The Volga-Urals basin is one of the largest oil-producing regions in Russia. Orenburg region is essential part of Volga-Urals basin and it represents most of the challenges encountered in the other parts of the basin. More than half the wells produce exclusively from carbonate formations and require periodic acidizing treatments to maintain economic production. Carbonate stimulation treatments are usually based on pumping of hydrochloric acid into the formation with use of special diverters to maximize zone coverage by acid treatment. Some of the formation properties in Volga-Urals basin create additional challenges for successful acidizing treatments, including significant reservoir pressure depletion, high water cut, formation heterogeneity with high-and low-permeability strikes, low reservoir temperature, crude with tendency for emulsion and sludge creation, and enormous thief zones created by previous acidizing treatments. In many cases, uncertainties in the formation properties make the stimulation more complex.Carbonate stimulation practices in the Orenburg region have continuously improved during the last 6 years. First, degradable balls and polymer self-diverting acid were introduced to improve zone coverage. Later, viscoelastic self-diverting acid was added to eliminate damage associated with the polymer, and a viscoelastic selective diverter was used to minimize the probability of an increase in water cut after the treatment. Finally, foam was placed as a diversion agent, with and without coiled tubing (CT), to account for reservoir pressure depletion and help in post-acidizing flowback. Many lessons were learned, resulting in recommendations for use of various technologies at different reservoir and wellbore conditions.In addition to the recommendations for technologies, a quality assurance and quality control (QAQC) standard was developed and implemented for acidizing treatments. Moreover, laboratory work such as core flow testing and proppant embedment tests has been undertaken to clarify post-treatment aspects.Most fields in the Orenburg region are old, which implies that quality of well candidates became worse from year to year. However, with continuous improvement in stimulation practices and rigorous quality control we manage to maintain production at economic level and even increase work scope compared with previous years. This paper presents the experience gained during the 6-year of acidizing treatments in Orenburg region with lessons learned, and best practices adopted.

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