Elastic Rock Heterogeneity Controls Brittle Rock Failure during Hydraulic Reservoir Stimulation

It is still an open question, to what extend brittle rock failure and associated microseismicity during hydraulic reservoir stimulation is related to elastic heterogeneity of the rock being fractured. We compare occurrence of microseismicity to elastic rock heterogeneity, obtained from sonic logs. Our observations suggest that rock heterogeneity controls the occurrence probability of brittle failure. We observe that the density of events is increasing with the Brittleness Index (BI) of the rock, which is defined as a combination of Young’s modulus and Poisson’s ratio. Because the BI has been introduced without physical justification, we evaluate its physical meaning and present a model to characterize the influence of elastic rock heterogeneity on the occurrence probability of rock failure. Our analysis is based on the computation of stress fluctuations caused by elastic heterogeneity of rocks. In the case of elastically isotropic rocks, our model suggests that the probability of rock failure increases with the BI. However, evaluation of a transverse isotropic model confirms that there are limitations in the applicability of the BI as a sweet spot indicator for hydraulic fracturing placement. Interpretation of the BI has to be done with great care, because its physical meaning varies for different reservoir locations.