Numerical Simulation of Near Wellbore Reservoir Stability during Gas Hydrate Production by Depressurization
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摘要: 天然气水合物藏降压开采是一个含相态变化的非等温物理化学流固耦合渗流过程。目前有关天然气水合物藏开采的研究集中于产能模拟,且没有考虑流固耦合作用影响,有关水合物分解形成的弱胶结、低强度、高孔高渗近井储层的稳定性研究尚未开展。为此,将水合物分解效应融合到渗流场与岩土变形场的耦合作用中,建立了天然气水合物藏气、水两相非等温流固耦合数学模型,引入出砂判别准则,开发了天然气水合物藏降压开采流固耦合储层稳定性分析软件,利用该软件对天然气水合物藏降压开采近井储层稳定性的一般规律进行了分析,并建立了井壁不出砂的临界生产压差分析方法。研究表明:水合物分解效应是影响水合物分解区储层稳定性的主要因素,流固耦合作用的影响较小;近井水合物分解区储层稳定性较差,其中井壁最小水平地应力方向储层稳定性最差,是出砂的优先位置;过渡区储层稳定性介于水合物分解区与原状储层之间。Abstract: Gas hydrate production from reservoir by depressurization is a non-isothermal flow process in porous medium involving hydrate phase change and fluid-solid coupling.The existing study on production of gas hydrate reservoirs is mainly limited to productivity simulation,without consideration of fluid-solid coupling effect.Due to hydrate decomposition,the near wellbore formation of hydrate bearing layers will have weak consolidation,low strength and high porosity and permeability.And the stability of the near wellbore formation shall be taken seriously but the relevant researches have not yet begun.Therefore,incorporating the effect of gas hydrate dissociation into the interaction between porous fluid flow and rock deformation,a gas-water two-phase non-isothermal fluid-solid coupling model was established and a corresponding finite element program was developed based on the sand production criterion.The general rule of the stability of near wellbore formation of hydrate reservoirs has been analyzed,and a method for critical drawdown pressure determination has been proposed.The research shows that the effect of gas hydrate dissociation is the dominant factor affecting the formation stability and the effect of the fluid-solid coupling on stability is negligible.The stability of near wellbore formation is relatively weak and the wellbore in the direction of minimum horizontal ground stress is in the worst in stability,where the sand production may occur first.The stability of the transient area is between the dissociation area and the undisturbed formation.
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