Alternatives for Draining Tight Naturally Fractured Gas Reservoirs: Horizontal Hole Drilling vs. Massive Hydraulic Fracturing

Abstract

ABSTRACT Tight gas reservoirs with matrix permeabilities in the microdarcy range sometimes contain zones of natural fractures with relatively high permeabilities. The development of these reservoirs requires great care in that it is essential to connect the wells with the natural fracture system in order to ensure economic well productivities. To assess the feasibility of developing such reservoirs, a study was undertaken to investigate the relative attractiveness of connecting a well to a vertical zone of high permeability either by a horizontal hole or by a massive hydraulic fracture (MHF). The first phase of the work assumed single-phase compressible flow and utilised Green's function theory to derive the transient pressure response in such complex situations. The results indicate that a horizontal hole draining a natural fracture zone compares well with connecting the well to the naturally fractured zone by a MHF, particularly in the case of short MHFs. The calculations also resulted in new principles for the interpretation of well tests for the two situations described above. For example, interpretation of the formation linear flow period when there are multiple fractures indicates fracture areas that are larger than the area of the MHF. The second phase involved numerical simulation, with mobile water both in the matrix and in the fracture systems taken into account. The results, which agree qualitatively with those of analytical methods, indicate under which conditions coning or cusping of water from the fractures into the well perforations might be expected.