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Abstract

A step-pressure test is introduced to evaluate stress sensitivity of the reservoir permeability. In this test the transient rate is measured against time at constant flowing pressures. The test is repeated several times at different pressure levels. The controlling permeabilities, thus calculated, are stress-sensitive within the radius of investigation.

Introduction

In a recent paper, Buchsteiner et al. (1993) provided a plausible correlation between effective stress (and, thus, pressure) and permeability in naturally fissured formations and the resulting permeability anisotropies. In the same work it was postulated that the magnitude and the directions of the permeability anisotropy may change during the life of the reservoir as the effective stresses change.

These phenomena could affect greatly the future well production. Furthermore, as horizontal wells gain an increasing share of all wells drilled (especially in fissured formations) their optimum direction may depend on the identification and evaluation of the stress-sensitivities of permeability and, within time, permeability anisotropy.

Stress-sensitive permeabilities and the inherent problems have been postulated early on (Elkins and Skov, 1960, Fatt and Davis, 1952). There has been considerable evidence of otherwise unexplained production rate decline (DaPrat, 1990) in fissured and naturally fractured reservoirs. Kranz et al. (1991), Engelder and Scholz (1981), Warpinski et al. (1991) and, of course, Buchsteiner et al. (1993) have suggested that natural fissure conductivity loss, associated with pressure drawdown and increase in the effective stress, is the main cause.

A number of studies and reported behavior in high-profile fissured reservoirs such as the Multiwell Experiment (MWX) in the Piceance basin in Colorado and the Ekofisk field in the North Sea have suggested that the reservoir permeability changes with time and it is decidedly anisotropic. (Branagan, 1985, Lorenz et al. 1986, Warpinski, 1991 and Teufel and Farrel, 1992).

Buchsteiner et al. (1993) extended and modified Walsh's (1981) model for fracture permeability vs. effective stress and applied it to field data from the MWX and Ekofisk reservoirs. The match was good and the results of that work is the basis of the proposed test presented here.

In their work, Buchsteiner et al. (1993) have shown that as the reservoir pressure declines and, hence, the effective stress increases, the reservoir permeability decreases. P. 359^

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