Numerical well testing of coalbed methane (CBM) reservoir
Numerical experiments and field applications proved that there exist percolation non-linearity and fluid multi-variability in low permeability CBM reservoirs. The percolation of fluid needs to overcome threshold pressure gradient, and klinkenberg effects will restrict the gas permeability. In addition, production enhancement and ultimate recovery improvement have given multi-branch horizontal wells the advantage over the vertical wells in many CBM marginal reservoirs. Moreover, Enhance Coalbed Methane (ECBM) recovery through injection of gases has been publicly proven, and can increase gas resources, however, its application in some actual field failed to address the good history matching. In this thesis, the numerical simulation and well testing problems encountered in the reservoir exploration and production are investigated. Firstly, a new dual porosity, single permeability model was developed, which reflects the high velocity non-Darcy flow that considers the threshold pressure, gas slippage and matrix shrinkage effects. It is solved using the fully implicit numerical method, a computer programme called COAFOR has been developed for this purpose. Secondly, an advanced non-analytical coupled CBM model is developed for predicting the flux in the CBM reservoir and single or multi-branch wellbore simultaneously. Thirdly, a coupled compositional triple porosity horizontal wellbore model for CBM reservoir considering the gas slippage and threshold pressure gradient effects is proposed with a newly developed permeability model. The simulator, called TRIPLE-COAL, was developed for this model. Finally, the new models developed in this thesis are validated by applying them into Heshun block, Yanchun South block and Zhijin block respectively. The history matching results checked the reasonability and accuracy of the models built in this thesis. The coupled multi-branch horizontal triple porosity model shows better matching result in Zhijin block than the coupled multi-branch horizontal dual porosity model in Yanchuan South block.