欠饱和煤储层渗透率动态变化模型及实例分析

开发过程中因受应力压实效应和基质解吸收缩效应的共同影响，导致煤储层渗透率发生复杂的变化。目前，已有诸多学者建立一系列的煤储层渗透率动态模型。然而，对欠饱和煤层气藏开发过程中的不同生产阶段，何种效应对煤储层渗透率起主导作用仍未达成共识。本研究在总结已有的渗透率变化模型的基础上，分析欠饱和煤层气藏开发过程中的降压解吸特征、有效应力效应、基质收缩效应和克林肯伯格效应，并对现有的渗透率模型进行改进与对比分析，以达到定量分析欠饱和煤层气藏储层渗透率变化规律的目的，最后通过鄂尔多斯东南缘韩城煤层气田实例分析煤储层渗透率动态变化特征及其主控因素。结果表明欠饱和煤层气藏开发过程中渗透率动态变化特征可以临界解吸压力划分为两个阶段，前一阶段仅为有效应力效应作用阶段，后一阶段则受有效应力效应、基质收缩效应和克林肯伯格效应影响，且后两种效应随着储层压力的降低而进一步显现。对比分析显示SD改进模型在描述欠饱和煤层气藏渗透率动态变化上优于PM改进模型。因此，借助SD改进模型对韩城煤层气井进行实例计算，分析结果显示煤储层渗透率改善效果依次为3#>11#>5#，区内煤储层渗透率改善效果取决于含气饱和度，而渗透率应力伤害受控于地解压差。

Permeability Variation Models and Case Studies for Undersaturated Coalbed Methane Reservoirs

Coal reservoir permeability is under very complicated change due to the combination of the effective stress effect and matrix shrinkage effect during the depletion of reservoir pressure. Nowadays a large number of models have been established to describe permeability variation with the depletion of reservoir pressure. However, no attempt has been made to draw enough attention to the difference of the effect of various factors on permeability variation in different production stages of unsaturated coalbed methane (CBM) reservoirs. This paper summaries the existing and common permeability models, and determines the relationship among various effects (effective stress effect, matrix shrinkage effect and Klinkenberg effect) and desorption characteristics of the recovery of unsaturated CBM reservoirs, then establishes two improved models to describe quantificationally permeability variation by improving the existing models, and finally discusses influence factors on permeability variation with a case study of Hancheng CBM field in Ordos Basin. Results show that the permeability variation during the recovery of unsaturated CBM reservoirs can be divided into two stages: the first one is that the permeability variation is only affected by the effective stress effect, and the second is that the permeability variation is affected by the combination of effective stress effect, matrix shrinkage effect and Klinkenberg effect. In the second stage, matrix shrinkage effect and Klinkenberg effect play much more significant role than effective stress effect, leading to the increase of permeability with depletion of reservoir pressure. A comparative study of the improved models indicates that the improved SD model is more sensitive to various parameters than the improved PM model and the improved models, and can describe dynamic permeability variation more exactly than the original ones. Thus, the case study, by improved SD model, on Hancheng CBM field shows that the increase in coal reservoir permeability is determined by gas saturation, and the damage in coal reservoir permeability is controlled by the difference between the initial reservoir pressure and critical desorbed pressure, and the descending order of the average increment is 3#, 11# and 5#.