Analytical and Numerical Solutions for Estimating the Gas In-Place for Abnormal Pressure Reservoirs

Abstract

SPE Member Abstract Gas reservoirs with abnormally high-pressure have 'been encountered all over the world. For these reservoirs, a straight line plot of P/Z Vs. Gp for the early production data and extrapolation to zero reservoir pressure project incorrect, IGIP, initial gas in place. The P/Z plot is based on the assumption that gas compressibility is the "Sole" reservoir drive mechanism. In abnormal pressure gas reservoirs, however, grain expansion, formation water expansion, and water influx from shale or small associated aquifer, in addition to gas expansion contribute significantly to the gas production. Several material balance models have been proposed to calculate the initial gas in place for abnormally high-pressure gas reservoir. The present study is concerned with analyzing the different material balance models used to estimate the IGIP for these kind of reservoirs. Therefore, it reviews the bases and assumptions on which these models have been developed, as well as discusses the strength and weakness of every model. In addition, the present study comprises calculations of the IGIP by analytical and numerical models of the material balance equations for eight case histories. The study shows that solution plot of Havlena and Odeh can be used to estimate the IGIP for abnormal pressure gas reservoirs without prior knowledge of the aquifer size or the formation compressibility. Moreover, the present investigation reveals that most of the material balance models analyzed in this study are sensitive to the value of the initial reservoir pressure and the early data. Unfortunately, this is the time when reliable estimate for the IGIP is vital for economic decision regarding the development of such gas reservoirs. Background Over the last two decades increased attention has been focused on the analysis of reservoir reserve, behavior, and possible driving mechanisms important to the production from abnormal pressure gas reservoirs. These types of reservoirs have been encountered all over the world. They are also called superpressured or geopressured. They have initial pressure gradients greater then 0.6 psi /ft and can occur at any depth but mostly found at depths above 10,000 ft. Abnormal pressure gas reservoirs have high porosity, high water saturation, and found in shale/sand sequences. Some of these reservoirs are very small, drained by few wells, and sometimes associated with small aquifers that has not been detected on seismic studies. In the United States, abnormally pressured gas reservoirs are concentrated in the Gulf Coast, Anadorko Basin, Delaware Basin and Rocky Mountain Area. In the Middle East, over pressured reservoirs found in Iraq, Iran and Saudi Arabia. Over pressured reservoirs plus abnormally high temperature have also been found along the Red Sea Region. Accurate determination of the initial gas in-place, IGIP, for gas reservoirs early in their production life is necessary in predicting future production and making economic decision regarding the development and production of such reservoirs. The industry standard method for the estimation of the IGIP for volumetric gas reservoir is the plot of P/Z Vs. Gp. This plot is used to estimate the IGIP by fitting a straight line thorough the early data and extrapolate to zero pressure. The plot is derived from the following equation; (1) P. 463