Melanie Grob

Inferring in-situ stress changes by statistical analysis of microseismic event characteristics

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 stre...

Statistical Analysis of Microseismic Event Locations and Magnitudes, and their Geomechanical Implications

Summary It is often postulated that many naturally occurring processes are characterized by power-law distributions and exhibit thus fractal properties. Two commonly estimated fractal dimensions in earthquake and rockfracture experiments are the so-called b and D values, representing statistical characteristics in the distribution of magnitude sizes (b) and spatial hypocenter locations (D), respectively. We analysed microseismic events recorded in 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 an 8-month period, ranging from extensional faulting (fractures opening), via a strike-slip regime, to finally compressive faulting (fractures closing). The fracture dimension D indicates spatially predominantly planar to spherical hypocenter distributions in the first and last stage, 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.