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在煤层气的初级生产过程中,为了获取较高的生产率,需要降低储层压力,储层压力下降对于煤层气的渗透率具有两个相反的效应:(1)储层压力下降,有效应力增加,煤层裂隙压缩闭合,渗透率降低;(2)煤层气解吸,煤基质收缩,煤层气流动路径张开,渗透率升高。Shi和Durucan、Palmer-Mansoori以及Gray等都建立了包含了基质收缩效应以及有效应力的影响的渗透率模型,其模型都基于以下两个关键假设:煤岩体处于单轴应变状态以及竖向应力恒定。为了检验上述两个假设的合理性,建立了一个考虑基质收缩效应以及渗流场-应力场耦合作用下的煤层气流动模型,对煤层气初级生产过程中渗透率的变化进行了耦合分析。分析结果表明:单轴应变的假设具有合理性,而竖向应力是随指向生产井的应变梯度的变化而变化的,其对于渗透率的变化具有重要影响,因此,竖向应力恒定的假设可能导致渗透率预测出现误差;上述渗透率模型都可能低估煤层气初级生产过程中渗透率的变化。
In the CBM primary production process, in order to obtain higher productivity, it is necessary to reduce the reservoir pressure. The reservoir pressure drop has two opposite effects on the CBM permeability: (1) the reservoir pressure is decreased and the effective stress is increased , Fractures of coal seam closed and permeability decreased; (2) Desorption of coalbed methane, shrinkage of coal matrix, expansion of coalbed methane flow path and increase of permeability. Permeability models incorporating the effect of matrix shrinkage and effective stress are established by Shi and Durucan, Palmer-Mansoori, and Gray et al., Whose models are based on two key assumptions: coal and rock mass in uniaxial strain and vertical stress Constant. In order to verify the rationality of the above two hypotheses, a model of coalbed methane flow considering the effect of matrix shrinkage and the coupling of seepage field and stress field was established, and the coupling analysis of permeability change during the primary production of coalbed methane was carried out. The results show that the assumption of uniaxial strain is reasonable and the vertical stress changes with the strain gradient pointing to the production well, which has an important influence on the change of permeability. Therefore, the assumption of constant vertical stress may Leading to errors in prediction of permeability; the above permeability models all may underestimate the change of permeability in the primary production of CBM.