Characteristic of anisotropic coal permeability and its impact on optimal design of multi-lateral well for coalbed methane production

Coal permeability is usually anisotropic and the permeability anisotropy ratio along the face cleats to the butt cleats can be up to 17:1 for some coals. The characteristic of the anisotropic coal permeability and its impact on the optimal well design for coalbed methane (CBM) production are important, but have not been well studied. This paper investigates this issue through numerical modeling and reservoir simulations. Various case studies are performed on the two commonly used multi-lateral well patterns including the quadlateral well and the pinnate lateral well to investigate the impact of permeability anisotropy ratio on the layout of the multi-lateral well. The results demonstrate that the optimal well direction of the quad-lateral well is parallel to the butt cleat direction as expected. However, the optimal main well angle of the pinnate lateral well is significantly affected by the permeability anisotropy ratio. The orientations of the branches of the pinnate lateral well are less sensitive than the branch numbers and this indicates that more gas recovery efficiency can be effectively achieved by drilling more branches but not by varying the branch orientations. These conclusions are drawn by using a widely used stress-based permeability model with the fixed permeability anisotropy ratio. In order to investigate the permeability anisotropy change during the CBM production, a strain-based and a stress-based coal permeability models for isotropic condition are improved to incorporate the permeability anisotropy and to quantitatively study the impact of permeability anisotropy ratio change on lateral well pattern. In order to be consistent with the previous permeability model, we implement the improved stress-based model into the reservoir simulation model. The results show that the permeability anisotropy is not only caused by the initial differences in structure and tortuosity of the coal cleats in the two directions, but also induced by the anisotropic mechanical and swelling properties during the CBM production. The permeability anisotropy ratio change during production may also have a significant impact on the optimal design of the multi-lateral well. (c) 2012 Elsevier B.V. All rights reserved.