In low-permeability gas reservoirs, horizontal wells have been used to increase the reservoir contact area, and hydraulic fracturing has been further extending the contact between wellbores and reservoirs. This paper presents an approach to evaluate horizontal well performance for fractured or unfractured gas wells, and a sensitivity study of gas well performance in low permeability formation. A newly-developed Distributed Volumetric Sources (DVS) method was used to calculate dimensionless productivity index for a defined source in a box-shaped domain. The unique features of the DVS method are that method can be applied to both transient flow and pseudo-steady state flow with a smooth transition between the boundary conditions. By describing the geometric dimensions of the sources, productivity index for a horizontal gas well or a longitudinal fracture along a horizontal well is readily calculated. For transverse fractures with infinite conductivity or uniform flux, the superposition principle is used for multiple sources in the system. Wellbore frictional pressure drop is coupled with the inflow calculation from fractures.

The study compares the productivity of horizontal wells versus horizontal well(s) with hydraulic fractures. The effects of parameters such as reservoir permeability and anisotropy ratio, horizontal well spacing and placement, and orientation of fractures along a horizontal wellbore are also presented in the paper. The objective is to optimize well design based on the maximum productivity and economic benefit. The results of the study show that the performance of horizontal wells can be very sensitive to the vertical permeability of the formation. In low vertical-permeability formation (high anisotropic ratio), fractures improve well performance significantly. In large drainage area, multiple-well placement, even with shorter wellbore length, increases productivity, compared with single-well placement. The study provides insight on optimization of production performance in low-permeability gas reservoirs.

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