rock types related to high-stand precursor facies in a carbonate setting, which is overprinted by diagenetic alteration in refluxed and recrystallized dolomites. Eight gross flow zones have been identified bounded by major sequence-stratigraphic surfaces that frame the background of genetically correlated units. Reservoir quality and vertical connectivity can be controlled further by placing properly distributed hydraulic fractures, vertically and laterally, to connect the prospective reservoir and existing natural-fracture networks to improve hydrocarbon recovery.

Proppant Distribution Among Multiple Perforation Clusters in Plug-and-Perforate Stages

Summary Plug-and-perforate (Plug-and-Perf) fracturing stages with multiple perforation clusters have become common practice in the industry. However, it is usually unclear whether the fluid and proppant are distributed evenly among all clusters. In this study, we present a method for computing the proppant distribution into each cluster in a fracturing stage. By integrating proppant transport into a multicluster hydraulic-fracturing model and implementing a simple screenout criterion, we show that the proppant distribution in a fracturing stage can be very uneven, with a strong bias toward the heel-side clusters even when the initial fluid distribution is uniform among all clusters. In this work, we define the efficiency of proppant transport into a perforation by the proppant-transport efficiency (PTE), which is defined as the mass fraction of proppant transported through a perforation relative to the total mass of proppant approaching the perforation. The dynamic proppant distribution in a fracturing stage is modeled with the PTE concept in three steps. First, a series of coupled computational-fluid-dynamics/discrete-element-method (CFD/DEM) simulations were performed to obtain PTE under controlled flow conditions. Then, the CFD/DEM simulation results were statistically analyzed to generate a PTE correlation as a function of wellbore, perforation, fluid, and proppant properties. Finally, the PTE correlation was incorporated into a multicluster hydraulic-fracturing model to compute the dynamic distribution of fluid and proppant among multiple clusters in a fracturing stage. Results from this work show that proppant concentration in the toe-side clusters can be several times higher than the injected concentration. This occurs because the high wellbore flow rate near the heel-side clusters provides proppant particles a large inertia sufficient to prevent them from turning into the perforations. Proppant concentration in the wellbore is thus increased as the slurry flows toward the toe side and the fluid preferentially leaks off from the heel-side perforations. The highly concentrated slurry increases the screenout risk of the toe-side clusters. Our modeling results show that if toe-side clusters screen out at early time in the proppant stage, fluid and proppant are redistributed to the heel-side clusters. In such a case, cumulative fluid and proppant distributions will be heel-biased. Simulation results are compared with field observations and are shown to be consistent with distributed-temperature-sensing (DTS) and distributed-acoustic-sensing (DAS) observations on proppant distribution made in three different studies. The method presented in this work provides a way to quantify proppant transport at a wellbore scale. It shows that the uneven proppant distribution among perforation clusters is a function of fluid, perforation, and proppant properties. An estimate of proppant placement in different perforation clusters can be computed for any pumping schedule and wellbore/perforation geometry with this method. This can be used to optimize perforation clusters that will result in a more-even distribution of proppant in each cluster.