Short-Term Buildup Testing
Authors: Barbe J.A. , Boyd B.L. 
Published: December 2007

This new testing method, developed for pumping wells, yields accurate quantitative values for wellbore transmissibility. Neither expensive pulling costs nor long afterflow periods limit its use, and in fact field pulling costs nor long afterflow periods limit its use, and in fact field applications have shown it to be a reliable, low-cost means of obtaining the reservoir information necessary for sound engineering recommendations.

Introduction

A newly developed buildup testing technique permits low-cost measurement of formation properties near the wellbore of a pumping well. This test requires that afterflow in the tubing-casing annulus be measured periodically with an echo-sounding device. The periodically with an echo-sounding device. The resulting afterflow data are converted to a quantitative value for wellbore transmissibility using the parametric type-curves proposed by McKinley. parametric type-curves proposed by McKinley. Values for wellbore transmissibility are not usually obtained during conventional buildup tests of pumping wells. This is because the mechanical equipment pumping wells. This is because the mechanical equipment in the well, afterflow, and fluid backflow into the formation cause problems that make the procedure expensive.

High Costs of Conventional Testing

In almost all cases, the rods and pump must be pulled to run a pressure bomb. For a typical 5,000-ft well, pulling and rerunning the rods and pump and running pulling and rerunning the rods and pump and running a pressure bomb can easily cost $900 or more. This combined with the associated lost oil production usually makes long-term buildup tests economically unattractive. Afterflow, a not uncommon problem, is the tendency of fluid to continue to flow into the wellbore after shut-in. In most West Texas fields, the low formation permeability and relatively large annular volume can cause afterflow to last from 60 to 70 hours, and in some cases, more than a week. The protracted afterflow necessitates long shut-in periods protracted afterflow necessitates long shut-in periods and seriously complicates data analysis. In conventional buildup testing, fluid that is in the tubing before the pump is pulled can cause trouble. For a typical 5,000-ft well, the volume of this fluid is approximately 20 bbl. As soon as the pump is unseated, the fluid drops onto the formation, distorting the early part of the buildup data. To grasp the magnitude of this distortion, consider the effect of 20 bbl of oil flowing suddenly into a formation and superimposing on a fluid flowing at a stabilized rate of 20 to 50 B/D.

Low-Cost Testing Procedure

Following is the procedure used in short-term buildup testing:

Production test the well. Install a pressure gauge to measure the casing pressure. pressure. While pumping, measure the fluid level in the well and record the casing pressure. Turn off the pumping unit; leave the casing and tubing valves open. Measure the fluid level and record the casing pressure at 15-minute intervals for 75 minutes. pressure at 15-minute intervals for 75 minutes.

The proper use of the instrument to obtain accurate fluid influx data is very important.

JPT

P. 800