天然气水合物降压开采储层内压力变化规律分析

Analysis for pressure variation of hydrate reservoir during gas production by depressurization

以往的天然气水合物开采数值模拟研究大多集中于产能结果分析,鲜见关于降压开采过程中储层内部压力变化规律的研究.本文基于质量守恒、能量守恒和达西定律,建立了水合物降压开采气、水和水合物三相一维模型,并首次在模拟计算中引入压降半径公式,利用有限差分法,采用IMPES(隐式压力-显式饱和度)方法求解模型.文中对比研究了井口压力、温度、渗透率和水合物分解对储层内压力变化的影响,结果表明:降压开采水合物导致井口附近形成低温区,可能引起水结冰和水合物再生的现象;储层绝对渗透率越大,压降在储层内的传播越快,总产气量也越大;水合物分解引起得水气两相相对渗透率变化和分解产生的水气是影响储层内压降传播速度的主要因素.

Extensive numerical simulation study of natural gas hydrate is mainly limited to gas productivity, which seldom analyzes the pressure variation of hydrate reservoir during gas production. A three-phrase, one-dimensional numerical model is developed to simulate gas production from hydrate reservoir with consideration of radius of pressure reduction for the first time. The model includes dissociation thermodynamics, kinetics, multiphase flow and conductive and convective heat transfer. A finite-difference scheme is used, which is implicit in pressure and temperature, and explicit in saturation （IMPES）. Well pressures, temperature, permeability and hydrate dissociation were investigated to evaluate the influence on pressure variation distribution. The results suggest that gas production by depressurization may cause considerable decrease of temperature, which favors the formation of ice and reformation of hydrate. Increased permeability comes with efficient pressure reduction and better cumulative gas production. The variation of relative permeability of gas and water and hydrate dissociation effect have significant impact on the efficiency of pressure reduction in hydrate reservoir.