A Case Study on Redevelopment of a Giant Multilayered Carbonate Reservoir Based on Modification of Ongoing Water Flood Program through Integrated Reservoir Modelling Studies

Abstract

Mumbai High North L-III reservoir is a highly heterogeneous multi-layered carbonate reservoir located on the continental shelf of Western India in Arabian Sea in about 250 ft of water depth. It is a saturated oil reservoir having about 4 billion barrels of initial oil in place and 1.7 trillion scf of initial gas cap gas in place. The field was put on production in 1976. Peripheral water injection was started in 1984. Couple of years later, up-dip water injection was also initiated in addition to peripheral water injection. The oil production rate from the reservoir peaked at about 120,000 bopd in 1986. The oil production rate in 1999 end was about 60,000 barrels of oil per day at about 60% water cut. This paper discusses various redevelopment options for the Mumbai High North L-III reservoir in detail, based on a detailed reservoir characterisation and reservoir simulation study for the field. A 780 layer fine grid geo-cellular model was prepared taking input from core data, log data and 3D seismic. This fine layer model was up-scaled to a sixteen layer model. A full field reservoir simulation study was carried out using the sixteen layer coarse up-scaled geological model. Saturation height functions were used to distribute initial saturations in the coarse model. Layer wise pseudo relative permeability curves were generated for use in reservoir simulation. Various redevelopemnt options studied indicate that there is a scope to improve the overall oil recovery from the multi-layered MHN L-III reservoir through infill drilling, enhancement and redistribution of water injection and proper completion strategy. A shift from line drive to pattern water injection is found to be highly beneficial. Introduction Mumbai High field is located about 160 km West-North-West of Mumbai city in the Arabian Sea on the continental shelf of Western India (Fig.–1). It is a gentle westward dipping anticline. It is bounded by a major fault trending NNW-SSE along the eastern margin of the field. A WSW-ENE trending graben divides Mumbai High field into Mumbai High North and Mumbai High South, two non-communicating reservoirs. Hydrocarbon is found in several limestone layers, one sandstone layer and in fractured basement patches in the field. The limestone layer L-III is the most prolific hydrocarbon bearing layer in Mumbai High field. This paper deals with redevelopment plan for Mumbai High North L-III reservoir. Mumbai High North (MHN) L-III is a multi-layered carbonate reservoir, deposited as a shallow marine and low relief carbonate platform. Platform growth was controlled by the lateral and vertical build-up of carbonate facies during periods of minimum clastic input. Carbonate deposition was occasionally disrupted by the sudden influx of terrigeneous clastics across the platform, resulting in the cessation of carbonate sedimentation and the deposition of shale. This cyclic deposition of carbonates and shale has resulted in the layered L-III reservoir seen in Mumbai High. The various sub-layers from top to bottom are known as A1, A2-I, A2-II, A2-III, A2-IV, A2-V, A2-VI, A2-VII, B, C and D. The field was discovered in 1974 and put on production in 1976. It is a saturated oil reservoir with gas cap gas and edge water in all the layers. Structure contour on top of Mumbai High North L-III reservoir is shown in Fig.–2. The L-III reservoir consists of several sub-layers. Different sub-layers of L-III have a common gas oil contact at 4313 ft. However the geological oil water contact varies from 4618ft for the uppermost sub-layer to 4467ft for the lower most sub-layer. However, the free water level at 4618 is common for all the sub-layers. The sub-layers of L-III are inter-bedded by shale layers and are in partial communication with each other at some places through holes in shale layers. The whole L-III is believed to be one hydro-dynamic unit.