Abstract

Fractured carbonate reservoirs are prone to premature water cut production at the early stage of water flood recovery. Conventionally completed long horizontal wells, suffering from high water cut through fractures need reliable, cost effective and out-of-the-box treatment solutions to control water entry, without damaging oil saturated segments. In such cases, polymer gel squeeze in the conventional way is not the option as the gel treatment needs damage protection of oil producing zones, which is a difficult task because of the complexity of fracture geometry and the enormity of the number of zones that may seek protection. This paper describes an approach in which mechanical zonal isolation devices is substituted by self-selective chemical isolation technique, designed for carbonate formation which can enable the water shutoff treatment to be performed without deployment of costly work-over rig. Through extensive laboratory investigation a three-chemical system is developed, to be placed in a sequence. The first fluid is designed to protect the low permeable and presumably zones of high oil saturation, by creating an impermeable filter cake while the water conductive fractures remain open for further treatment. This would be followed by a second fluid, designed to invade, solidify and seal off the fractures. The third fluid is a bio-enzyme chemical breaker designed for uniform degradation of the filter cake on matrix zones created by the first fluid and bring the well back to production. In laboratory simulated core flood studies, excellent production profile is achieved. Depending on the fracture geometry, effective brine permeability is reduced by 74 – 91% while the oil effective permeability is reduced only by 12 – 17%. Finally, the paper discusses the key elements for field implementation in terms of fluid composition, pumping pressure, flow rate and onsite monitoring, and also suggests the downhole equipment necessary to improve the success rate.

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