Abstract
A large number of models have been established to describe permeability variation with the depletion of reservoir pressure to date. However, no attempt has been made to draw enough attention to the difference in the effect of various factors on permeability variation in different production stages of unsaturated CoalBed Methane (CBM) reservoirs. This paper summarizes the existing and common permeability models, determines the relationship between 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 quantificationally describe permeability variation, and finally discusses the effects of various factors (gas saturation, cleat porosity, Poisson’s ratio and shrinkage coefficient) on permeability variation. The results show that permeability variation during the recovery of unsaturated CBM reservoirs can be divided into two stages: the first one is that permeability variation is only affected by the effective stress effect, and the second is that permeability variation is affected by the combination of the 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 the effective stress effect, which leads to an increase in permeability with depletion of reservoir pressure. Sensitivity analysis of parameters in the improved models reveals that those parameters associated with gas saturation, such as gas content, reservoir pressure, Langmuir volume and Langmuir pressure, have a significant impact on permeability variation in the first stage, and the important parameters in the second stage are the gas content, reservoir pressure, Langmuir volume, Langmuir pressure, Poisson’s ratio, Young’s modulus and shrinkage coefficient during the depletion of reservoir pressure. A comparative study of the improved models indicates that the improved SD model has a greater sensitivity to various parameters than the improved PM model and the improved models describe permeability dynamic variation more exactly than the original ones.
Highlights
Coal reservoir physical properties are affected by several factors and show dynamic variation through the recovery of CoalBed Methane (CBM)
Permeability variations are controlled only by the effective stress effect when the reservoir pressure is above the critical desorption pressure, while its variations are controlled by the effective stress effect and by the matrix shrinkage effect and the Klinkenberg effect when the reservoir pressure is below the critical desorption pressure
Once reservoir pressure decreases below the critical desorption pressure, the permeability simulated by the improved Palmer and Mansoori model (PM) and Shi and Durucan model (SD) models does not decrease again and maintains a low value
Summary
Coal reservoir physical properties ( permeability) are affected by several factors and show dynamic variation through the recovery of CoalBed Methane (CBM). It is widely thought that coal can be substantially unsaturated with thermogenic gas if not augmented by hydrocarbon migration or late-stage bacterial methanogenesis [45,46,47,48,49,50,51,52,53,54,55,56,57,58] In consideration of this fact, we improved the existing permeability models in this paper to make them more suitable for describing permeability dynamic variation in unsaturated CBM reservoirs based on the production performance of those reservoirs, and discuss the sensitivity of various parameters, including the gas content, initial reservoir pressure, Langmuir volume and pressure, Poisson’s ratio, Young’s modulus, Langmuir strain, and Klinkenberg coefficient, to permeability dynamic variation in the recovery of unsaturated CBM reservoirs
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