“Rigid-elastic chimera” pore skeleton model and overpressure porosity measurement method for shale: A case study of the deep overpressure siliceous shale of Silurian Longmaxi Formation in southern Sichuan Basin, SW China

Abstract

Based on analysis of pore features and pore skeleton composition of shale, a “rigid elastic chimeric” pore skeleton model of shale gas reservoir was built. Pore deformation mechanisms leading to increase of shale porosity due to the pore skeleton deformation under overpressure were sorted out through analysis of stress on the shale pore and skeleton. After reviewing the difficulties and defects of existent porosity measurement methods, a dynamic deformed porosity measurement method was worked out and used to measure the porosity of overpressure Silurian Longmaxi Formation shale under real formation conditions in southern Sichuan Basin. The results show: (1) The shale reservoir is a mixture of inorganic rock particles and organic matter, which contains inorganic pores supported by rigid skeleton particles and organic pores supported by elastic-plastic particles, and thus has a special “rigid elastic chimeric” pore structure. (2) Under the action of formation overpressure, the inorganic pores have tiny changes that can be assumed that they don't change in porosity, while the organic pores may have large deformation due to skeleton compression, leading to the increase of radius, connectivity and ultimately porosity of these pores. (3) The “dynamic” deformation porosity measurement method combining high injection pressure helium porosity measurement and kerosene porosity measurement method under ultra-high variable pressure can accurately measure porosity of unconnected micro-pores under normal pressure conditions, and also the porosity increment caused by plastic skeleton compression deformation. (4) The pore deformation mechanism of shale may result in the “abnormal” phenomenon that the shale under formation conditions has higher porosity than that under normal pressure, so the overpressure shale reservoir is not necessarily “ultra-low in porosity”, and can have porosity over 10%. Application of this method in Well L210 in southern Sichuan has confirmed its practicality and reliability.