From Rock Physical Data to a Geomechanical Model - A Reservoir Safety Study

We did multi-stage triaxial compressive strength tests in conjunction with 3D in situ stress determinations (applying the RACOS® technique) on Middle Jurassic silt- and sandstones from the North Sea Basin which are the caprock of a hydrocarbon reservoir. The achieved data served as basis for a geomechanical safety model of a hydrocarbon reservoir / hypothetical underground gas storage. The rock mechanical tests revealed the siltstone to be slightly stronger and stiffer than the sandstone. Of particular interest for the geomechanical model (based on the centroid model of Finkbeiner et al. 2001) are the angles of internal friction measured at post-failure conditions (φresidual) which lie between 27° (siltstone) and 32° (sandstone). RACOS® investigations showed that the stress field in the reservoir is of normal faulting nature with a total vertical stress S1 of ca 61 MPa and the minimum total horizontal stress S3 of ca 49 MPa (oriented NNW-SSE). For our geomechanical model we applied beside φresidual , S1 and S3 the pore pressure within the caprock Ppc of roughly 32 MPa. Based on these data our model shows that pore pressure within the reservoir can be up to 10 MPa above Ppc without reactivating inactive faults within the caprock.