Zunpeng Hu

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: As the exploitation of oil and natural gas has progressed, hydraulic fracturing has become a primary method for increasing oilfield production. Simultaneous hydraulic fracturing of several perforation clusters, by employing limited-entry methods, has become standard in horizontal well stimulation. However, challenges such as unclear fracture identification and limited monitoring methods persist in hydraulic fracturing. This study integrates an optical frequency domain reflector with true triaxial fracturing of multilevel horizontal wells to develop a physical simulation system for monitoring fractures in a laboratory setting via distributed fiber optics. By employing fiber optics, dynamic monitoring of fractures during the fracturing process of multilevel horizontal wells can be achieved. The results indicate that monitoring with distributed fiber optics can clearly record data and accurately determine the initiation points of fractures. The strain data induced by the fractures on the fibers can be interpreted to deduce the fracture width. If a fracture deflects upon encountering a fiber, the fiber will exhibit tensile strain within an abnormal range. When using distributed fiber optics for monitoring fractures in multilevel horizontal wells, it is crucial to optimize the placement of fractures to ensure that the signals detected by the fibers are complete and avoid signal loss. In this paper, we demonstrate the feasibility of using distributed fiber optics for fracture monitoring in multilevel horizontal well fracturing experiments, overcoming the limitations of current single-method approaches to monitoring laboratory true triaxial hydraulic fractures.

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.