Lessons learned from the integrated multifarious study of a poorly performing yet great quality oil reservoir: an example from Assam Basin, India

Abstract

A bafflingly low recovery factor (<10%) of a sandstone reservoir (average porosity 20%, permeability 500 md) was investigated. This situation was attributed to complex fluid movements in response to aggressive gas production, a strong bottom-water aquifer, and reservoir heterogeneity. Some higher productivity wells are situated in high seismic amplitude regions. Material balance studies indicated the presence of an initial gas cap, strong bottom-water drive, and major upward fluid movement due to excessive early gas production. Detailed analyses of well logs and pressure-production-PVT data confirmed temporal movement of contacts. Oil segregation inferred from geochemical analysis indicated reservoir heterogeneity, also supported by seismic amplitude variations. This situation, combined with differential drawdowns, resulted in substantial non-uniform distribution of moved oil into the initial gas cap, causing unpredictable reservoir behavior, well failures, and early water breakthroughs. Remedies include shutting down wells with high gas-oil ratios and water cut, and modeling studies to locate moved oil. This study reinforces: (1) criticality of periodic integration of multifarious dataset, (2) reservoir monitoring, especially ones with large gas caps, and that (3) for maximum recovery from similar reservoirs, oil should be produced first, carefully managing initial gas cap- and water-drive energies, followed by remaining gas production.