Augmentation of Well Productivity With Slant and Horizontal Wells (includes associated papers 24547 and 25308 )

Abstract

This paper presents an equation to calculate the productivity of horizontal wells and a derivation of that equation using potential-fluid theory. This equation may also be used to account for reservoir anisotropy and well eccentricity (i.e., horizontal well location other than midheight of a reservoir). The theoretical predictions were used to calculate the effective wellbore radius and the effective skin factors of horizontal wells. Laboratory experiments with an electrical analog were also conducted. These 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 two to five times greater than unstimulated vertical- or slant-well productivities, depending on reservoir anisotropy. Conversely, in a 400-ft (122-m) -thick reservoir, slant wells perform better than horizontal wells if vertical permeability is less than horizontal permeability. Horizontal wells perform significantly better than vertical wells in reservoirs with gas cap and/or bottomwater. This study reports an equation to compare horizontal-more » and vertical-well gas-coning tendencies. The results indicate that horizontal wells are suitable for reservoirs that are thin, show high vertical permeability, or exhibit gas- and water-coning problems. The equations reported should be useful in initial evaluation of a horizontal-well drilling proposal.« less