Abstract

This work embodies our recent experiences with pumping well testing using the acoustic method. Three areas of testing are examined here: first, computation of changing liquid density throughout the liquid column at every time step; second, interpretation of the test using simultaneous downhole pressure and flow-rate data; third, static and dynamic response of the acoustic device.

An algorithm is developed to incorporate the gas/oil mass transfer effect (solubility) so that accurate gas velocity and in turn improved liquid density in the annulus can be computed. The effect of gas solubility is to reduce the gas volume and hence the total downhole volumetric flow-rate (BHF), without affecting bottomhole pressure (BHP) significantly. Transient interpretation improves when the gas solubility is accounted for.

A review of some of the wellbore hydrodynamic models and correlations is attempted in this study. The Godbey-Dimon hydrodynamic model is modified to simulate the moving liquid-column during a buildup test.

Transient interpretation of many field tests indicates that long-duration wellbore storage is the dominant feature in all cases. In general, the use of BHF together with BHP aids early-time interpretation. However, in pumping wells, complexity in well/reservoir flow and uncertainty in early-time-rate estimation do not allow the benefit of such analysis in every situation. Hence the need arises for long shut-in times in many cases. Nonetheless, the examples show that the acoustic method does produce interpretable quality data.

Field examples are used to examine both the static (accuracy) and dynamic response (resolution) of the acoustic device. The available literature suggests that the acoustic method can faithfully reproduce the absolute static bottomhole gauge pressure. This study shows that the results from dynamic tests (buildup) compare very favorably with those from downhole measurements.

Introduction

Testing of sucker-rod pumping wells using an acoustic well sounding device dates back some 20 years. Ref. 1 attempts to summarize the relevant works published until 1985. The major drawbacks of this technique lay with the acoustic device being manual and that estimation of the changing density in the annular liquid column was somewhat complicated. The complication arises from the fact that the gas/liquid ratio entering into the wellbore declines with shut-in time, precipitating a variable density liquid column both as a function of the column height and test time. Often the liquid column is a major contributor to the calculation of BHP; thus, the need for an accurate estimation of the column's density.

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