天然气水合物降压开采沉积物压缩效应数值模拟研究

深海天然气水合物降压开采过程中,沉积物的压缩会改变储层的物理力学特性,进而对天然气的开采效果产生显著影响.为揭示沉积物压缩效应下井周围储层物理力学特性演化规律,本文建立了考虑沉积物压缩效应的理论模型,通过COMSOL模拟研究了不同初始固有渗透率、初始水合物饱和度和井底压力条件下的降压开采中生产井周围储层的物理力学特性演化规律以及开采效果.结果表明:受沉积物压缩的影响,水合物分解区的渗透率随着与井筒距离的增加先增加后减少;产气与产水速率由零立即上升至峰值,然后迅速下降,并且考虑沉积物压缩时的产气与产水速率比不考虑时低;在水合物完全分解区,渗透率的大小与有效应力成负相关关系,未分解区渗透率的大小与水合物饱和度成负相关关系;井底压力越小,有效应力越大,生产井周围储层的渗透率下降越明显;初始水合物饱和度对产气与产水的影响存在拐点,饱和度拐点位于0.25与0.35之间,高水合物饱和度并不代表储层开采效果好,产气速率的高低还与储层的渗透率有关,高水合物饱和度储层的渗透率较低,产气速率较低;储层初始固有渗透率较高时显著促进了开采效果,但储层变形量较大增加了储层的不稳定性.

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.