Accurate estimation of the initial gas in-place for high-pressure gas reservoirs is quite often difficult because of the effect of formation compressibility and the uncertain presence of water influx from small, associated aquifer or adjacent shales.
This paper presents a material balance solution for estimating the initial gas in-place and predicting the prevailing production mechanism for high-pressure gas reservoirs. The material balance solution incorporates water influx from aquifer, water influx from shale, formation expansion, connate water expansion, and formation of condensate. Application of the proposed solution to four case histories of high-pressure gas reservoirs shows that this solution would successfully estimate the IGIP after production of 15% of the initial gas in-place and predict the prevailing reservoir production mechanism. In comparison with the available methods, the proposed solution requires no prior assumptions about the formation compressibility, aquifer size, or volume of adjacent shales.
Accurate estimation of the initial gas in-place (IGIP) plays an essential role in the evaluation, analysis, prediction of future performance, and making economic decision regarding development of gas reservoirs. Estimation of IGIP is also needed for planning long term gas contracts and commitments to supply gas to users.
The principle methods for predicting IGIP are the volumetric method and the material balance method. The volumetric method is based on geological data to define the reservoir areal extent and also on core and log data to define the reservoir rock properties and distribution of fluids inside the reservoir. The volumetric method provides a sketchy estimate for the IGIP, specially in the early history of the reservoir, and provides neither prediction of the future production as a function of the reservoir pressure nor interpretation of the reservoir producing mechanism. However, the material balance method is based on pressure-production data for estimating the initial gas in-place. The simplest method is to plot P/Z Vs Gp and extrapolate to zero-pressure. The method is derived from the material balance equation with the assumption that gas expansion is the sole driving mechanism responsible for gas production from gas reservoir. This assumption is valid for low pressure, completely sealed off "Volumetric" gas reservoirs. However, if the gas reservoir is in contact with an aquifer or a massive amount of uncompacted shale, the pressure drop in the gas reservoir, may cause water influx because of gas production. As a result of this pressure support from water influx, the extrapolation of the P/Z Vs Gp of the early data to zero pressure is not a valid means to estimate the IGIP. Further, if the reservoir initially has abnormally high formation compressibility, as observed in some high pressure gas reservoirs, the rate of pressure drop may increase with gas production. This is due to the fact that the compaction of the reservoir rock will provide pressure support at the high pressure level. Some of these abnormal pressure gas reservoirs contain retrograde gases. If the reservoir pressure drops below the dew-point pressure, retrograde condensation occurs. Because the liquid condensate has lower compressibility than the gas, the rate of pressure decline would accelerate. Also, the formation of condensate inside the reservoir rock might reduce the gas permeability resulting in less gas production and hence an accelerated pressure drop in a unit's gas production.
Several material balance methods have been proposed to estimate the initial gas in-place for abnormally high pressure gas reservoirs.