Between December 2003 and February 2005 eight wells were stimulated in Tengiz field in Kazakhstan using a viscoelastic diverting acid system to evaluate the effectiveness of this system in achieving diversion and zonal coverage in large limestone reservoirs. The viscoelastic diverting acid system was pumped through coiled tubing in three of these wells and bullheaded in five other wells for comparison between both methods of placement. Pre- and post-job production logs acquired in five wells provided analysis of changes in the production profiles. In one of the wells, the formation was stimulated first with 15% HCl through coiled tubing and then with the viscoelastic diverting acid system bullheaded down the production tubing; production logs were acquired after each treatment.
The results from comparison of pre- and post-job production logs clearly show a change in the production profile after the stimulation with the viscoelastic diverting acid system, with a significant increase in production from the least prolific zones.
Conclusions from this field trial are that viscoelastic diverting acid systems can achieve full zonal coverage in stimulating large limestone reservoirs, whether conveyed by coiled tubing or bullheaded from surface. The use of viscoelastic diverting acid systems allows oil recovery from the most damaged zones. Oil recovery from these damaged zones was not previously achieved with conventional stimulation methods, owing to the tendency for stimulation fluids to migrate to the most permeable zones. Viscoelastic diverting acid systems are more effective in achieving diversion than coiled-tubing-conveyed methods using conventional linear acid systems.