Evaluation of Reserves for a Number of Small, Partially Shut-In Gas Fields in Northern Tennessee
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AbstractThis paper presents analysis techniques for post-transient gas flow at a constant flowing bottomhole pressure. Reservoir parameters can be estimated from production decline curve analysis using analytical solutions which are formulated in terms of real gas pseudopressure analysis.These techniques were applied to a field study of two small, low productivity gas reservoirs in Northern Tennessee. The productivity data for this field study was analyzed using decline curve analysis. The types of decline curve analysis used were: rate-time graphical, rate-time and cumulative-time non-linear least squares. Decline curve analysis by the use of type curves was attempted, but data scatter caused significant deviation of results when compared to the other types of decline curve analysis.The decline curve analysis results were then interpreted using an analytic reservoir model which did include pseudopressure analysis. This analysis was verified by a simulated real gas example which attempted to model producing conditions similar to those encountered in the field. The overall objective of the field study was to find correlations of reservoir parameters to measured production data. Several rate-reserve, rate-permeability/porosity-area, and rate-permeability-thickness (kh) correlations were obtained from the results of the decline curve analysis. The best correlations were based on an average rate, calculated from a well's first three months of production.The purpose of this paper is to present a straightforward approach for obtaining reservoir parameters from production decline curve. This approach could be used for single well analysis or it could, as to be shown in this work, be extended to a field study of several reservoirs.Without this type of analysis we would be forced to use more tedious material balance methods which would require average reservoir pressure estimates as a function of time. This would be virtually impossible for the small operator and it could be quite unprofitable for a major operator since a well has to be shut-in for extended periods to obtain average reservoir pressures. Therefore, the ability to estimate reservoir parameters from production data is needed.The first approach used to analyze production decline curves was to fit the production data with some empirical model such as an exponential or hyperbolic equation and to extrapolate that equation to some economic limit. Nind gives a thorough development of these empirical relations, which are summarized in Table 1. Recently, Fetkovich has developed a type curve for analyzing production decline curve data. The transient portion of the type curve is based on analytical model while the depletion or post-transient portion of the type curve has an analytic exponential stem and several empirical hyperbolic stems. This is a sound approach for analyzing production data, but since our rate data are quite scattered, non-unique matches were obtained when this method was applied.Since type curve matching could not be used, we chose to use the data fitting techniques. A very efficient solution algorithm proposed by More, et al., was chosen to solve the data fitting problem in a non-linear least squares sense.P. 173^
