Numerical simulation on sand production in the exploitation process of gas hydrates based on skeleton failure of reservoirs

Abstract

Gas hydrates have become a new focus in resource exploration and development. However, the sand production problem in the exploitation process of gas hydrate reservoirs greatly restricts the commercial-scale operation of hydrates. In this paper, by introducing a plastic softening stage, a thermal-fluid-solid coupled mathematical model for sand production based on the skeleton failure and phase change of hydrates in reservoirs is established. By using the model, the sand production process of reservoirs at the wellhead under depressurization is truly simulated and the dynamic change of gas hydrates in hydrate reservoirs is analyzed. In addition, the mechanism and change law of sand production are analyzed mainly from the perspective of rock mechanics. Research result shows that the propagation rate of the pore pressure is always larger than the reduction rate of the hydrate saturation. When the pore pressure reduces to a point below the equilibrium pressure of hydrates, the hydrates are not immediately dissociated. The hydrates are dissociated only when the drawdown pressure exceeds a certain range; the hydrates are rapidly dissociated in the early phase. With the growth of the exploitation time, the reduction rate of the hydrate saturation obviously slows down; the sand production is first found at the position with the minimum horizontal principal stress and it hardly extends along the circumferential direction but constantly causes an erosion effect inwards the wellhead. The sand production undergoes three stages: initial, large-scale and stable sand productions. The large-scale sand production stage should be concerned in the exploitation process, during which it is the most possible to induce the large-area skeleton failure and further trigger the large-scale sand production.