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Two methods exist for the estimation of Gas Initially In-Place – volumetrics (probabilistic or deterministic) and performance (material balance equation or numerical simulation). The more appropriate method will depend on the maturity of the project reservoir. For potential accumulations with limited information, the GIIP estimate may be volumetric-based using probabilistic methods, covering the range of possible outcomes with P90, P50 and P10 outcomes as low, best and high estimates. For fields where more subsurface data are available, the preferred method will generally shift towards volumetric-based GIIP estimates using deterministic low, best and high cases. Once a field is in production and sufficient data are available, a performance-based GIIP estimate should be established, including the appropriate range of uncertainty. Over the years, the tendency has been to over-rely on volumetric-based estimates with little attention paid to performance based GIIP update leading to sub-optimal gas field development. In this paper, we took a case-study reservoir having or tending towards negative reserves in course of its production life to underscore the need for timely update of GIIP based on performance data as an aid to optimal development of a gas field. Reservoir A has volumetrically based GIIP of 1.6 Tscf respectively leading to prediction of early End of Life (EOL) or negative reserves. When compared with performance based GIIP and updated volumetric GIIP of 2.04 Tscf to match performance data, the issue of early EOL or negative reserves was ameliorated and ultimate recoveries were optimized. Performance based GIIP estimate is reliable when premised on appropriate choice of methods (P/Z material balance for depletion reservoirs, aquifer influx model in water drive gas reservoirs material balance model or dynamic simulation) integrating all available data as illustrated in the case studies. To allow comparison between volumetric-based (static) and performance-based (dynamic) GIIP estimates and to understand the potential difference between the two, it is recommended that a current volumetric GIIP estimate should be maintained throughout the life of the field in addition to an up-to-date performance-based GIIP estimate to aid optimal gas field development. |
