Sequential Propagation of Multiple Fractures in Horizontal Wells
Authors: Wang Tianyu , Yong Yuning , Zhai Jiaheng , Tian Shouceng , Zhou Xiaoxia , sheng Mao 
Published: August 2024

ABSTRACT:
Simultaneous fracturing and zipper fracturing of horizontal wells has rapidly evolved to the development of unconventional oil and gas. The fracture interference when multiple fractures propagate needs further study. This paper experimentally investigates the sequential propagation of multiple fractures in horizontal wells. Eight artificial rocks, each with dimensions of 400 mm ×400 mm × 400 mm and equipped with two horizontal wells, are sequentially fractured. The effects of the horizontal stress difference, the fracturing sequence, and the arrangement of perforations were discussed. The results show that previously created fractures exhibit a noticeable attraction effect on laterally created fractures, with horizontal stress difference emerging as a pivotal factor. A lower horizontal stress difference strengthens the attraction effect of previously created fractures on laterally created fractures. During staggered fracturing, laterally created fractures are simultaneously affected by the attraction effect of previously created fractures and the extrusion effect of the adjacent propagating fracture. Staggered fracturing of multiple perforation clusters proves effective in enhancing the stimulation of perforation clusters and achieving balanced fracture initiation and propagation. This research yields valuable insights into fracture propagation, guiding and optimizing the design and implementation of multiple staged fracturing in horizontal wells.

1. INTRODUCTION
Tight oil reservoirs are abundant with enormous resources globally, which are characterized by extremely low porosity and permeability. Multi-stage hydraulic fracturing is the main technology for extracting tight oil reservoirs nowadays. In comparison with conventional hydraulic fracturing, the application of multi-stage hydraulic fracturing with horizontal wells is limited by its high cost, hindering the efficient development of tight oil resources. Therefore, simultaneous fracturing and zipper fracturing technology has rapidly evolved, aiming to reduce cost and enhance production efficiency. This technology allows for the simultaneous or sequential hydraulic fracturing of numerous horizontal wells, effectively reducing the overall cost of hydraulic fracturing.
Previous scholars have conducted extensive research to investigate fracture propagation and interference when multi-stage hydraulic fracturing of several horizontal wells. Manchanda and Sharma (2013) investigated the fracture interference in multi-perforations fracturing with geomechanical computations. The results show that a longer time between successive stages of a horizontal well reduces the effect of stress shadow and fracture interference, leading to a more efficient fracture network. (Li & Zhang, 2017) studied the effects of perforation cluster spacing, well spacing, and fracturing sequence of multi-wells on fracture complexity. The results show that the appropriate well spacing should be carefully selected to avoid the interconnection between the fracture tips and take advantage of the shear stress caused by the fracture tips. The fracturing sequence of multi-well completion is very important. (Manchanda et al., 2020) simulated the dynamic three-dimensional propagation of multiple non-planar fractures in multiple wells and capturing the stress interference between the fractures. (Wu et al., 2022) established a three-dimensional model to reveal the mechanism of fracture initiation and propagation, which found that the fracture initiation pressure of multi-cluster fracturing is higher than that of single-cluster fracturing, and the stress shadow increases with the decrease of FS (fracture spacing). (Cui et al., 2022) used a method that can deal with non-uniform fractures, and established a semi-analytical model of fracture failure in multi-level well sites. The well spacing has an effect on the occurrence time of the well interference state. (Tang et al., 2023) established a DFN-FEM (combined Discrete Fracture Network and Finite Element Method) integrated numerical model for multi-well hydraulic fracturing. The results show that under the smaller natural fracture stiffness, the larger the induced stress range, the higher the fracturing hit rate. Different fracturing sequences between multiple wells show different induced stresses between wells, and the staggered fracturing sequence can greatly reduce the fracturing hit rate. (P. Yang et al., 2023) established a fully coupled multi-well fracturing model considering three-dimensional extended fractures and proppant transport. The comprehensive model considers hydraulic fracture interaction (i.e., stress shadowing effect), gravity proppant settlement, etc. The numerical results show that the geometry of fracture propagation under the condition of multi-well fracturing completion is affected by multiple stress interference mechanisms, such as asymmetric fracture propagation caused by strong inter-well stress interference and heel offset caused by inter-stage stress interference. (Peng Yang et al., 2023) established a 3D multi-well pad fracturing numerical model coupled with fracture propagation based on the displacement discontinuity method, and the fracture propagation and proppant distribution during multi-well fracturing are investigated by taking the actual multi-well pad parameters as an example. Fracture initiation and propagation during multi-well pad fracturing are jointly affected by a variety of stress interference mechanisms such as inter-cluster, inter-stage, and inter-well, and the fracture extension is unbalanced among clusters, asymmetric on both wings, and dipping at heels. Zipper fracturing produces more uniform fracture propagation. (Li et al., 2023) proposed a proppant migration model to realize the dynamic propagation simulation of multiple fractures and the change state of each fracture flow distribution. (Liu et al., 2023) established a fluid-solid coupling numerical model that comprehensively considers dynamic flow distribution, dynamic fracture propagation and propagation, and stress interference. The results show that by effectively controlling perforation friction, the stress interference generated in the minimum direction of horizontal principal stress can be balanced, and the balanced extension of multiple fractures can be promoted. At the same time, with the increase of injection flow rate and the decrease of cluster spacing, the degree of stress interference is gradually serious. (Yang et al., 2024) developed a comprehensive model for fracture propagation and proppant transport in multi-wellsite fracturing, which not only considers the key physical processes such as stress shadowing, dynamic velocity distribution in wellbore, fluid leakage, perforation erosion, fracture evolution, proppant bridging and gravity sedimentation, but also couples the simulation of three-dimensional plane propagation fractures and proppant transport. Compared with the traditional fracturing model, he has good computational efficiency and good application prospects. For laboratory experiments, (Guo et al., 2023) used 13 cubic shale samples (300 × 300 × 300 mm), they investigated the effect of in-situ stress, treatment parameters, and different patterns of multi-well fracturing (staggered or tip-to-tip) on fracture propagation. A high injection rate generates higher net pressure in fracture, and the hydraulic fracture can propagate and cross the bedding plane easily.