Sand production evaluation during gas production from natural gas hydrates

摘要

Natural gas hydrates are a potential energy resource that can ease the energy crisis, and their development has attracted an increasing amount of worldwide attention. Submarine natural gas hydrates are mainly stored in the shallow strata of deep water, and the reservoirs are poorly cemented and have low strengths. In addition, the dissociation of natural gas hydrates during the development process can greatly reduce the strength of these reservoirs. As a result, sand production easily occurs. Based on the geomechanical characteristics of submarine hydrate reservoirs, this study analyzed the changes in the reservoir characteristics caused by the coupling effect of a multiphysical field and the dissociation of natural gas hydrates during the development of natural gas hydrate reservoirs. A numerical simulation was used to analyze the characteristics that determine the plastic yielding of the rock surrounding the well and the laws of sand production during gas production from hydrate reservoirs. The results showed that with the increase in the pressure drop during production and the difference between horizontal in situ stresses in the reservoirs, the stress concentration on the surrounding rocks of the wells will increase, enabling the reservoirs to more easily produce sand. Furthermore, an increasing pressure drop during production also promotes hydrate dissociation and lowers the strength of the rock surrounding the well, causing large-scale plastic yielding in the rock surrounding a well; consequently, this causes sand production. Moreover, with higher bottom-hole temperatures and reservoir permeabilities, the natural gas hydrates will dissociate faster, which accelerates the strength reduction of the reservoir, and sand will be more easily produced. During the initial period of production, the intensity of sand production has no defined relationship with the initial hydrate saturation. However, as the production time increases, especially after the dissociation of the hydrates near the well, and with higher initial hydrate saturation, the sand production issue becomes more serious. These research results provide a theoretical basis for the strategic formulation of sand control measures during natural gas hydrate production.