A dynamic predictive permeability model in coal reservoirs: Effects of shrinkage behavior caused by water desorption

Forecasting production in coal reservoirs accurately has been of growing interest in the industry. Permeability of coal reservoirs is a key parameter affecting coalbed methane production. In terms of current literature, the dynamic variation of coal permeability mainly depends on the effect of effective stress, coal matrix shrinkage caused by gas desorption and gas slippage during coalbed well drainage. The volume occupied by water in coal matrix was overlooked and its effect on matrix desorption-shrinkage-effect has not been investigated ever. In this work, on the basis of S-D stress-permeability model (Shi and Durucan, 2004), a dynamic prediction model of coal permeability fully considering the effect of effective stress, coal matrix shrinkage caused by gas desorption, gas slippage and coal matrix shrinkage caused by water desorption was established. The dynamic variation of permeability in coalbed methane development was revealed. The reliability of proposed model for permeability variation was successfully verified through the excellent agreements compared with experimental results. In addition, comparisons among the commonly utilized models and the new model indicated that the predicted permeability from the new model is overestimated than other models, such as P-M, S-D, and C-B models. The large differences between the new model and traditional models explicitly stress the necessity and significance of considering the matrix shrinkage effect caused by water desorption on coal permeability, which should not be neglected. Furthermore, based on the proposed permeability model, the influence of relative humidity in coal matrix was seriously paid attention. The four effects for the dynamic permeability of coalbed methane well were compared and analyzed. Results illustrated that relative humidity has an important effect on coal permeability performance, which will increase the dynamic permeability in coal reservoirs. Accordingly, the effect of coal matrix shrinkage caused by water desorption will increase the dynamic permeability in coal reservoirs. The gas slippage has a little effect on dynamic permeability in coal reservoirs.