Augmentation of Well Productivity Using Slant and Horizontal Wells

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Abstract
This paper presents an equation to calculate the productivity of horizontal wells and a derivation of that equation using potential fluid flow theory. This equation may also be used to account for reservoir anisotropy and well eccentricity (i.e., horizontal well location other than mid-height of a reservoir). The theoretical predictions were used to calculate the effective well bore radius and the effective skin factors of horizontal wells. Laboratory experiments using an electrical analog were also conducted. The present laboratory experimental data and also the laboratory data available in the literature show good agreement with the theoretical equation, indicating its accuracy. The paper also compares vertical, slant, and horizontal well productivity indices, assuming an equal drainage area. In addition, the comparison also assumes an equal reservoir contact area for slant and horizontal wells. The results show that in a 100 ft (30.48 m) thick reservoir, horizontal well productivities are 2 to 5 times greater than unstimulated vertical or slant well productivities, depending upon reservoir anisotropy. Conversely, in a 400 ft (122 m) thick reservoir, slant wells perform better than horizontal wells if vertical permeability is less than the horizontal permeability. Horizontal wells perform significantly better than vertical wells in reservoirs with gas cap and/or bottom water. This study reports an equation to compare horizontal and vertical well gas coning tendencies. The results indicate that horizontal wells are suitable for reservoirs which are thin, show high vertical permeability or exhibit gas and water coning problems. The equations reported in this paper should be useful in an initial evaluation of a horizontal well drilling proposal.

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