ASSESSMENT OF THE STRESS STATE OF A RESERVOIR BASED ON THE DYNAMICS OF MICROSEISMIC EVENT OCCURRENCE DURING FLUID INJECTION

The study examines the possibility of estimating in-situ stress state parameters from the temporal evolution of microseismic events accompanying fluid injection into a porous reservoir. The approach is based on statistical modeling of microseismicity induced by pore pressure increase in a rock mass under frictional equilibrium. The relationship between the shape of event count versus pore pressure curves and the stress tensor parameters, as well as the characteristics of the orientation distribution of potential slip planes, is analyzed. It is shown that the evolution of microseismic activity is characterized by the presence of local maxima corresponding to tangency between Mohr circles and the Coulomb failure criterion, which makes it possible to relate their positions to the parameters of the stress state, in particular to the magnitude of the intermediate principal stress. It is established that under moderate anisotropy of slip plane orientations and a sufficient number of recorded events, the intermediate principal stress and the Lode–Nadai coefficient can be recovered with high accuracy. In contrast, at high anisotropy of orientations, the local maxima become poorly distinguishable, leading to a significant reduction in the informativeness of the microseismic activity curves. The obtained results demonstrate that the shape of microseismic activity curves can be used for a preliminary assessment of the stress state and horizontal stress magnitudes from microseismic monitoring data, as well as for evaluating the reliability of such estimates depending on the quality and volume of observations.