Publication Rights Reserved This paper was presented at the 40th Annual Fall Meeting of the Society of Petroleum Engineers of AIME, to be held in Denver, Colorado, October 3–6, 1965. Permission to copy is restricted to an abstract of not more than 30 words. Illustrations may not be copied. The abstract should contain conspicuous acknowledgment of where and by whom the paper is presented. Publication elsewhere after publication in the JOURNAL OF PETROLEUM TECHNOLOGY or the SOCIETY OF PETROLEUM ENGINEERS JOURNAL is usually granted upon requested to the Editor of the appropriate journal, provided agreement to give proper credit is made. Discussion of this paper is invited. Three copies of any discussion should be sent to the Society of Petroleum Engineers Office. Such discussions may be presented at the above meeting and, with the paper, may be considered for publication in one of the two SPE magazines. Abstract This paper demonstrates techniques for determining gas reserves and for predicting performance of gas-condensate reservoirs where cycling is employed. Calculation of original gas in place is accomplished through the application of a simple transient gas energy balance equation. The volumetric gas balance utilizes past production data and bottom-hole pressures taken over the life of the project. Reservoir and fluid data are derived from other sources. Flow potential is obtained from pressure buildup and drawdown tests. Conformance of the reservoir is predicted through the use of a relaxation technique which is easily programmed on digital computers and which has many and varied applications to the secondary recovery of oil and gas. The use of the procedure is here applied to a gas-cycling project to predict condensate recovery based on future volumetric sweep of the reservoir by cycled gas. The procedure is equally adaptable to water-drive or waterflooded oil reservoirs. An actual reservoir is studied using field data. Results are shown in the form of graphic representation of sweepout and of condensate recovery vs time. These data are then used, in the actual study as the basis for economic analysis of the gas-cycling project. The reservoir under study is represented as a grid in which the Darcy flow at each point is summed to zero in the four x-y directions. Provision is made for either injection or production wells being located at any of the points in the grid. The volumetric sweep efficiency is determined by application of the relaxation procedure sequentially to adjust reservoir pressure or potential at each point in the reservoir matrix until a predetermined residual for two successive calculations at every point is not exceeded. Computer output is produced in the form of a map of the potential distribution throughout the reservoir at final relaxation of the matrix for the imposed conditions. From this map isopotential and flow lines are produced and the position of the injected fluid front calculated. From this data, condensate recovery is predicted and the economics of the project can be studied. Advantages of the program are simplicity and versatility.