| Abstract: | Although it has long been realized that gas recovery from a water-drive gas reservoir may be poor because of high residual gas saturation left in the water-invaded portion of the reservoir, it appears that only limited information on the subject has been available until recently. This study was made to: (1) develop methods for forecasting the unsteady-state performance of water-drive gas reservoirs, (2) investigate the quantitative effect of various reservoir parameters which may influence and control the recovery of gas from such reservoirs, and (3) look into the possibility of obtaining a correlation between the residual gas saturation and other reservoir rock parameters. The study was performed in the main with a high-speed digital computer so that performance of water-drive gas reservoirs could be evaluated for a large variety of conditions. Results indicate that gas recovery from such reservoirs may be very low in some cases; perhaps as low as 45 per cent of the initial gas in place. Gas recovery under water drive appears to depend in a significant way upon: (1) field production rate and manner of production, (2) aquifer properties, (3) volumetric displacement efficiency, and (4) residual gas saturation behind the water invading the gas reservoir. The manner of estimating water influx in a water-drive gas reservoir can vary considerably. Examples are: the steady-state method, the Hurst modified steady-state method, and various unsteady-state methods such as those of van Everdingen-Hurst, Hurst and Carter-Tracy. In this study, the Carter-Tracy water-influx expression has been used mainly, but results have also been obtained by using a rigorous superposition method. The comparison was found to be excellent. Multiple regression analysis techniques were used to study the relationships between imbibition residual gas saturation and parameters such as porosity, permeability, and initial gas saturation. Data points used in this study included published and unpublished determinations of imbibition residual gas saturation. Results of the several attempts to correlate the data indicated that it was not possible to develop a general correlation of high accuracy. However, several useful regression equations for the residual gas saturation were developed. In regard to the performance of water-drive gas reservoirs, it appears that in certain cases gas recovery can be increased significantly by controlling the field production rate and manner of production. For this reason, an early investigation of water influx should be made in particular gas reservoirs to permit adequate planning to optimize the gas reserves. |
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