Abstract

Poor recovery of oil rims is a common challenge for most operators. The reasons are inherent subsurface risks and capital intensive EOR and IOR projects. This paper presents a similar case of reservoir with oil rim where a cost effective method was employed to increase oil recovery. The idea was to inject high pressure gas from a shallower gas reservoir into zone with oil rim.

The case study is about B Field located in Sindh Province of Pakistan discovered through drilling of well B-1 in 1989. The well encountered hydrocarbon bearing Lower Goru X&Y Sands. Formation testing results and Open-hole logs showed X Sand to be gas bearing, whereas, Y Sand found thin oil rim overlain by gas cap. B-1 was put on production from Y Sand and it flowed at around 300 bopd with 500 scf/stb GOR. However, rapid decline in oil rates and reservoir pressure was witnessed due to gas coning. Based on engineering and G&G analysis, it was decided to re-pressurize oil rim in Y Sand using gas of X Sand at virgin pressure. Considering Y Sand to be laterally extensive body, effect of gas injection at B-1 should be observed at wells B-2 and B-6 in the same structure.

Nodal analysis was performed to estimate injection rate while tank models were used to estimate additional recovery. The X Sand was then perforated and cross-flow established. Performance of gas injection was monitored through surveillance at B-2 and B-6 that recorded increasing Y Sand pressures. Success of in-situ gas injection was established as considerable improvement in oil recovery was achieved.

In-situ gas injection, wherever applicable, could be an efficient technique to maintain reservoir pressure of oil reservoir without additional surface footprints or facilities thus incurring marginal capital expenditure. The efficacy of this technique could be easily monitored through regular surveillance and well production behavior.

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