This paper identifies re-stimulation opportunities in existing horizontal wells with existing multistage hydraulic fracturing to increase oil production in a tight carbonate formation, offshore Black Sea. Pilot candidates were screened and ranked through the following decision criteria described in this paper, coupling reservoir, production and completion parameters. Favorable pilot candidates were verified by numerical simulation, which also highlighted potential areas of by-passed oil for future sidetracks. In an era of low oil prices, it is increasingly difficult to justify drilling new wells, especially in the offshore environment. A strategic shift towards revamping and workovers has made operators of tight and unconventional reservoirs to focus on restimulation. Many factors define success herein, and the key to finding the right candidates. Synergy between numerous parameters, combined often in indexes or drivers by their nature, is used to score and prioritize existing well potential and associated risks. On the other hand, reservoir and its understanding still play a major role and prevail over solely statistical methods. Top scoring refracturing candidates in this work were simulated in both full field and sector models. Real post-job fracture geometries and newly initiated fractures were critical inputs into unstructured reservoir simulation grids to ensure that the identified targets of the restimulation pilot wells are realistic and achievable. Recent multistage stimulation jobs in this field accidentally led to several "frac hits" (cross-well communication initiated while pumping a hydraulic fracturing treatment), which were confirmed by tracer analysis in the adjacent wells. Subsequent offset well behavior had both positive and negative effects, thus enabling the quantification of gains from restimulation, where possible, and intrinsic well interference. The choice between refracturing old or new wells, and finding the balance were the major pitfalls in this work. Wells, drilled 5-7 years ago and stimulated with typically 3 stages, had sub-optimal completions for refracturing, however were placed in the better quality rock, therefore displayed higher initial production. Delineating the reservoir bodies from seismic inversion and integrating with the simulation model, highlighted favorable well placements and historically it was proven to reduce initial water cut by 2-3 times. Recent and more extended wells were positioned in the tighter, "saddle" zones, drilled and completed couple of years ago. They had due drawdown management, flatter decline, plus less mechanical and operational risks. Pay in these extended wells was recognized as a major uncertainty when history matching flowing bottomhole pressures. Results of the statistical scoring and decision criteria, to enable selection of refracturing candidates, are presented in this work, along with the integration and coupling these outcomes with the applied reservoir simulation. Both sets of results were used to find new opportunities for the mature field redevelopment.

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.

This work presents the entire case history process from the recognition and identification of a potential candidate for multistage hydraulic re-stimulation. This includes operational preparations, execution, and how post-treatment results influenced the rollout of a pilot. While the recognition of the potential candidate was a coincidence by intersecting a stimulated area from one well to a neighboring well, resulting in sustainable higher production, the study phase to identify the full field potential and candidate well identification was executed in a structured way. A primary challenge was the proper preparation of the well, while keeping the overall costs manageable. Dedicated supply vessel has been used to accommodate stimulation equipment which was used to execute hydraulic stimulation treatments in Black Sea. Hybrid designs have been designed to carry 20/40 RCP proppant, which was pumped in four cycles. Since reservoir has been accessed by originally opened sleeves with addition of new hydrajet cuts along horizontal section the need of special degradable diverter was required to ensure good lateral coverage and proppant distribution. Together with the treatment, hydrocarbon and water sensitive tracers were pumped, allowing an allocation of flow per cycle. After shutting in the well to allow the resin coated proppant to cure, the well was cleaned out with energized fluid and returned to production Within four hydraulic re-stimulation cycles, a total of 300 tons of proppant with approximately 2,000 m3 of fluid were pumped and successfully diverted. End of cycle ISIPs rose by more than 20 bars and back-calculation of volume of the diverter stages allowed identifying the diversion effect. During the initial flow phase, very high water cut was observed, exhibiting good cleanup of the treatment. After 2 weeks, the rate stabilized at double the pre-treatment rate and slightly above the conservative prediction. Multistage hydraulic re-stimulation is not well utilized in Europe thus far and has not been applied offshore. In this mature field, the entire process from study to execution and post-job analysis was strongly cost driven, but resulted in the potential for six more hydraulically stimulated wells in this field.