The Use of Static and Dynamic Material-Balance, Advanced Decline Curve and Rate-Transient Analysis Methods for Estimating Original Gas-In-Place in Tight Gas Wells and Matured Gas Reservoir in Awali Field

Abstract Estimation of original gas-in-place (OGIP) early in reservoir life is of paramount importance to assess economic viability of field development of any gas reservoir. This paper discusses the estimation of OGIP in gas wells in two tight gas reservoirs in the Bahrain field located in the Arabian Basin. Due to low permeability, both vertical and horizontal wells were drilled, and produced through multi-stage fracturing technique to increase reservoir contact area to achieve higher productivity. An appraisal project is planned to drill additional wells and gather extensive surveillance data to evaluate initial well productivity and long-term production performance. Therefore, early, and accurate determination of OGIP of the existing wells is crucial for further appraisal and future development planning of the reservoirs. We also estimate the original gas-in-place of a matured gas reservoir producing for the last fifty years for potential infill development and compression to maximize the ultimate recovery. We use multiple analytical techniques such as dynamic material balance (DMB), flowing material balance (FMB), and transient-PI methods besides conventional static material balance analysis. This study also presents a systematic procedure for estimating OGIP in a volumetric system when both rate and flowing pressure data are gathered at the well-head. Log-log diagnostic plots are used to ascertain boundary-dominated pseudo-steady (PSS) flow period. Static and FMB/DMB calculations are done on the same cartesian plot and corroborated with transient-PI method and Blasingame/Agrawal type curve analysis. Additionally, cartesian analysis of rate-normalized pressure versus material-balance pseudo-time (Palacio and Blasingame, 1993) and "quadratic" production-based rate-cumulative empirical equation (Johnson et al. 2010) are used to estimate OGIP to support dynamic/flowing material-balance results. Performance data can be adequately fitted in Blasingame's and Agarwal-Gardener log-log type curves suggesting good coherence between measured rates and the estimated bottom-hole flowing pressures. The OGIP estimated by different techniques in tight gas wells are in good agreement instilling confidence on the analysis methods. The analytical history match provides a consistent agreement between the measured reservoir/flowing pressures and calculated pressures using OGIP obtained from the different material balance analysis. Conventional material balance (MB) analysis of the matured gas reservoir indicates volumetric reservoir based on driving indices and fluid expansion accounts for almost all the energy supporting reservoir withdrawal. The OGIP estimated using Cole and Havlena-Odeh straight-line plots and static material balance (P/Z) method compare favorably. The multiple evaluation techniques described here can be used for reliable estimation of OGIP in both tight gas and conventional matured gas reservoirs. Integrating these multiple approaches ensures a robust and comprehensive framework for OGIP estimation, enabling better reservoir management and production optimization.