Geophysical Understanding of Microseismicity
Geo-mechanical Modeling of Fracture Strength Distribution in Elastically Heterogeneous Rocks
We analyze the influence of elastic heterogeneity on stress and fracture strength distribution in rocks. We simulate the distribution of elastic modules inside a reservoir rock as a 3D fractal random medium according to parameters obtained from sonic well logging data. Using an ABAQUS finite element stress
analysis model we determine the stress field inside the rock volume. By applying geo-mechanical considerations we then compute the fracture strength distribution and analyze relations between elastic modules, stress state and fracture strength. The stress modeling analysis performed in this paper suggests that the stress state in elastically heterogeneous rocks can be highly heterogeneous. We find strong dependencies between elastic modules, stress state and fracture strength, which can be applied to predict the stress distribution in hydrocarbon and geothermal reservoirs and the occurrence probability of fluid injection induced seismicity.
Acoustic Emission in Sandstone Samples: Pore Pressure Pulses and Kaiser effect
Rocks show a memory for previously applied stress (Kaiser effect) i.e. AE is mainly observed when the applied stress exceeds the previously applied one. This effect is also observed in field and laboratory for pore pressure (apparent Kaiser effect with respect to pore pressure). Here we study AE activity in water-saturated pre-stressed Flechtingen sandstone samples subjected to changes in pore pressure. Especially, we investigate the reason for the apparent Kaiser effect with respect to pore pressure by means of Mohr Coulomb analysis. We show that microseismicity provides a basis for computing the internal friction coefficient. We show that it is necessary to consider effects on microscale (μm − mm scale) and plasticity effects, because we work under pre-failure conditions.
Microseismic Back Front Signatures for Nonlinear Fluid-Rock Interaction
We explore back front signatures of spatio-temporal seismicity evolutions accounting for nonlinear fluid-rock interaction. For pressure-dependent transport properties our aim is to better understand post-injection induced seismicity. For this we solve nonlinear diffusion equations for a time-dependent injection source. Based on the obtained pore-fluid pressure evolutions within the medium we generate synthetic clouds of microseismic events. We then analyze spatio-temporal dynamics of post-injection induced seismicity.