The Importance of Water Influx in Gas Reservoirs ABSTRACT R. G. AGARWAL R. AL-HUSSAINY JUNIOR MEMBERS AIME H. J. RAMEY, JR. MEMBER AIME A lthough it has long been realized that gas recovery from a water-drive gas reservoir may be poor because of high residual saturations under water drive, it appears that only limited information on the subject has been avail- able until recently. This study was performed to show the quantitative potential importance of water influx. Results indicate that gas recovery 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 an important way on: (1) the production rate and manner of production; (2) the residual gas saturation; (3) aquifer properties; and (4) the volumetric displacement efficiency of water invading the gas reservoir. The manner of estimating water-drive gas reservoir re- covery can vary considerably. Examples are: the steady- method, the Hurst modified steady-state method, and I'arious unsteady-state methods such as those of van Ever- dingen-Hurst, Hurst, and Carter-Tracy. The Carter-Tracy water influx expression was used in this study. In certain cases, it appears that gas recovery can be in- creased significantly by controlling the production rate and manner of production. For this reason, the potential importance of water influx in particular gas reservoirs should be investigated early to pel mit adequate planning to optimize the gas reserves. INTRODUCTION In recent years, the economic importance of natural gas production has become increasingly apparent. This has been evidenced by more intensive exploration efforts aimed at gas production, and exploitation of both deep, as well as low-permeability gas reservoirs. Technical developments such as deep-penetration fracturing have made develop- ment of such formations economically feasible. Unfortu- nately, water influx has forced abandonment of a number of gas reservoirs at extraordinarily high pressures. Although reservoir engineering methods for estimating water influx have long been available, it appears that ap- plication of these methods to the water-drive gas reservoir has been sporadic.'-' Available methods for estimating wa- ter influx which can be applied to the water-drive gas res- ervoir problem include 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.' Interesting applications of these solu- Original manuscript received in Society of Petrolewn Engineers office May 11, 1965. Revised manuscript of SPE 1244 received Sept. 3, 1965. Paper presented at SPE Annual Fall Meeting held in Denver, Colo., Oct. 3-6, 1965. 'References given at end of paper. 1336 TEXAS A&M U. COLLEGE STATION, TEX. tions to gas reservoir and the aquifer gas-storage problems have appeared recently!,l2·" The experimental study of residual gas saturations un- der water drive by Geffen et al. in 1952 indicated that residual gas saturations could be extremely high" A value of 35 per cent of pore volume is often used in field prac- tice when specific information is not available. The study of Geffen et al. showed that residual gas saturation might be much higher in some cases. Naar and Henderson con- cluded that the residual non-wetting phase saturation un- der imbibition should be about half of the initial non- wetting phase saturation.'o The Naar and Henderson result that residual gas saturation under water influx should be about half the original gas saturation is recommended as an estimate if laboratory measurements are not available. Thus, it is clear that a considerable portion of the ini- tial gas in place might be trapped in a' water-drive gas reservoir as residual gas at high pressure. A full water-drive would result in loss of residual gas trapped at initial reser- voir pressure. Consideration of transient aquifer behavior leads to the conclusion that high-rate production of water- drive gas reservoirs could result in improved gas recovery by reduction of the abandonment pressure. However, there appears to be little quantitative information on this pos- sibility. One of the few advantages of water-drive. gas produc- tion appears to be improved deliverability through water- drive support of the reservoir pressure. There may also be an advantage in higher condensate recovery caused by pressure maintenance for gas-condensate water-drive res- ervoirs. In view of the preceding, this study was made to assess the potential importance of water-drive in gas reservoir engineering. The Carter-Tracy approximate water-influx expression was used because this equation offers some ad- vantages in hand-calculation which do not appear to have been generally recognized.' However, calculations were per- formed in the main with a high-speed digital computer to permit evaluation of the effect of water-drive under a large variety of conditions. CALCULATION METHOD Water-drive gas reservoir performance can be estimated in a manner completely analogous to oil reservoir calcula- tions: a materials balance is written for the reservoir, and a water influx equation is written for the aquifer. Simul- taneous solution provides the cumulative water influx and the reservoir pressure. When reservoir performance data (gas produced and reservoir pressures) are available, it is usually possible to match performance data to determine the initial gas in place and the water influx parameters- JOURNAL OF PETROLEUM TECHNOLOGY Downloaded from http://onepetro.org/JPT/article-pdf/17/11/1336/2217846/spe-1244-pa.pdf by guest on 12 June 2022