Drill stem tests (DST's) often provide unexpected or incongruous results when compared with data obtained from electric well logs, samples, and/or cores. Commonly, DST recoveries from damaged productive zones consist of a combination of gas at low producing rates, oil or gas cut drilling mud, or mud filtrate water. In many cases, DST's such as these have only investigated the reservoir in the ncar-wcllbore area, in which the relative permeability to the productive in situ reservoir fluid has been altered due to the invasion of drilling fluids. Frequently, the damage has been so extensive that the true formation capability, relative to the productive in situ reservoir fluid at initial conditions, has not been either investigated or evaluated.
Many service company and evaluations engineers interpret DST's as described above to represent either productive wet zones, or zones with at best "average", or more often, "relatively low" permeability. When the DST has only investigated the damaged, or invaded zone, such interpretations are correct, but only with respect to the relative permeability of the altered reservoir to the produced fluid. In most cases, this will understate the relative permeability to the in situ reservoir fluid. Often, the buildup portion of the DST is dominated by wellbore storage effects, and no evaluation of permeability, especially to the in situ productive fluid, can or should be made.
Included in this paper are three case histories from Western Canada which demonstrate the importance of recognizing the impact of invasion on DST results and interpretation.