Shale failure processes and spatial distribution of fractures obtained by AE monitoring

Abstract

The study of the shale rocks' fracturing characteristics reveal general guidelines for hydraulic fracturing processes in shale reservoirs, and the microseismic information provides a direct reflection of the fracturing process. Acoustic emission (AE) testing of rocks in the laboratory can be used for the application of microseismic information acquired in the field. Therefore, uniaxial compression experiments were performed to study AE characteristics during the failure process in shale rocks. In this study, the number of AE hits, along with the energy, a-values and b-values, were used to characterise the AE data and to describe the rock's rupture behaviour. The testing results indicated that the AE rates and energy values increased markedly when slippage occurred and these parameters reflect the generation of micro-cracks in the rock. The b and a-value curves can be divided into four stages corresponding with the fracture processes. The changes in the b and a -values can indicate the distribution characteristics of cracks within the rock. Based on this theory, the AE characteristics from dry and water-saturated shale samples under uniaxial compression were compared to demonstrate that water diffuses pressure in the rock and promotes fracture network generation. The b-values in the elastic-yield stage increase along with the brittleness index, and the b-values in the post-peak stage can offset the limitations of brittleness indexes used in evaluations of reservoir fracability. Exploring the law of rupture for shale under pressure provides a foundation for microseismic-based quantitative evaluation of the hydraulic fracturing stimulation efforts in shale reservoirs; therefore, this knowledge has both scientific and engineering significance.