A Complex Morphologically Regular Pore Network Model to Study Water Retention Curve of Hydrate-Bearing Sediments

Water retention curve essentially determined by pore throat morphology, wettability, pore connectivity and so on has a close relationship with many physical properties of hydrate-bearing sediments. Figuring out its accurate dynamic evolution regularity is of significant importance to the efficient development of gas hydrate deposits. However, most currently used hydrate-bearing networks for capturing the dynamic evolution of water retention curve possess over simplified pore throat cross-sections, resulting in ambiguous evolution law. In this work, a regular hydrate-bearing network with complex pore throat morphology combining circles, squares, arbitrary triangles, regular n-cornered star, and regular polygons in the pattern of grain-coating hydrate is firstly constructed. Then, the capillary entry pressure of different pore throat morphology in the presence of hydrate and process of primary drainage are respectively introduced. Afterwards, primary drainage is carried out in the established network based on invasion percolation. The dynamic displacement characteristics and water retention curves are relatively obtained. Furthermore, factors influencing the dynamic displacement characteristics and evolution of water retention curves in hydrate-bearing sediments such as pore throat cross-section, wettability, coordination number and initial aspect ratio are investigated in detail. Results indicate that the capillary entry pressure increases with increased hydrate saturation due to the reduction of effective pore throat radius caused by hydrate occupation. The number of gas invaded pore bodies and throats grows small with the increase of hydrate saturation at the same capillary pressure, causing large water saturation. The water retention curve evolves to an increasing direction with increased hydrate saturation during primary drainage. Pore throat morphology plays a significant role in capillary entry pressure, the number of gas invaded pore throats at the same capillary pressure, fluid configuration at the same pore throat cross-section, and gas-water spatial distribution, resulting in great difference of water retention curves. With the decrease of wettability to aqueous phase, the capillary entry pressure grows small, and the number of gas invaded pore throats becomes large, resulting in small water saturation at the same capillary pressure. Meanwhile, the proportion of piston-like displacement without water film turns large, leading to large connate water saturation when all water-filled pore throats that satisfy the criteria for gas invasion are invaded. In addition, the number of gas invaded pore bodies and throats increases at the same capillary pressure with increased coordination number, causing small water saturation. At the same time, the proportion of piston-like displacement with water film becomes large, resulting in small connate water saturation. And the water retention curve evolves to the direction of large values with the increase of coordination number. However, the initial aspect ratio has little impact on dynamic displacement characteristics and water retention curves through changing the generated pore body radius while the throat radius is kept constant. This work provides a novel insight into dynamic displacement characteristics and evolution of water retention curves in hydrate-bearing sediments.