Experimental Study on the Change of Resistivity of Synthetic Methane Hydrate Under Different Saturation and Clay Composition Conditions

The electric characteristics of a hydrate reservoir are the basis for evaluating porosity and saturation. Because drilling hydrate core samples are unstable at ambient temperature and pressure, and deep-sea drilling is very expensive and limited, simulation experiments on synthetic methane hydrate sediment samples in a laboratory are important tools for understanding the physical properties of marine gas hydrate sediments. This paper quantitatively analyzes the relationship between hydrate saturation and reservoir resistivity test experiments to study the formation and decomposition characteristics of hydrates of different saturation under different temperatures and clay content conditions, which is similar to the China Sea target area. The experimental device used in this study is a self-designed and developed methane hydrate reactor. Methane hydrates were generated under low temperature and high-pressure conditions using high-purity methane gas and deionized water with natural sea sand simulated sediments with particle sizes of 0.18 ~ 0.25 mm and 0.425 ~ 0.85 mm. It is composed of three types of experiments: (1) variation characteristics of resistivity during the formation of methane hydrates under different conditions of temperature; (2) influence of different hydrate saturation on the resistivity during the formation process; (3) effect of clay components on the resistivity of methane hydrates. In the process of hydrate formation, the general trend of resistivity changes under different hydrate saturation conditions is the same. The results show that: (1) When the methane hydrate saturation is low, the resistivity increases slowly, which is because the hydrate is first generated in the large pores. However, with the formation of a hydrate, the salt in the formation water is analyzed, and the mineralization of the formation water increases, which plays a role in resistance; (2) With the further increase of hydrate saturation, the resistivity increases significantly, and the growth rate is larger, which is because the hydrate gradually changes from dispersed type to cemented type, and starts to block the throat, resulting in the rapid increase of resistivity; (3) When the hydrate saturation is 20 to 60%, the resistivity of the hydrate is basically the same under the conditions of no clay and 10% clay, indicating that clay has no effect on the resistivity of hydrate. When the hydrate saturation is greater than 60%, the clay increases the resistivity of the hydrate to a certain extent. The accuracy of the experiment is verified in two wells in the South China Sea, answering questions about the low resistivity of the hydrate reservoir in the area.