Modelling of Fracture Strength Distribution in Elastically Heterogeneous Rocks

Modelling the stress conditions inside hydrocarbon and geothermal reservoirs is important to predict fracture behaviour during the injection of fluids. We analyse the influence of elastic heterogeneity on stress and fracture strength distribution in rocks. Therefore, 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 analyse relations between elastic modules stress state and fracture strength. The stress modelling analysis performed in this paper suggests that the stress state in elastically heterogeneous rocks can be highly heterogeneous. Our modelling study according to elastic heterogeneity derived from sonic well log data along the KTB main hole results in a broadly distributed fracture strength between -10 to 20 MPa. We find strong relations 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.