Reservoir Stress Path for Underground Gas Storage

Injection or production from porous reservoirs respectively increases or decreases reservoir pore pressure that correspondingly lower or raise the effective stresses. The pore pressures vary with fluid volumes displaced depending on permeability, flow rates, and time. During fluid injection or retrieval, the magnitude and spatial distribution of pore pressures will depend critically on the formation permeability. This consequent changing of effective stresses is of particular concern to underground gas storage (UGS) projects subject to seasonal variations of fluid pressures with the modern demands may result in higher frequencies of filling and discharge. Already, a number of publications have attempted to predict the reservoir’s stress path which is normally presented as changes to the horizontal stresses. In general, for the stress path the total stresses (not the effective stresses) are considered. Here, using a generic numerical model we first investigate the influence of well locations on the reversibility of storage pressures and stresses within the reservoir. The generic model was then adapted to study more specifically using the actual depth, reservoir thickness, and material properties of an existing UGS site in the Bavarian Molasse Basin, Germany. ... mehrThe reservoir is inclined and bounded by an impermeable fault zone and intersected with two boreholes used for injection and retrieval. We simulated various injection and production scenarios with the two wells at different locations within the reservoir. Our results show that the pressure distribution and especially the reversibility of pressure changes depends on the distance of the wells to the fault zone, the production and injection history of the individual wells and on the inclination of the reservoir horizon.