Field Cases of a Zero Damaging Stimulation and Diversion Fluid from the Carbonate Formations in North Kuwait

Abstract

Abstract The high permeability contrast seen in the producing zones of the oil wells of North Kuwait Mauddud formation makes the uniform stimulation of this carbonate formation a challenge. To achieve diversion, polymer based fluids were used earlier, but with limited success. Recently, a non-polymeric system containing a Visco-Elastic Surfactant based Self Diverting Acid (VES-SDA) was used to divert and effectively stimulate these pay zones. Production logs run before and after the stimulation treatments indicates stimulation of the entire perforated intervals. Stimulation using this new system on the first 17 wells resulted in a production increase of about 30,000 BOPD, much higher than that expected from conventional treatments. The wells that were not producing earlier after several conventional treatments are now producing naturally after treatments using this non-damaging system. Introduction North Kuwait Mauddud formation consists of six main lithology sections with permeability ranging from 3 to 400 mD (Figures 1, 2, Table 1)1. This high permeability contrast in conjunction with thick reservoir layers makes the uniform stimulation of this carbonate formation difficult when using conventional matrix stimulation fluids. Effective diversion is the key for the success of stimulation treatments to achieve uniform production from all pay zones. Without diversion, acid tends to seek the path of least resistance and enters only a small portion of the interval being treated. Chemical diverting agents temporarily block the more permeable section of the interval, forcing the acid into damaged and/ or less permeable areas. In multi-layered reservoir containing zones with different injectivities due to different permeabilities and severity of damage, stimulation fluid diversion is highly recommended. Conventional stimulation treatments use regular acid or retarded acids2,3 in conjunction with chemical diverters including foams4 to fully stimulate long, non-uniform carbo-nate formation. The most commonly used chemical diverters are polymer based5, and are associated with induced formation damage6. To perform stimulation of the entire zone, a new chemical diverter with a solids-free self-diverting acid was recently developed7. The base fluid for the system is HCl, and it stimulates and diverts automatically based on in situ viscosification. The fluid is non-damaging and on breaking, it leaves no residue in the formation. The VES-SDA provides a solution for the heterogeneous carbonate reservoirs, and eliminates the concern of ineffective stimulation. Less friction pressure experienced while pumping this new acid system compared to other diversion systems provides more pumping rates and better treatment effectiveness. The simplicity of the fluid together with the flexibility in designing the treatment, make the execution easier and less cumbersome. Fewer tanks and equipment are needed and hence, fewer footprints are required for space short offshore operations. Stimulation by coiled tubing was shown to be the best tool for acid placement and to get maximum coverage8,9. The producing zones in the Mauddud formation are completed on the short string of dual completed wells. The difficulty of using coiled tubing to acidize short strings makes bullheading of chemical diverter the only choice to perform these treatments. Because of the non-damaging nature and effective-ness as a diverting agent, this VES fluid was chosen to stimulate the oil wells of North Kuwait Mauddud formation. This paper presents the first application worldwide on the use of VES self diverting acid technology in stimulating carbonate formations. Background Maximizing oil and gas recovery is one of the most complicated, but interesting tasks in the oilfield industry today. A chemical solution for uniform production is important in managing the recovery efficiency, allowing an efficient sweep of the hydrocarbons to increase the hydrocarbon recovery.