Horizontal Drainhole Drilling Techniques Used for Coal Seam Resource Exploitation

Abstract

ABSTRACT Methane gas contained in coal represents a potential new gas supply. Transformation of this large gas-in-place resource to reserves will depend on developments of modified and/or new completion methods to achieve long-term economic production. Coal reservoirs typically have very low matrix permeability with gas being desorbed from the matrix to the wellbore through a system of natural fractures or cleats. Due to low permeability, coal reservoirs usually require hydraulic fracture stimulation to achieve economic production. Investigations have shown that hydraulic fractures usually propagate parallel to the maximum stress or face-cleat direction and therefore may not adequately access an anisotropic reservoir. A more effective stimulation technique may be a horizontal borehole placed perpendicular to the maximum permeability direction. This paper discusses the short-radius and medium-radius directional drilling techniques that have been used to successfully place horizontal drainhol es into coal seams.