Evaluation of a Depleted Oil Reservoir for Gascap Blowdown Using 3D Simulation Models: A Case Study of Chrome Field Development

Abstract

Gas Cap Blowdown (GCBD) involves a process of depressurizing the gas cap of an oil reservoir to produce the associated gascap volumes, more often after the oil rim has been depleted of significant oil production. Thus, the optimal development plan for such reservoirs carries both large technical and commercial uncertainties and a robust technical evaluation is required to underpin commercial decision. In most moderate to strong aquifer drive reservoirs, the displacement of gas as the oil rim moves upwards into the gas cap causes residual oil losses with trapped gas. This paper details the integrated study of a gascap development of oilrim reservoir initially developed by twelve (12) oil wells and with over 40 years of production history; and overlained with an undeveloped huge gascap with M-factor, m>1.6. The reservoirs comprise of number of communicating blocks by a series of nonsealing synthetic faults and a number of nonsealing transverse faults of up to 200 ft throw. Full scale 3-D simulation models; using the Shell's proprietary 3D dynamic modelling application; Modular Reservoir Simulator (MoReS) were deployed to assess subsurface uncertainties and history match reservoir performance of the reservoirs. The impact of resaturation of original gas cap volume and timing of gascap blowdown on the ultimate recovery of the gascap were exhaustively evaluated. Optimal development of the gascap was assessed against a range of technical uncertainties which then formed the basis for development of other reservoirs in the field that were not subject to the full uncertainty assessment. From the results of the subsurface uncertainty analysis and extensive history match, three (3) gas wells targeting the gascap were proposed to develop the reservoir and optimum timing of gas cap development was evaluated, while concurrently producing the remaining oil volumes with exising wells. In addition, the results from sensitivity analysis evaluated different timing for commencement of gascap blow down and the associated incremental recoveries. Also, for residual oil in the presence of gas, reduced range of values upto 30% of the oil-to-water residual saturation results due to the different blow down timings.