Stress sensitivity of carbonate gas reservoirs and its microscopic mechanism

Abstract

In order to evaluate the stress sensitivity of carbonate reservoirs, a series of rock stress sensitivity tests were carried out under in-situ formation temperature and stress condition. Based on the calibration of capillary pressure curve, the variable fractal dimension was introduced to establish the conversion formula between relaxation time and pore size. By using the nuclear magnetic resonance (NMR) method, the pore volume loss caused by stress sensitivity within different scales of pore throat was quantitatively analyzed, and the microscopic mechanism of stress sensitivity of carbonate gas reservoirs was clarified. The results show that fractures can significantly affect the stress sensitivity of carbonate reservoirs. With the increase of initial permeability, the stress sensitivity coefficient decreases and then increases for porous reservoirs, but increases monotonously for fractured-porous reservoirs. The pore volume loss caused by stress sensitivity mainly occurs for mesopores (0.02–0.50 μm), contributing more than 50% of the total volume loss. Single high-angle fracture contributes 9.6% of the stress sensitivity and 15.7% of the irreversible damage. The microscopic mechanism of the stress sensitivity of carbonate gas reservoirs can be concluded as fracture closure, elastic contraction of pores and plastic deformation of rock skeleton.