Gas–Water Characteristics of Tight Sandstone in Xihu Sag, East China Sea Basin under Different Charging Models

Abstract

The Xihu sag has two main oil−gas fields: Huagang Gas Field and Pinghu Oil Field. The Huagang formation is the reservoir of the Huagang Gas Field in the Central Tectonic Zone, while the Pinghu formation is the reservoir of the Pinghu Oil Field in the Western Slope Zone. In this paper, which mainly focusses on the Huagang formation, we conducted gas-driven water displacement–magnetic resonance imaging (GWD-MRI) experiments to simulate the charging characteristics of the sandstone migration layer, centrifugal magnetic resonance (Cen-NMR) experiments to simulate the short-term rapid trap charging process, and semi-permeable baffle (SPB) charging experiments to simulate the slow trap accumulation process. The results indicate that a start-up pressure exists for migration layer charging, where the start-up pressure for a core with a permeability of 0.3 mD is about 0.6 MPa. Our experimental simulations confirm that a planar front of changing water saturation exists, which has a width of about 1–1.5 cm. Migration layer charging is mainly influenced by two actions: the drive effect and the carrying effect. The drive effect can reduce the water saturation to 70–80%, while the carrying effect can further reduce the water saturation by 5–10%. The water saturation in the rapid charging scenario is mainly affected by the petrophysical characteristics of the rock: if the porosity is high, the water saturation is low. The water saturation decreases significantly with the increase in centrifugal force when the centrifugal force is small; however, when the centrifugal force is greater than 0.8 MPa, the water saturation decreases slowly. In the slowly charging trap experiment, the water saturation was basically stable at 40–50%, which matched the measured water saturation of the airtight cores well (ranging from 40–55%), and the petrophysical characteristics of the rock did not have a significant effect on the final water saturation.