Impacts of water flooding on pore structure of sandstone reservoirs—case study of Wang Guantun oilfield, Bohai Bay Basin, China

To study the variation mechanism of pore structure of sandstone reservoir in different stages of water flooding, water flooding experiment and a series of parallel tests are performed on typical continental sandstone samples from the Bohai Bay Basin of China, including thin sections (TS), scanning electron microscopy (SEM), X-ray diffraction (XRD), rate-controlled mercury injection (RMI), and X-ray computer tomography (CT). SEM results showed that three pore types existed in sandstone reservoir, including inter-granular pores, dissolution pores, and pores within clay aggregates. By comparing the mineral composition and pore throat characteristics of samples, we observed the variation characteristics of pore structure in different stages of water flooding are different, and the potential factors that affect these variations mainly were the original pore structure of rocks and the content of swelling-intend mineral. For the reservoir with larger pore size and more smectite, pore spaces were reduced, and pore throats were blocked by the clay swelling and particle migration in the early stage of water flooding. As water injection volume increases, the injected water plays a major role to wash tiny particles away, which results in pore spaces increasing and some closed pores re-open. For the reservoir with smaller pore size and less smectite, the dominant mechanism in the early stage of water flooding was pore space cleaning—tiny particle was washed away from the pore path, which results in pore size increasing. As water injection volume increases, clay swelling reaction becomes to be the critical factor, reducing the pore size. This study makes clear the variation regularity and the variation mechanism of pore structure of continental sandstone reservoir in different stages of water flooding based on the combination of CT scan and water flooding experiment.