This paper presents a systematic method for the direct estima-tionof gas-in-place and reserves using only gas productiondata. The fundamental concept used in this work is "quadra-tic"rate-cumulative relation, which is given as:qg=qgi-DiGp+1/2Di/GGp2…………………………………………… (1)Where we define the decline constant, Di, for this case as:Di=(2qgi)/([1-[(pwf/zwf)/(pi/zi)]2])G …………………………………………….. (2)The basis of our method is the development and application ofseveral rate-time/cumulative production-time plotting func-tions which are derived using Eq. 1 (see Appendix A). Eq. 1is itself derived from a rigorous coupling of the gas materialbalance equation for a volumetric dry gas reservoir and amodified version of the stabilized gas flow equation for a gaswell producing at a constant bottomhole pressure. Theproposed procedure is designed to simultaneously match theindependent plotting functions (developed from Eq. 1), wherewe note that each function exhibits unique characteristics (e.g.,a straight-line trend) which can be used as an extrapolationmechanism for estimating gas reserves.In this work we utilize a "spreadsheet" approach in which thedata and plotting functions are linked by "global" modelparameters, thus ensuring consistent evaluations. We verifyour methodology using a simulated case and we then illustratethe application of this method using a field case. Althoughtheoretical considerations limit the rigorous use of the pro-posed methodology to reservoir pressures less than 6,000 psia,we have successfully applied of our method in practice forreservoir pressures as high as 10,000 psia.
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