Fracture Propagation Characteristics of Shale Oil Reservoir in Multi-Cluster Fracturing Experiments Based on Acoustic Emission Monitoring

Abstract

ABSTRACT: Stage-spacing and cluster-spacing techniques have to be employed to stimulate multiple hydraulic fractures to increase the generation of fracture networks. It is hard to investigate the fracture propagation in Lucagou Formation (the shale oil reservoir in Xinjiang Province, China) by numerical simulations because it is difficult to obtain the complex mechanical properties of Lucaogou shale required in the numerical models. The propagation mechanism of multiple fractures in Lucaogou formation is still unclear. Here we show the novel experimental process to investigate the propagation of multi-cluster fractures in the Lucagou shale reservoir and use the acoustic emission (AE) to monitor the dynamic process of hydraulic fracturing. The results show that the fracture interference starts from the initiation stage and is seriously affected by the bedding structure. The fracture interference caused many shear failures of the rock and hindered the fracture height. More clusters per stage lead the fracture interference occurs near the wellhead. A decreasing injection rate can reduce fracture interference. The experimental results provide basic and detailed data for the study of hydraulic fracturing in the Lucaogou shale oil reservoir. 1. INTRODUCTION The shale oil reservoir of the Lucaogou Formation in Xinjiang Province has few natural fractures (Ishida et al., 2017). This makes it difficult to form complex fracture networks by hydraulic fracturing (Li et al., 2020). Therefore, stage-spacing and cluster-spacing techniques have to be employed to stimulate multiple hydraulic fractures to increase the complexity of fracture networks (Luo et al., 2018). However, the growth of multiple fractures can be easily restrained when the stage-cluster spacing is short (Manchanda et al., 2020). This is a problem caused by the interference of strong stresses among multiple fractures. Previous numerical simulations by Wu and Olson (2015) indicate that the "stress shadow" produced by the old fractures could significantly affect the propagation of new fractures. This phenomenon is called fracture interference. Although increasing the cluster spacing can be a direct method to reduce fracture interference, excessive cluster spacing reduces the efficiency of hydraulic fracturing. The optimization of cluster spacing is based on the study of fracture interference. The numerical simulation is applied to study the factors affecting fracture interference, such as perforation friction, rock mechanical properties, and fracturing fluid viscosity (Liu et al., 2020; Xiong et al., 2020). However, there are only few researches on fracture interference caused by the thin interbed, which is mainly limited by the construction of a physical model of thin interbedded. Yang et al. (2020) studied the influence of heterogeneity on multi-stage fracture propagation, but the heterogeneity distribution characteristics in their model cannot reflect the layered characteristics of thin interbeds. Zou et al. (2017) established a layered bedding structure using discrete elements and discussed the influence of the number and physical properties of bedding on hydraulic fracture propagation. However, the fractures in his numerical simulations are plane fractures, which cannot fully reflect the fracture interference caused by non-plane fractures. The field fracturing and experiments of the Lucaogou Formation show that there are a large number of non-planar fractures after fracturing (Huo et al., 2019; Zhang et al., 2021a). At present, it is difficult to explain the problem of multi-cluster fracture propagation due to the influence of thin interbeds by theoretical and numerical methods. Therefore, it is necessary to study the law of fracture propagation under multi-cluster fracturing in Lucaogou Formation through experiments.