Pressure buildup tests on pumping wells usually have significant afterflowfollowing surface shut in. The measurement and use of this afterflow data hasbeen the subject of much interest as its application can lead to an earlierdetection of a semi-lop, straight line, a more accurate description of thereservoir model, and confirmation of other analysis techniques. This workdemonstrates that the use of afterflow rates derived from acoustically measuredliquid levels can be used to interpret pressure buildup tests for pumping oilwells. Several different superposition methods are demonstrated with fieldexamples.
Pressure buildup data is needed to analyse the character of a reservoir and theproductivity of a pumping well. However. the rods and pumps must be removedfrom the well in order to place downhole pressure recorders. This actionresults in fluid bring dumped onto the formation and loss of early time data.Recorders are sometimes pulled too early, resulting in lack of radial flow dataneeded for semi-log analysis of permeability, skin and average reservoirpressure. Because the time of wellbore storage is often quite long for lowpermeability reservoirs, conventional analysis of such data by type curvematching does not always give conclusive results.
An alternative procedure to obtain pressure buildup data for 3 pumping well isto determine the liquid level in the annulus acoustically, measure the surfaceannulus pressure, and calculate the bottomhole pressure.. The procedure allowsone to monitor the buildup from the very start of the test. The method alsogives direct measurement of the wellbore storage coefficient and both liquidand gas afterflow during the test, This technique is described byBrownscombe1.
The use of afetrflow and pressure data during a buildup test has been thesubject of current interest. Convolution2–4 and variable rate5–6 examples are techniques used to reduce well bore storage effects andprovide for reservoir descriptions at an earlier time of buildup. Thesetechniques have primarily been demonstrated with wells producing at arelatively high rate and largely with one phase (cil or gas). The use ofsuper-position techniques for multi-phase pumping wells must consider that boththe bottomhole pressures and afterflows are derived indirectly wellbore andreservoir crossflow coupled with changing wellbore storage coefficients andmulti-phase afterflows does introduce new variables into the analyis. However, consideration of these effects and the measured data quality can producepractical results.
Pumping wells are often hydraulically fractured to provide for improvedproductivity. Pressure transient data will diagnose the effectiveness of thefracture job bur early time data is often lost or masked by wellbore storage.The use of the acoustic well sounder provides for an alternative method toanalyse transient data and identify flow regimes during buildup.
This work demonstrates the use of pressure and afterflow data derived fromacoustic methods to analyse buildup tests. Several different superpositionmethods are demonstrated with field examples. The use of rate-normalizationtechniques is also presented.
