52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: Two 0.3 m × 0.3 m × 0.3 m cubic blocks of shale and coal were used for hydraulic fracturing experiments under true tri-axial stress conditions. The shale block used was highly homogeneous and without visible fractures, while the coal block contained a host of natural fractures. The mechanical and hydraulic properties of both rocks were characterized through multi-stage triaxial tests, Brazilian disk tests, and porosity and permeability measurements. A true tri-axial rock testing machine equipped with loading, pump and acoustic systems was used in the experiment. The acoustic system uses 48 transducers with active sources to repetitively generate and receive ultrasonic P/S wave pulses to reveal fracture initiation and growth. Before the experiment, initial seismic response of both blocks was recorded under hydrostatic stress conditions to characterize anisotropy and heterogeneity of the blocks as reference. Silicon oil was injected centrally into both blocks to create a hydrofracture under deviatoric stress conditions and the load, displacement, pump pressure and volume, and seismic response during the injection process were recorded. Results from two blocks are being compared in terms of hydrofracture geometry and seismic features.
Hydraulic fracturing is both an efficient and widely used technique to enhance permeability and increase productivity in unconventional gas reservoirs and in releasing gas pressure to combat coal and gas outburst hazards in coal mines. In this context, understanding fracture initiation, propagation, and its interaction with other fractures, plays a significant role both in estimation of simulated rock volume (SRV) in unconventional reservoirs and in prevention of mining hazards.
Including the rock properties and the operational procedures, there are different parameters affecting the hydraulic fracturing performance. There is a considerable amount of research on understanding the hydraulic fracture behaviour in different rock formations (Guo et al., 1993; Akrad et al. 2011; Ding et al., 2012; Fischer and Warpinski, 2012; Zhang et al., 2013; Padin et al, 2014; Li et al., 2015; Stoechert et al., 2015; Liang et al., 2017; Zhu et al., 2017). However, most of the experimental research was conducted on isotropic and homogeneous rock or synthetic samples, which resulted in unrealistic estimations.
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