Production Performance Analysis of Horizontal Drainage Wells for the Degasification of Coal Seams
Authors: Ertekin Turgay , Sung Wonmo , Schwerer Fred C. 
Published: December 2007

The production performances of horizontal drainage wells for the degasification of coal seams have been investigated with a multidimensional, two-phase coal-seam degasification model. The model is written in rectangular coordinates, can handle a number of horizontal wells, and operates in a one-, two-, or three-dimensional (1D, 2D, or 3D) mode. The model accommodates multiple horizontal boreholes originating from a common vertical shaft or horizontal boreholes being drilled from the peripheries of the reservoir. In consideration of the relatively thin but peripheries of the reservoir. In consideration of the relatively thin but large lateral extent of coal seams, flow dynamics around a horizontal borehole is described in elliptic flow geometry. The nonlinear system of equations generated by the finite-difference approximation is solved fully implicitly with Newton's iteration.

A series of computer runs was conducted systematically to demonstrate the relative significance of different reservoir and fluid characteristics on the negative and positive decline features of horizontal drainage wells. Along these lines, the effects of coal-seam properties-such as thickness, porosity, permeability, and sorption characteristics-and horizontal borehole parameters-such as hole diameter, penetration length, and positioning of the borehole-have been isolated to understand the role of each individual parameter on the production performance of horizontal drainage wells.

Contrary to the design of a vertical fracture, the orientation of the horizontal borehole with respect to face and butt cleats is controllable. Results of another series of runs are presented for different orientations of horizontal boreholes with respect to the principal directions of the natural fracture network to maximize the recovery of methane from coal seams through horizontal boreholes.

Introduction

Several techniques have been developed for producing methane from virgin coal seams undergoing mining activities. The methane production strategies in advance of mining operations include horizontal production strategies in advance of mining operations include horizontal boreholes drilled from vertical shafts, stimulated or unstimulated vertical wells, and slanted holes drilled from the surface. In active mine-working sections, methane can be drained through small-diameter horizontal holes before the mining of the coal.

The original permeability of the coal seam is predominantly a result of the presence of a cleat system that is distinguished as face cleat and butt cleat. The principal permeability directions are usually assigned along these two cleat systems. The face cleat is continuous throughout the reservoir and capable of draining large areas.

The main advantage of the horizontal-borehole production scheme is that the direction of the borehole can be controlled with respect to the principal permeability directions of the coal seam, as shown in Fig. 1. Also, the penetration length of a horizontal borehole is determined by the capacity of the available equipment.

In this paper, results of a series of numerical exercises conducted to better our understanding of the production characteristics of horizontal boreholes are presented. The numerical simulator used accounts for multimechanistic flow of gas in anisotropic and heterogeneous coal seams and uses quasisteady-state sorption phenomena. An elliptic flow geometry in the vicinity of the horizontal phenomena. An elliptic flow geometry in the vicinity of the horizontal borehole was assumed because coal-seam thickness is considerably less than the penetration length of such boreholes. This assumption of elliptic flow geometry is consistent with the physical system under consideration because of the highly pronounced permeability anisotropicity that exists in coal seams. In the case permeability anisotropicity that exists in coal seams. In the case of thin coal seams, the model was run in the 2D mode. The 3D version of the model was used in thick coal seams when multiple boreholes were placed in the vertical direction.