Abstract

Two commonly estimated fractal dimensions, called b and D values, represent statistical characteristics in the distribution of magnitude sizes (b) and spatial hypocenter locations (D) of microseismic events, respectively. We establish that the values of these two dimensions are related to specific stress regimes. Through the case study of a heavy-oil field drained using cyclic steam stimulation, we infer that the measured temporal variation in fractal dimension b is most likely due to significant changes in the local stress regime over a seven-month period, ranging from extensional faulting (fractures opening), via a strike-slip regime, to finally compressive faulting (fractures closing). The fracture dimension D indicates predominantly planar-to-spherical hypocenter spatial distributions in the first and last stages, but changes to a more linear-to-planar spatial pattern in the intermediate strike-slip regime when the vertical stress is anticipated to be in between the maximum and minimum horizontal stresses. These changes could be due to localized pore-fluid overpressure. A statistical analysis of the microseismic event locations and their magnitudes is therefore a useful method to understand reservoir geomechanics and thereby facilitate its management.

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