Abstract

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.

Introduction

Tengiz field is a large carbonate reservoir located in Kazakhstan. Producing intervals in the wells can be as long as 800 m, at depths varying from 3800 m to 5500 m, producing 47°API oil with an average of 16% hydrogen sulfide (H2S) in the gas, with 11,600-psi reservoir pressure and an average temperature of 118°C. Producing media varies from natural fractures and fissures to matrix flow, with average permeability in the range of 1 md to 10 md.

Zonal coverage during acid stimulation of the wells is a key issue because historically most of the acid tends to be injected into the higher permeability layers or the first-stimulated zones. In an attempt to overcome this problem and achieve diversion, the injected acid, conveyed by coiled tubing, was spotted in front of all producing intervals. Starting late in 2003, a new approach used a viscoelastic diverting acid system to achieve diversion. This document discusses the results of the first wells treated with the viscoelastic diverting acid system and compares their results with those of the conventional methods used previously.

Discussion

Before December 2003, the predominant procedure for stimulating wells in Tengiz was the use of 15% HCl conveyed by coiled tubing. Spotting the acid in front of the various reservoir layers provided diversion of the acid. Results of laboratory tests, in which viscoelastic diverting acid systems were injected through limestone cores, suggested that viscoelastic diverting acid systems would provide diversion during the stimulating of limestone reservoirs. Based on this, it was decided to apply this system in Tengiz field and analyze the results of the stimulation treatments to evaluate their effectiveness. Whenever possible, pre- and post-job production logs and pressure transient tests were performed to better evaluate the results of the stimulations.

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