Unconventional Resources (Jan 2022)
Petrophysical experiment-based evaluation method for the saturation of gas hydrate
Abstract
The seas around China contain abundant non-diagenetic gas hydrate deposits. The traditional logging and evaluation techniques based on conventional oil and gas no longer satisfy the evaluation requirements of offshore gas hydrate layers, and it remains difficult to calculate the hydrate saturation accurately, which seriously restricts the accurate evaluation of the resources. Petrophysical experiments are an important means to study gas hydrate. By simulating the hydrate formation conditions in sediments under natural conditions, the study of various petrophysical properties of gas hydrate is essential to the exploration and development of hydrates and the prediction of reservoir parameters. In this study, a tetrahydrofuran (THF) solution, montmorillonite, and natural sea sand were selected to simulate the growth of gas hydrate based on its characteristics in the study area, and in situ CT scanning experiments were carried out at different hydrate formation stages to determine the microscopic morphologies of gas hydrates within the pore space. In addition, the acoustic wave and resistivity of hydrates with different saturations were measured. Forward modeling of a differential effective medium (DEM) model was conducted for the hydrate reservoirs with different saturations, and the gas hydrate morphology was analyzed. Thereafter, the hydrate saturation predictions based on the petrophysical experiments were analyzed. The results of these analyses indicate that the microscopic morphology of hydrates in the study area is primarily pore-filling form. Based on the saturation estimations using the Archie's law, Wood equation, time-average equation and the DEM method, and under the constraint of the acoustic and electrical measurement results of the petrophysical experiments, an electric acoustic joint model was built by integrating the resistivity and acoustic waves. The estimation results indicate that the joint electro–acoustic model is the best method for calculating the saturation in the study area.
