Abstract

McKinley-type curves that include the effects of short production times (i.e., less than 120 minutes) have been developed for pressure buildup test analysis. These curves allow improved analysis of drill-stem test data in undamaged wells with kh/Mu F less than 25,000 md-ft-psi/ cp-ft3. The use of the standard McKinley curves in these situations would lead to the erroneous interpretation that the zone is moderately damaged with flow efficiencies of about 60 percent. In stimulated or severely damaged wells, the short production time effects are masked and the standard analysis provides results with acceptable accuracies.

Introduction

A simple, direct method for analyzing pressure transient test data taken during the afterflow period has been developed by McKinley. This method, utilizing the type curve approach to well test analysis, provides a means to obtain: provides a means to obtain:

  1. value for well flow efficiency,

  2. numerical value of the near well bore transmissiblity, and

  3. estimate of the bulk formation transmissibility.

The type curves originally presented by McKinley can be used for analysis of pressure drawdown, buildup, and fall-off data. For analysis of buildup data, their use is based on the assumption that the rate of pressure change before shut in is small compared to the pressure change before shut in is small compared to the rate of pressure change during the buildup period. This assumption is valid for the majority of field situations. However, for tests having short drawdown times, such as some drill-stem tests, this assumption may be violated and errors in analysis are possible using the original type curves. The effect of the short drawdown time is to reduce the rate of pressure increase with time during the buildup period. As a result, the buildup curve for an undamaged well may be similar to one for a well with formation damage and thus is subject to misinterpretation. This effect is most pronounced in high-storage wells completed in low-transmissibility formations.

To minimize the error associated with finite draw-down, type curves have been generated for flow times of 15, 30, 60, and 120 minutes. These curves are similar in appearance and, with slight exception, are used in the same manner as the original long flow time curves.

GENERATION OF THE TYPE CURVES

The short flow time buildup type curves were generated using the same basic equations (Eqs. 1 and 2) and approach as those described in Ref. 1. Buildup solutions were computed following a constant surface production rate, q, of duration tF. The Ei-function was production rate, q, of duration tF. The Ei-function was used as the unit rate pressure response function with rw2/eta = 2.88 × 10-2 minutes (the same value used for the long flow-time type curves of Ref. 1). For ease of presentation and to avoid confusion in the use of the buildup type curves, they were normalized to the long flow-time curves by defining the flow time correction factor, C, for each flow period, tF, such that

(1)

That is, the one-minute value of for a particular buildup response is the same as the for the corresponding long flow response of Ref. 1. The buildup curves were then plotted as log delta t versus log.

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