Selective Multi-Stage Stimulation in Open-Hole Completion in a Carbonate Formation Using Dynamic Diversion: Case History in UAE

Abstract

Achieving successful stimulation with optimum production performance in open hole horizontal completions is complex in carbonate reservoirs. When stimulation becomes necessary in openhole completions, methods to perform multistage fracturing with proper isolation, are limited. Effective application of dynamic diversion provides a fit for purpose solution to this challenge, allowing to selectively place fracturing treatments, reaching the productivity enhancement targets. Prior to applying this technique a series of formation testing needs were conducted, including step rate test and leak off test. Outputs of the tests help optimize pumping schedule and strength of the acid mixture. The method involves dynamic isolation while performing dual injection of acid through Coiled Tubing (CT) and water-based fluid through annulus. A precise fracture initiation is established through the stagnation pressure developed when the CT fluid is jetted at high velocity. The pressure generated act as barrier replacing mechanical isolation (Surjaatmadja 1998, Surjaatmadja et al. 1998). Dynamic diversion is achieved with a specially designed downhole Hydrajetting tool, which generates a specific pressure drop across its nozzles. Down-hole acid mixing is highly influenced by leak off rate along the open-hole section, as well as the maximum allowable rates through CT and Annulus; this is particularly critical for surface preparation of the fracturing acid. Treatment acid is pumped through the CT at highest possible pumping rate within the pressure limitation of the CT. Fresh water based fluid is pumped in the annulus at 80% of the fracture gradient. Reservoir stimulation with this technique allowed completing 20 treatments in less than 18 hours, proving its remarkably efficiency. With proper depth correlation, the project targeted the sweet spots as initially designed based on petro-physical and geomechanical properties of the reservoir along with the presence of natural fractures network. Post-treatment rates observed during the initial production suggested the method is effective for acid fracturing of carbonate reservoirs with gas-condensate. Post-fracture simulations involving treating pressures matching and initial production analysis, support these observations. Recommendations are focused on technologies to perform additional measurements confirming fractures geometry achieved and contribution to production. Beyond isolation, the petrophysics and geomechanics of the zone of interest as well as the surrounding formations represented additional challenges. Leak off along the horizontal and depleted adjacent layers required an accurately engineered pump schedule. Several combinations of rates, volumes and fluid types had to be simulated to obtain the optimum design. The paper summarizes the design processes, selection criteria, challenges, and lessons learned during the planning & execution phases. It will also pave the way for future development of tight carbonate reservoirs which are available in company's portfolio currently undeveloped due to insignificant well productivity with important in place volume.