Development of Management System to Efficiently Solve the Scale Formation Problem PCP
Reservoir pressure at the oil and gas field was maintained by water injection into dedicated wells. As the volume of available water was not adequate for pressure maintenance, several source wells completed with PCPs were additionally drilled to provide the required water volume. Severe blockage of PCPs in the source wells occurred, and workovers were required 2-3 times per year. It was initially assumed that CaCO₃ deposits were responsible for blocking the PCPs. For this reason, citric acid was applied to remove CaCO₃ deposits. However, this treatment was unsuccessful.
CMSProdex was invited to solve the problem. A risk assessment was conducted using the Water Chemistry Model – Modul Scale. The results showed that CaCO₃ was not responsible for PCP failures or blockages, but rather the formation of Ca-sulfate and BaSO₄ salts. After selection and application of an appropriate scale inhibitor, monitoring demonstrated stable PCP operation without the need for workovers.
Project background
Reservoir pressure at the oil and gas field was maintained by water injection into dedicated wells. The most important requirements for injection water were sufficient volume to meet daily injection needs throughout the field life and acceptable injection water quality.
Water collected from different sources was treated at oil and water treatment plants to meet necessary quality for injection. As the volume of collected water streams was insufficient for pressure maintenance, additional wells producing water were drilled to provide the designed water volume for daily needs.
Challenge
The main problems in the source wells were as follows:
- Severe blockage of PCPs source wells
- Rods covered with deposit
- Some deposit on the tubing walls
- Frequent workovers - 2-3 times per year
Solution
CMSProdex was invited to solve the problem. The main task was to develop a scale management system to provide effective scale control and reduce PCP failures. The scale management system comprised Phase I – Risk Assessment, Phase II – Development of a Proactive Management Strategy, and Phase III – Monitoring, Validation, and Optimization.
The main findings of Phase I were:
- Wells operated discontinuously, with operation adjusted to meet daily water requirements.
- Batch treatment with citric acid during inactive period was applied
- Water analysis: TDC 150–160 g/l, Ca²⁺ 4200–6100 mg/l, Mg²⁺ 1700–2400 mg/l, SO₄²⁻: 2400–3000 mg/l, HCO₃⁻: 25–200 mg/l, High Fe-content, oxygen was found in water
- Calcium and barium sulfate, iron sulfide and carbonate and Ca-citrate deposits were identified
- SRBs colonies were detected, although no SRBs were present at the start of source-well operation.
Scaling tendency calculations were performed using WCM program, including scale type, scale probability and severity, and the overall threat index. The results can be summarized as follows:
- Negative saturation index for calcium carbonate. Calcium precipitation was unlikely.
- Positive saturation index for calcium sulfate. Scaling severity was estimated as moderate. The overall scaling threat was estimated as high.
- Strontium sulfate scale was not likely
- Barium sulfate precipitation is likely. Scaling severity was estimated as moderate. The overall scaling threat was estimated as high.
Conclusion of the Risk assessment: During the so called “inactive, conservation process” of the well, severe PCPs blockage resulted primarily from CaSO₄ and BaSO₄ formation, with contribution from Ca-citrate and FeS deposits.
The management strategy phase included selection of an appropriate scale inhibitor. Following inhibitor selection, a field trial was conducted together with a monitoring program.
Results and Impact
Monitoring program after the field trial and the full scale application have shown the following:
- Smooth PCPs operation
- No deposit was observed
- No need for workover in source wells
It was also identified that well stimulation with citric acid caused precipitation of Ca-citrate downhole. Ca-citrate formed from Ca-ions in the source water and precipitated as a solid residue. In addition, citric acid dissolved in water served as a favorable substrate for sulfate-reducing bacteria (SRB). SRBs likely utilized citric acid as a nutrient, producing H₂S, which reacted with Fe²⁺ ions to form FeS, further contributing to the problem. Undissolved Ca-citrate and FeS impaired injection water quality.
Lessons Learned / Key Takeaways
- Detailed risk assessment is essential
- Develop an effective, efficient, reliable and economically justified management system is critical
- Investment in proper scale management delivers long-term operational and economic benefits.
Broader Significance / Sustainability
The implementation of a structured scale management system can significantly reduce equipment failures arising from the scale formation, workovers, and chemical misuse, resulting in improved operational efficiency and lower environmental impact. By minimizing unnecessary interventions and extending equipment run life, the approach supports sustainable field operations through reduced resource consumption, lower emissions associated with workovers, and optimized chemical usage.