NUMERICAL SIMULATION INVESTIGATION OF SEDIMENT COMPRESSION EFFECTS CAUSED BY DEPRESSURIZATION PRODUCTION OF NATURAL GAS HYDRATE

The sediment compression during the depressurization production of deep-sea gas hydrates can alter the physical and mechanical characteristics of the reservoir, which can have a significant impact on the production results of the reservoir. In order to reveal the evolution of the physical and mechanical characteristics of the reservoir around the production well under the sediment compression effects, in this paper, a theoretical model considering the sediment compression effects is established, and the evolution of physical and mechanical characteristics of reservoir around well and the production results during depressurization production under different initial intrinsic permeability, initial hydrate saturation and bottomhole pressure conditions is investigated by COMSOL numerical simulation. The results show that the permeability in the hydrate decomposition area increases and then decreases with increasing distance from the well due to the influence of sediment compression. The rates of gas and water production rise immediately from zero to a peak and then decline rapidly, and are lower when sediment compression is compression is considered than when it is not. In the area of complete hydrate decomposition, the magnitude of permeability is negatively correlated with the effective stress and in the undecomposed area the magnitude of permeability is negatively correlated with the hydrate saturation. The lower the bottomhole pressure, the higher the effective stress and the more significant the decrease in permeability around the production well. There is an inflection point for the effect of initial hydrate saturation on gas and water production, and the inflection point for saturation is between 0.25 and 0.35. High hydrate saturation does not mean good reservoir recovery, because the rate of gas production is also related to the permeability of the reservoir. Reservoirs with high hydrate saturation have lower permeability and lower gas production rate, while high initial intrinsic permeability of the reservoir significantly contributes to the production results, but the larger reservoir deformation increases the instability of the reservoir.