Determining Average Reservoir Properties From Gathering-Line Transient Analysis for a Multiwell Reservoir

Abstract

The increased importance of underground gas storage has prompted the development of a new method for finding the average reservoir properties of a multiwell gas reservoir. This method provides the properties of a multiwell gas reservoir. This method provides the same information as a conventional pressure buildup analysis, but eliminates the need for monitoring the pressure buildup of each well. Introduction With the increased importance of underground gas storage, gas companies need accurate statements of gas in place as a function of pressure and as a measurement of reservoir performance. Since gas-storage reservoirs operate all year, the time available to obtain the data for a reservoir analysis is limited to a relatively short time period between the injection and the withdrawal cycles. In the past, an isobaric map of the short shut-in pressure of each well was used to arrive at an average reservoir pressure. This method did not give any indication of pressure. This method did not give any indication of reservoir performance, and, in tight reservoirs, it was seen that the short shut-in pressure would not give the stabilized reservoir pressure. Because of these reasons, a new method was sought to give the needed information. The principal method for estimating a formation's performance and pressure in a gas well is the analysis of performance and pressure in a gas well is the analysis of shut-in bottom-hole pressure-buildup data. Applying this method to a gas well requires that the well be produced to pseudosteady state and that it be shut in for produced to pseudosteady state and that it be shut in for a length of time sufficient to obtain a clearly defined straight line on the plot of bottom-hole pressure vs log (t + Delta t)/t. From the slope of this straight line and from other obtainable data, the effective permeability, flow efficiency, skin factor, and reservoir pressure at infinite shut-in time can be estimated. Using this process as a base, Matthews et al. developed a method for calculating the average reservoir pressure in a bounded reservoir - a reservoir with no water drive. Their procedure requires that each well's rate and pressure buildup be known before applying the method. This would require additional expenditures not normally necessary in a gas-storage field operation. From the foregoing discussion, it is apparent that it is desirable to have an alternative method for obtaining the same data as derived from the conventional buildup analysis. The Matthews et al. method is not practical for a gas-storage reservoir, but it does offer a solution to the problem. Using the same principle used by Matthews et al. for finding the average reservoir pressure, a new and practical method for estimating average reservoir properties is presented in this paper. This new method properties is presented in this paper. This new method does not require that the pressure buildup and rate of each well be known. The required pressure data are obtained by observing the transient behavior of the main header line with all producing wells open to the gathering lines. The new method yields the same information as the conventional pressure-buildup analysis, but avoids the need for shutting in each well and measuring the pressure buildup. Theory Using methods similar to those used by other authors, it is shown in the Appendix that the actual pressure drop at any point in a multiwell reservoir will be the sum of the pressure drops of the individual wells. pressure drops of the individual wells. JPT P. 835