P/Z Analysis of a Mature Gas Condensate Field, Offshore Trinidad

Abstract

Graphical application of the gas Material Balance Equation (MBE), P/Z plots, have typically been used to predict the gas in place (GIP) of volumetric reservoirs. Few reservoirs however, are truly volumetric and several workers have developed procedures to correct such P/Z plots, particularly in over pressured regimes, for the pressure maintaining effects of rock collapse and/or shale water influx. The paper examines and discusses the applicability of eight (8) such procedures to the mildly overpressured, relatively low yield, gas condensate Kiskadee Field in which the original GIP is known from late life P/Z data and in which pressure maintaining effects on the early life P/Z data were not previously recognised. Production, bottom hole pressure, PVT and pore volume compressibility data are used to analyse the early and late life P/Z data of each identified fault block and to determine the extent and causes of apparent early life pressure maintenance as well as observed variations in the degree of pressure maintenance in individual blocks. A modification is proposed to the Bourgoyne procedure that accurately corrects the early life P/Z data of mildly overpressured reservoirs. A further modification is proposed to correct late life P/Z data for the effects of decreasing shale water influx and increasing water saturation. This further modification is successfully applied and results in GIP estimates that are within 3 % of the known volume. Introduction An estimate of the Original Gas In Place (OGIP or Gi) for a volumetric gas reservoir can be obtained from volumetric gas material balance considerations and yields:P/Z=Pi/Zi-(P i/ZiGi)Gp . . . (1) This linear relationship is the expression of a constant volume reservoir and assumes that rock and water expansion are negligible and that there is no net movement of gas into or out of the reservoir volume of interest. A material balance plot of P/Z vs. Gp for a volumetric, depletion drive gas reservoir generates a straight line of slope -(Pi / ZiG i) with an intercept of Pi / Zi for Gp=0. Extrapolation of the straight line to the Gp axis i.e. P/Z=0, yields the OGIP. Many gas or low yield gas condensate reservoirs do not yield linear plots but instead may curve upwards or downwards2. The constant volume assumption is then not valid. In addition to bad data, an upwards or downwards curving P/Z plot may be due to a number of reasons inclusive of water influx, drainage/leakage into the reservoir, subsidence/compaction drive, expanding oil rim (upwards curving) or retrograde condensation, drainage/leakage out of the reservoir, competitor overproduction and/or overpressured reservoir (downwards curving). Retrograde condensation is usually not significant enough to result in downwards curvature13. Downward curving plots usually plot more accurately as a continuous curve but are generally approximated by two (2) linear segments with distinct slopes that yield a good fit to the end points of the data set with a poor match in the area of intersection9. The slope of the early life data (the first slope) extrapolates to an Apparent Gas In Place (AGIP). The late life data plots with a higher slope and extrapolates to the OGIP. Dependent on the severity of the downward curvature, extrapolation of early life data may yield grossly inflated estimates of the GIP with consequent deleterious effects on the ability of the Operator to efficiently manage and produce the field. This is particularly important where long-term contractual arrangements are in force for the supply of gas from the field as pertains for the majority of Trinidad's East Coast gas reservoirs. Kiskadee Field The Kiskadee Field is located 40 kilometres off the eastern coast of Trinidad in approximately 270' of water. The field was discovered in 1977 with the drilling of two (2) exploration wells that tested gas-condensate from the primary reservoir sands at 15,500' subsea (Kiskadee Sand). The sands are mildly overpressure (0.55 psi/ft), Pliocene age, shallow marine, deltaic deposits with an average porosity of 25% and permeabilities within the cleaner sands of between 50 to 100 mD. The field lies along the gently dipping, northwest plunging structural nose of a major (Cassia) anticlinal structure. Primary structural elements are northwest trending normal synthetic faults, downthrown to the northeast, that divide the field into four (4) productive fault blocks (FBs) that are as shown in Figure 1.