A Study of Anomalous Pressure Build-Up Behavior

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Abstract
Published in Petroleum Transactions, AIME, Volume 213, 1958, pages 44–50. Abstract In one field in South Texas, approximately 75 per cent of the pressure build-up results show a characteristic "hump" (i.e., the pressure builds up and then falls off) which makes interpretation by standard methods impossible. Correlation of size and time of hump with formation permeability, well productivity index, and method of completion led to the tentative conclusion that the humps were caused by segregation of gas and oil in the wellbore after closing-in. This conclusion was confirmed by performance of simple laboratory bubble-rise experiments, by theoretical bubble-rise time calculations, and by a detailed calculation of PVT behavior in the wellbore of a particular well on which accurate surface and bottom-hole pressure measurements were made. The hump behavior has since been found to occur in many other fields. The cause, however, is not the same in all cases. In some of these the hump is traceable to leaks in the tubing which allow influx of gas from the annulus after closing-in. In other cases the hump is traceable to leaks in the device separating pay horizons in dually completed wells. It is concluded that the recording of both surface and bottom-hole pressures is desirable in wells which show an anomalous build-up behavior. A number of field examples is discussed where use of both sets of measurements enables the cause for anomalous behavior to be found, and a reasonable interpretation of bottomhole pressure to be made. Introduction Theoretically the pressure build-up in an infinite reservoir should be a linear function of ln [(t + ?t) /?t] where t is the production time and ?t is the closed-in time. Some of the variations from this behavior are well known, such as the curved portion immediately after shut-in which results from after-production and skin effect, and the flattened end portion which results from boundary effects in a limited reservoir. The effect of stratified producing zones and irregular geometrical drainage patterns may also contribute unusual characteristics to build-ups.

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