Influence of Hydraulic Heterogeneity of Rocks on Pore Pressure Changes Induced by Reservoir Stimulations

We interpret borehole logging measurements from the German Continental Deep Drilling Program (KTB) to construct a hydraulic diffusivity profile for the central part of the main borehole. The profile indicates strong fluctuations of diffusivity ranging over three orders of magnitude. We find that the variations can statistically be described by a log-normal distribution and by a power-law scaling in the corresponding power spectrum. We generate 2D fractal random media which obey the statistical properties of the derived diffusivity log as representations of hydraulically heterogeneous rock. We numerically simulate the diffusion of pore pressure perturbations caused by a fluid injection in such a rock model. We compare the result of the modelling with a model where a homogeneous distribution of an effective hydraulic diffusivity (equal to the mean of the log-normal distribution of diffusivity variations) was applied. We find that the magnitude of pore pressure changes is affected, in particular in the vicinity of the injection borehole where regions of strong enhancement as well as reduction exist. Another feature is the development of flow paths along which the injection pressure is transported. We assume that these flow paths reflect higher permeable fractures embedded in the low porosity rock matrix.