20 October 2023| By Abdullah Hussein
Barium sulfate (BaSO₄, barite) is one of the most challenging scale types commonly encountered in oil and gas fields. Due to its physicochemical properties and its severe impact on production, barite scale can quickly become a major operational problem when it accumulates within the production system.
In general, barite is characterized by:
- Extremely low solubility (≈ 2.3 mg/L in water),
- High mechanical strength, making it resistant to conventional removal methods,
- Rapid formation and accumulation rates (for example, a North Sea well experienced a production decline from 30,000 BPD to zero within 24 hours due to barite scaling),
- Tendency to concentrate naturally occurring radioactive materials (NORM), resulting in radioactive scale deposits.
Fig.1: Scale sample comprises mainly BaSO4, SrSO4 , the sample also was radioactive.
In many cases, chemical removal techniques are ineffective at dissolving barite, which forces operators to rely on mechanical methods. In severe situations, neither chemical nor mechanical approaches are successful, leaving cutting and replacement of the affected sections as the only viable option.
Commercial BaSO₄ dissolvers are available; however, their application is often limited by high cost (particularly when large volumes are required), slow dissolution kinetics, and stringent operating conditions, such as elevated temperatures.
A more cost-effective alternative is the use of polyaminocarboxylate (PAC) chelating agents, such as EDTA, DTPA, CDTA, and NTA (Fig. 2). These chelants have demonstrated successful barite dissolution in several oil and gas field applications.
Fig. 2: Examples of polyaminocarboxylate
PACs have been used across a range of oilfield applications and, when applied as scale dissolvers, offer several advantages:
- Relatively low cost compared to specialty dissolvers,
- Non-corrosive behavior,
- Broad applicability, with effectiveness against BaSO₄, CaCO₃, CaSO₄, and some sulfide scales,
- Ease of handling with minimal safety and environmental concerns,
- Flexible formulation, with dissolution efficiency tunable through catalysts and additives.
While reported PAC dissolution efficiencies vary in the literature, performance is highly dependent on proper application design and execution.
Practical Guidelines for Cost-Effective Barite Removal Using PACs
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Laboratory Analysis
- Identify scale composition, including mineral polymorphs.
- Assess layer-by-layer scale distribution.
- Measure physical properties (e.g., thickness and hardness) to support cleaning strategy design.
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Dissolver Selection
- DTPA is generally the most effective chelant but is also the most expensive.
- Lower-cost alternatives (e.g., EDTA) may be sufficient in some cases.
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Conduct laboratory dissolution testing (Fig. 3) to determine:
- Optimal PAC concentration,
- Catalyst requirements,
- Temperature and soaking time,
- Potential side effects or operational risks.
Fig. 3: Dissolution of barite scale sample using PACs
3. Performance Enhancement with Catalysts
PAC efficiency can be improved using suitable catalysts, such as:
- Oxalate,
- Thiosulfate,
- Glycolate,
- Maleate,
- Succinate,
- Phosphonates.
4. pH Control
- Maintain dissolver pH above 10, ideally in the range of 10–11.5.
- Avoid pH values above 11.5, as contact with produced water may cause secondary precipitation.
5. Temperature Management
- Temperatures above 60 °C significantly accelerate dissolution kinetics.
- Downhole heat can often be leveraged, or external heaters and heat exchangers used for surface systems.
6. Soaking Time
- Typical soaking periods range from several hours to a few days, depending on scale thickness and age.
- Amorphous barite generally dissolves faster than well-crystallized deposits.
7. Pre-Treatment
- Flush the system with an organic solvent to remove oil films coating the barite surface.
- If acid-soluble components are present, apply inhibited acid to weaken the scale structure, followed by a thorough water flush to prevent PAC–acid interference.
8. Field Execution
- Soak the dissolver for the predetermined duration; repeat treatments for thick or aged scale.
- Circulate or agitate the fluid where possible.
- Combine chemical treatment with mechanical assistance (e.g., wireline tools, pigging, or coiled tubing) to maximize scale removal efficiency.
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Further reading
Chemical removal of barium sulfate scale