This paper identifies restimulation opportunities in existing multistage completed horizontal wells with plans for a customized refracturing solution applying breakthrough stimulation and diversion processes to increase oil production in a tight carbonate formation, offshore Black Sea. Because operators are shifting strategy in a low oil price environment from new well drilling toward well interventions, refracturing is gaining more focus, particularly for tight and less conventional reservoirs. Many potential candidates also have suboptimal completions for refracturing, so the challenge for operators is selecting the best candidates and designing a refracturing treatment for improved well performance in these complex situations. This paper describes the well screening and selection process for the restimulation of existing horizontal wells with multistage openhole completions.

During Phase 1 of the project, pilot candidates were ranked using a weighted average score of specific decision criteria for evaluating the refracturing potential. The goal of the screening process was to identify wellbores with the most bypassed reserves and to determine the root cause. Top candidates demonstrated bypassed reserve potential because of large completion spacing and lower average permeability than was originally estimated. The design process emphasized identifying areas where incremental oil could be accessed by substantially increasing total exposed conductive surface area and placing new fractures between existing using novel approaches to refracturing incorporating flow diverting technology. The application of an engineered pressure-managed design approach optimized proppant cycles, and flow diverting refracturing methods were a fundamental component in recognizing that the restimulation pilot was realistic, achievable, and justified economically. By dynamically managing and adapting proppant schedules, diverter volume fractions, and total materials pumped over time, new induced fracture surface areas can be reliably created in the most economic manner.

Phase 2 consisted of executing the refracturing operation on the selected pilot well, which had been originally hydraulically fractured in 2009. A repressurization procedure of the reservoir was performed before the main treatment to equalize pressure depletion along the lateral and ultimately enhance the coverage of newly fractured zones along the wellbore. The refracturing treatment on the pilot well consisted of four proppant cycles with application of engineered pressure management to improve fracture initiations and flow distribution. A degradable particulate diverter technology was used as primary isolation of each fracturing cycle.

Restimulation results of the pilot well demonstrated technical and production success, with huge potential to implement this technology during the next phase of field revitalization (Phase 3). This pilot project has proved that the combination of a well selection process aimed at finding unstimulated and bypassed reservoir volume and the application of customized technical solutions for refracturing can be successfully applied to increase recovery factors and identify new opportunities in mature field redevelopment.

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