Mumbai Offshore fields are consisting of heterogeneous limestone reservoirs. Some of the reservoirs consist of tight carbonate with poor wellbore transmissibility. With the maturity of the fields, improving production from tight reservoirs has become the main focus for sustaining overall production. However, the responses of conventional stimulation techniques such as matrix acidization with self-diverting acid, deep penetrated acid, conventional acid fracturing etc. are not very encouraging in a tight reservoir, which calls for novel stimulation approach.

To address the issues, reservoir properties, well completion strategies and previous stimulation practices were analyzed in collaborative approach. Based on analysis, a different stimulation approach Closed Fracture Acidizing (CFA) was identified as the potential technique. Closed Fracture Acidizing reopens previously created fracture system with a pre pad fluid pumped at high rates. It involves creating fractures with a viscous fluid, letting it close followed by pumping a suitable acid system at matrix rate for etching the fractures. To improve treatment efficiency vis-à-vis better etched conductivity, different techniques such as leak-off control additives, effective diversion system, viscous acid systems etc. are applied.

For application of the technique, some low producing wells completed in Mukta pay of Heera field, A1 and L1 layers of Mumbai High field and Panvel pay of B-192 field were identified. The well specific treatments were designed by using Cross-linked Guar gel for creating fractures and a customized foamed acid system for etching of fracture faces. Foamed acid has been selected in these treatments for better exposure of acids in the entire fracture network and effective flow back of the treatment fluids. The foamed acid was customized by using a viscoelastic surfactant (VES) as foaming as well as viscosifying agent. Converting the acid system to stable foam greatly controlled fluid leak-off, increased the effective fluid volume and retarded the reaction rate, which is desirable in carbonate reservoirs. The various additives such as cross-linker, breaker etc. were optimized at onboard laboratory in the well stimulation vessel.

The jobs have been implemented in ten wells under constant monitoring of treatment parameters during fracturing, closure and acidization. Post CFA jobs indicate 670 bbl/day of oil production from a non-flowing well and significant improvement of productivity in other wells.

This paper describes the details of Closed Fracture Acidizing technique, treatment design, lessons learned during execution and results. The novelty of the approach over the conventional stimulation techniques in tight carbonate reservoirs are the treatment methodology and use of foamed acid system for better acid etched fracture conductivity.

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