Yunpeng Wang

Summary: Low-frequency (LF) distributed acoustic sensing (DAS) (LF-DAS) facilitates the analysis of hydraulic fracture propagation by detecting strain variations in neighboring wells during the fracturing process. While earlier studies have established a theoretical foundation for interpreting simple fracture-induced optical fiber strains, field observations, particularly in geologically complex conditions, have exposed the intricacies of optical fiber strain patterns. The current comprehension of optical fiber strain evolution remains inadequate for interpreting fracture propagation dynamics. To address this, we use a 3D complex fracture propagation simulator to calculate optical fiber strain and strain rate using the 3D displacement discontinuity method. Through numerical simulations, we investigate the strain and strain rate evolution associated with various fracture types, including planar hydraulic, natural, and bedding-plane fractures. We also apply our findings to field data from shale gas fracturing in the Sichuan Basin, comparing the derived strain and strain rate patterns with LF-DAS monitoring results. Our findings indicate that, in horizontal wells, planar fractures with fixed and unrestricted heights produce similar strain and strain rate patterns, complicating the identification of fracture height. Natural fractures with nonperpendicular dip angles exhibit strain and strain rate patterns that align with their dip direction. The relative contributions of natural and hydraulic fractures to overall strain dictate the orientation of the strain pattern, which can be predominantly inclined or horizontal. When the strain contribution from natural fractures is minimal, the primary fracture characteristics are more discernible, overshadowing natural fracture features. In cases where 90° natural fractures extend, LF-DAS strain signals in horizontal wells exhibit symmetrical compression-tension patterns that correspond to the natural fracture length. The copropagation of hydraulic fractures and horizontal bedding in the absence of fracture-fiber interaction results in LF-DAS strain and strain rate signatures akin to those observed in vertical well monitoring. The field case study confirms the presence of natural fracture propagation strain patterns, corroborating the geological data of the region. This study lays a theoretical groundwork for interpreting complex strain signals and sets the stage for more precise fracture geometry inversion.

Summary: The interaction between natural fractures (NFs) and hydraulic fractures (HFs) in 3D space poses significant challenges to numerical simulation. The interaction behaviors between HFs and NFs in 3D space include crossing, offset, stopping, and bypassing. Many existing numerical simulators are 2D, which inherently limits their ability to account for the vertical growth of fractures. Consequently, they are unable to model the bypassing behavior effectively. In this paper, a mathematical model for the propagation of HFs and NFs in a naturally fractured reservoir is established. The displacement discontinuity method (DDM) is used to solve the rock deformation, while the finite difference method (FDM) is utilized to solve the fluid flow within fractures. The undirected graph structure is used to represent the complex fracture network, and a dynamic adjustment of grid connectivity method is implemented to describe the process by which HFs bypass NFs. By integrating with graphics processing unit (GPU) computing, an efficient 3D simulator for HFs and NFs propagation is developed. The accuracy is verified against analytical solutions and reference solutions. Subsequently, a series of numerical studies on the bypassing behavior are conducted. The primary findings are as follows: (1) The simulator can accurately capture the interactions between HFs and NFs in 3D space, including crossing, arresting, and bypassing behaviors. (2) The bypassing behavior is characterized by three distinct processes—intersection, height growth of HF, and bypassing. (3) During the intersection stage, both the injection pressure and maximum NF width increase. During the height growth stage, the pressure is relatively stable, while the maximum NF width continues to increase. These two stages together result in the “storage” phenomenon. Once the bypassing behavior occurs, both the pressure and the maximum NF width decrease sharply, leading to a “release” phenomenon. Additionally, the “storage” and “release” phenomena may impact proppant transport. (4) Given the presence of bypassing behavior, it is essential to consider the NFs in 3D fracture simulators. The simulator and its findings can provide valuable insights for field design.

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.