Effect of pore-throat structure on air-foam flooding performance in a low-permeability reservoir

Abstract

In this study, effect of pore-throat structure on air-foam displacement performance in a low-permeability reservoir has been experimentally examined. More specifically, porosity and permeability of samples collected from the Upper Triassic Yanchang Formation of the Chang 6 low-permeability oil reservoir in the Ordos Basin, China, were measured, and then their petrological and petrophysical properties as well as pore and throat size distributions were determined by integrating thin-section analysis, x-ray diffraction analysis, high-pressure mercury injection (HMI) and rate-controlled mercury injection (RMI) tests, and NMR technique. Based on the ultimate displacement efficiency and the measured washing efficiency, the underlying recovery mechanisms associated with air-foam flooding in two sub-member samples which have different pore-throat distributions (i.e., sample C6-1: Concentrated throat distribution and sample C6-2: Dispersed throat distribution) are rather different, implying that the timing for converting water injection to air-foam flooding shall be different as well. Also, the difference of throat-distribution characteristics between sub-members C6-1 and C6-2 is found to impose a great but different influence on the air-foam displacement performance by improving their oil washing and microscopic sweep efficiencies, respectively. This finding is of a great significance to help design, evaluate, and optimize the injection-production strategies in a low-permeability reservoir with different pore-throat distributions.