INFLUENCE OF TIDAL PHENOMENA ON INTERPRETATION OF PRESSURE BUIILD-UP AND PULSE TESTS

Abstract

Bottom-hole pressure tests conducted in the Kingfish oil reservoir (located in Gippsland Basin - Offshore Victoria) during 1974 and 1975 using a high sensitivity surface recording electronic bottom-hole pressure gauge indicated the presence of sinusoidal pressure oscillations in the reservor. The oscillations are of the order of 0.1 psi in amplitude and their frequency suggests that they are in some way related to tidal phenomena.Although the oscillations do not affect production, they do influence interpretation of pressure build-up and pulse tests. Interpretations of both late time pressure build-up behaviour and pulse tests of small response magnitude and long time lags are considered to be particularly susceptible to errors due to these oscillations if they are not recognized and corrected for. Interpretations of early time pressure build-up data and pulse tests of definite response and relatively short time lags are not regarded as being significantly affected.The physical mechanism causing these pressure oscillations in the reservoirs is not known. However, one of the various possible hypotheses is that the Latrobe Formation sands could be outcropping on the ocean floor at abyssal depths southeast of Kingfish and that the pressure transients generated by changes in the hydrostatic head due to surfate tides are transmitted hydraulically to the reservoir. If this hypothesis is proved to be valid it could influence pressure performance predictions of Gippsland Basin reservoirs.