Abstract

The characteristics of deep formation "damage" and low permeability appear quite similar on a formation test conducted in a normal manner. As a result, with such tests, a good reservoir with deepdamage appears quite similar to a poor reservoir with low permeability. In the past this has led to the abandonment of potential producers, on one hand or to the unnecessary expenditures for heavy stimulation in other wells that offered no real prospects. It was, therefore, necessary to develop a technique removing this weak area of test interpretation. Amethod is described whereby positive identification can be made between deep "damage" and low permeability. This method takes advantage of a deviation from the Horner assumption in his theory of superposition.

The identification of formation parameters from drill stern test data is normally confined to the radius of investigation created duringthe flow period. Under drill stern test conditions where the flow rate is very low, an expanding dominant transient is created by the afterflow extending the radius of investigation during the shut-in period. An extended shut-in, in many cases, will thus allow an inspection of the permeability at a great distance from the well bore. Data received at thesurface by the "Tele-Pressure" method during the course of a test can be used to indicate the shut-in time needed to acquire conclusive data. Also, a re-test designed with data obtained from an original test of a formation has been successfully used to obtain the desired information.

Field results are given to support this new evaluation technique. Mathematical support for the technique is presented in the appendix.

Introduction

It has been recognized for some time that a properly conducted drill stern test will yield data that can be utilized in the estimation of various reservoir parameters. The accuracy of the calculations is dependent upon good correlation between the test conditions and the assumptions which were used in obtaining the particular analytical procedure. All of the available interpretation methods assume that the formation has the same and constant characteristics (homogeneous) throughout the length and thickness of the particular zone under consideration. Since a homogeneous formation is virtually non-existent in nature, it is expected that some inaccuracies will occasionally result from this assumption. Sometimes the resulting test data from improperly conducted tests will completely misrepresent true reservoir conditions, primarily through improper interpretations.

It has not been uncommon in the past to observe drill stern tests of formations that appeared to offer no promise for practical completion and yet the operator set pipe, stimulated the zone and made a commercial producer. At the other end of the spectrum, however, similar data, particularly low recovery, may be noted on a drill stem test of another well, the operator set pipe, stimulated the zone, and ended up with a dry hole. With some formations there appears to be no definite criteria to utilize in making the decision on whether or not to attempt a completion.

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