Located in northern Russia, the Timano-Pechora province covers a large variety of oil and gas fields with different formation properties and characteristics, including formation temperatures (BHST) ranging from ~ 40°C to ~100°C, permeabilities of 20 to 400 mD, and the presence of 1% to 3% H2S. Many wells have long perforated intervals of up to 80 to 100 m and produce heavy oil with high wax appearance. Stimulation treatments in the region also face such operational challenges as harsh surface weather conditions, difficulty in flowback because of low formation pressure, and long shut-in times. Because most of the wells have electric submersible pumps (ESPs) in place, well interventions are often limited to straddle packer or coiled tubing operations. Some wells are remotely located, with only 2 to 3 months of road accessibility, and there is limited information available for the wells to be treated. The supply and purity of acids, as well as inhibitors added by the manufacturer, can also present potential problems such as incompatibility with other additives and formation of emulsions.

To address the challenges of acidizing in the Timano-Pechora region, a comprehensive laboratory and field approach was applied. To achieve better zonal coverage in these heterogeneous carbonate reservoirs, a viscoelastic surfactant (VES)–based self-diverting acid system was deployed, consisting of a VES and HCl. The new system self-diverts, triggered by the increases in viscosity as the acid dissolves calcite and dolomite. The viscosity of spent fluid is reduced by produced hydrocarbon, leaving no solid residue to cause formation damage. Field results demonstrate the effectiveness of the new acid system. The simplicity of the system makes it the fluid of choice, especially in sour environments. The absence of metallic crosslinkers in this system eliminates problems associated with sulfide precipitation in sour wells.