Abstract

We use the general analytical solution (Acuna, 2020) for pressure transient in unconventional wells to study the discrepancies in flow regimes frequently seen between buildup test and drawdown log-log diagnostic plots obtained from Rate Transient Analysis (RTA). These occur because the effect of the extended drawdown is stronger and occurs earlier in unconventional wells than in vertical wells with radial flow. The apparent flow regimes seen in the buildup response may include radial flow but are only artifacts that frequently lead to test misinterpretation and general confusion. A method to extract the drawdown response from buildup data is proposed and tested. The technique is demonstrated with simulated data where the actual response is known. The application of the method shows how these artifacts disappear and the response becomes comparable to drawdowns obtained by Rate Transient Analysis (RTA). The effect of hydraulically connected neighboring wells is also examined with a simple model of three identical wells. Cases with one, two or all wells shut-in are presented as well as a case with variable pressure and flow. Among the learnings found are that while wells can be analyzed individually when they flow simultaneously, when one or more of them are shut-in, the entire group of connected wells must be included in the analysis. This paper provides an easy way to achieve that based on the idea or partial shut-ins. Finally, a sequence of buildups for a single well is also presented to show the variations that result from test to test can be misinterpreted as variations of reservoir properties with time even when reservoir properties never change. This is due to a stronger extended drawdown effect in early buildups compared to late buildups.

Introduction

Pressure buildups are the preferred test for pressure transient analysis in conventional reservoirs. In unconventional wells, however, the analysis of flowing conditions using Rate Transient Analysis (RTA) offers several advantages. One of them is that the low permeability of unconventional reservoirs causes very long pressure transients, and a pressure buildup may need long shut-in times with large production losses. RTA on the other hand does not have any production loss as the well simply continues production.

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