Probability of brittle rock failure during hydraulic fracturing of conventional and unconventional reservoirs

Abstract

For interpretation and inversion of microseismic data it is important to understand, which properties of the reservoir rock control the occurrence of brittle rock failure and associated seismicity during hydraulic stimulation. This is especially important, when inverting for key properties like permeability and fracture conductivity. Although it became accepted that seismic events are triggered by fluid flow and resulting perturbation of the stress field, the magnitude of perturbations, capable of triggering failure in rocks, can be highly variable. Parts of the rock, which are unlikely to fracture, will act as flow barriers after termination of stimulation. We compare occurrence of microseismic events at the Cotton Valley gas field to elastic rock heterogeneity, obtained from sonic logs. Our observations suggest that heterogeneity of the rock formation controls the occurrence of brittle failure. In particular, we observe that the density of events is increasing with Brittleness Index (BI) of the rock, which is defined as a combination of Young's modulus and Poisson's ratio. We evaluate the physical meaning of the BI and characterize the influence of elastic rock heterogeneity on the probability of rock failure. Our analysis is based on the computation of stress fluctuations caused by elastic heterogeneity of rocks. We find that stress changes necessary to open and reactivate fractures in rocks are strongly correlated to fluctuations of elastic moduli. A crucial factor for understanding seismicity in unconventional reservoirs is the role of anisotropy of rocks. We evaluate a VTI model corresponding to a shale gas reservoir in the Horn River Basin to understand the relation between stress, event occurrence and elastic heterogeneity in anisotropic rocks.