Abstract

Located in northern Russian, the Timano-Pechora province covers a large variety of oil and gas fields with different formation properties and characteristics. These include formation temperatures (BHST) ranging from ~ 40°C to ~100°C, permeability 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 from 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. In addition, there is limited information available for 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. It consists 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.

Introduction

Timano-Pechora oil and gas province is located in Northern Russia spanning across Komi republic, Arkhangelsk and Nenets regions (see regional map in Fig. 1). This remote region covers a large variety of oil and gas fields with different formation properties and characteristics, formations and challenges. Although exploration drilling was started as early as1890, the first light oil field (Chibyuskoe field) was only discovered 40 years later in 1930, which was followed with heavy oil field (Yaregskoe field) in 1932. The region currently has more than 75 fields representing over 230 different carbonate and sandstone formations. Those oilfields, both brown and green, are owned by several government and private E&P companies. While the brown fields are mostly on third development stage, the green fields are located extremely remotely with almost no road accessibility.

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