Response characteristics of coal subjected to hydraulic fracturing: An evaluation based on real-time monitoring of borehole strain and acoustic emission

Abstract

With the increasing consumption of conventional oil and gas reservoirs for gas recovery/production, unconventional reservoirs, such as coalbed methane, shale gas, and gas hydrate, have become very popular in recent times. In this regard, hydraulic fracturing is an effective technique commonly used for enhanced coalbed methane recovery. In previous studies, the fracture morphology was described by comparing the fracture morphology before and after hydraulic fracturing from a macroscopic perspective. Because fracture initiation and subsequent networks of fractures are formed instantly when coal mass is subjected to hydraulic fracturing, it is almost impossible to acquire complete information about fracture initiation by only analyzing the change in hydraulic pressure and fracture morphology. In this paper, a triaxial experimental system was developed to simulate hydraulic fracturing using raw coal and briquette coal samples, respectively. The borehole wall strain observed during hydraulic fracturing was plotted (borehole wall strain curves) and the acoustic emission response was also obtained. In addition, the fracture behaviors during hydraulic fracturing were analyzed. Our results show that the response of coal subjected to hydraulic fracturing can be divided into the following four stages: microcrack formation, fracture initiation, unstable crack propagation, and fracture closure. The borehole wall strain curves effectively reflected the deformation and failure of borehole wall. Acoustic emission response can thus be utilized to identify the orientation of fractures during hydraulic fracturing. The combination of the two methods offers an effective option for clarifying the fracture initiation and instability mechanism near the borehole subjected to hydraulic fracturing.