A challenging problem confronting reservoir engineers these days is the estimation of the initial gas in-place and prediction of the performance of high pressure gas reservoirs because of the effect of formation compressibility and possible water influx from small associated aquifer or embedded shales. Several material balance models have been proposed to estimate the initial gas in-place and predict the prevailing production mechanisms for such reservoirs. However, most of these models have prior assumption about either the formation compressibility or the aquifer size.
This paper presents a material balance solution plot. The solution includes gas expansion, reservoir condensate expansion, rock expansion, formation water expansion, water influx from associated aquifer, water influx from embedded shales, and gas evolved from formation water.
Application of the material balance solution plot described in this paper shows that this solution plot successfully estimate the initial gas in-place and predict the prevailing reservoir production mechanism. Comparison of the new solution explained in this paper with the others shows that the new solution is more simple and accurate than the others.
In the last two decades, an increasing number of gas reservoirs have been discovered at great depths. Most of these gas reservoirs have significantly abnormally high pressure. Estimation of the initial reserve from pressure-production data and predicting the driving mechanisms for such gas reservoirs is difficult because of the effect of the formation compressibility, the uncertainty of water influx from small associated aquifer with these reservoirs, water influx front embedded shales, and the unknown amount of gas evolved from formation water. Production mechanism for these reservoirs is very controversial. P/Z versus Gp plots, for these reservoirs show double slope or down word curvature. Some authors1–5 attribute this curvature to formation expansion or pore collapse. Others6–9 attribute this downward curvatures to water influx from associated aquifers, or to water influx from embedded shales10,11 or a combination of all12–14.
Several material balance models have been presented for estimating the initial gas-in-place for high pressure gas reservoirs2-4,8,9,11-16. A review and analysis of these models was presented by Elsharkawy17. These material balance models use pressure-production data to estimate the initial gas in-place. They assume prior knowledge of the formation compressibility and/or the aquifer size to estimate the initial gas in-place. Other methods, however, simultaneously estimate the initial gas in-place and quantitatively indicate the prevailing reservoir driving mechanisms.
Lately, Elsharkawy14 presented a material balance solution plot for high pressure gas reservoirs to estimate the initial gas in-place without prior knowledge of the formation compressibility and/or the aquifer size. This solution treated the aquifer associated with the gas reservoir as a tank and account for the rock and formation water expansion. The proposed model did not account for gas evolve from formation water and account for the expansion of the reservoir condensate by using the two phase gas deviation factor instead of the single phase below the dew point pressure.
In this paper, reservoir condensate expansion and gas evolved from formation water are included in the material balance solution. The new solution is used to estimate the initial gas in-place and predict the prevailing reservoir mechanism.