Influence of unconnected pores on effective stress in porous geomaterials: Theory and case study in unconventional oil and gas reservoirs

Unconventional oil and gas reservoirs are mostly characterized by unconnected pores and abnormal pore pressures. These conventional effective stress theories, such as Biot's theory, cannot display the quantitative relationship between unconnected pores' micro characteristics and the macro stress applied to the solid structure. These parameters related to the unconnected pores should be quantitatively incorporated in the effective stress formula. In this paper, two modified models were proposed to evaluate the solid framework's real effective stress and deformation. The effects of the unconnected pore pressure, unconnected porosity, and unconnected pore fluid's compressibility on effective stress and volumetric strain were theoretically investigated. It was found that the increase of unconnected pore pressure can decrease the effective stress and volumetric strain. For an abnormal high pore pressure formation, the growth of unconnected porosity can reduce the effective stress but increase the volumetric strain. The decrease of the unconnected pore fluid's compressibility can increase and decrease the effective stress and volumetric strain for abnormal low and high pore pressure reservoirs, respectively. Two case studies on the abnormal pore pressures induced by under-compaction and hydrocarbon generation were conducted. The results showed that the pore pressure exhibits significant increases when the unconnected pores and unconnected pore fluids are proactively compressed in the processes of undercompaction and hydrocarbon generation, respectively. The abnormal high pore pressures in these tight sandstone reservoirs in the Sichuan Basin, Tarim Basin, and Bohai Bay Basin can be theoretically generated when the pore space is compressed to 95%-84% of the initial pore space. The model can evaluate the effective stress and deformation for reservoirs with considerable unconnected porosities. In turn, it can also interpret the abnormal pressure generation induced by the changes of unconnected porosity and pore fluid's compressibility.