Comparative Evaluation of Vertical and Horizontal Drainage Wells for the Degasification of Coal Seams

Abstract

Summary A two-phase, single-well, numerical simulator capable of modeling the performance of either vertical or horizontal wells (drilled from a shaft bottom) has been developed. For vertical wells, the model allows for the prediction of postfracture performance. The hydraulically induced fractures can be specified as either infinite conductivity or finite (constant) conductivity, so that conventional, as well as massive, treatments can be modeled. Although the simulator allows for multiple fracture wings, only the classic, two-wing, hydraulic fracture is considered in this paper. For coal seams, the formation to be modeled is a dual-porosity reservoir. The natural fracture network of the coal seam (the butt and face cleat) has been assigned an effective porosity and permeability. A time-dependent, quasisteady-state sorption term has been included to describe the phenomenon of gas desorbing from the formation matrix and diffusing into the fracture network. The model was used to sift through the many parameters associated with the two production schemes to determine those that affect production most significantly. Where possible, these parameters, along with their effects, are put into dimensionless form. Type curves for estimating recovery were generated from these dimensionless groups. Finally, a comparison between the performance of vertical and horizontal boreholes was made with actual field data.