Scaling mechanisms describe the fundamental processes that lead to the formation and deposition of inorganic scales in production systems.
Understanding the mechanism behind scale formation is essential to selecting the correct modeling approach and mitigation strategy.
There are two main types of scaling mechanisms:
- Self-scaling (single-fluid scaling)
- Scaling caused by brine incompatibility (multi-fluid scaling)
1. Self-Scaling (Single-Fluid Scaling)
Definition:
Self-scaling occurs when a single brine experiences changes in temperature and/or pressure, that drive it toward supersaturation with respect to certain mineral phases.
Mechanism:
As pressure and temperature vary along the production or injection path, the solubility of salts changes. For example:
- Temperature changes may reduce the solubility of different salts (e.g. barite can form as temperature drops across the well).
- Gas exsolution (CO₂ or H₂S coming out of solution) alters pH and ionic strength, which can for example promote carbonate scale formation.
- Water Evaporation or condensation caused by changes in temperature and pressure affect the brine concentration — leading to localized supersaturation.
Examples:
- Calcite (CaCO₃) scale forming due to CO₂ degassing as near wellbore pressure drops.
- Halite (NaCl) scale caused by water evaporation in high-temperature gas wells.
Figure 1 illustrates an example of a scale risk profile for an artificial lift well equipped with an ESP. Under reservoir conditions, the produced fluids are in equilibrium with the calcite formation rock, with a saturation ratio (SR) of 1.
As pressure decreases in the near-wellbore region, CO₂ degasses from the fluid. This degassing increases the pH, shifting the carbonate equilibrium and promoting calcite precipitation. The risk of CaCO₃ scale formation is further intensified in the vicinity of the ESP motor, where elevated temperatures reduce calcite solubility and enhance precipitation tendencies.
Although additional depressurization occurs at surface conditions, the accompanying temperature decrease increases calcite solubility, helping to maintain the mineral in solution and limiting further precipitation topside.
Figure 1: Calcite SR and excess solute from reservoir to water re-injection
2. Scaling Caused by Incompatibility (Multi-Fluid Scaling)
Definition:
Incompatibility-induced scaling occurs when two or more brines with different ionic compositions are mixed.
The interaction between reacting ions from different sources leads to the formation of scale.
Mechanism:
When mixing brines, cations and anions that were previously soluble in separate waters may combine to form insoluble compounds. For instance:
- Mixing a barium-rich formation water with a sulfate-rich seawater can produce barium sulfate (BaSO₄) scale.
- Mixing of calcium-rich and carbonate-rich brines can lead to calcium carbonate (CaCO₃) precipitation.
The most common examples of incompatible water mixing are:
- Produced water reinjection (PWI) where incompatible waters mix downhole.
- Manifold or pipeline mixing of fluids from multiple wells with different ionic signatures.
OLI Studio: ScaleChem can predict the mass of solids formed and identify the specific mineral phases that precipitate as a result of mixing.
Figure 2 illustrates an example of barite scale formation resulting from the mixing of incompatible brines at different points within a production manifold. The field is under seawater injection, and Wells 1–6 produce fluids with varying concentrations of barium and sulfate due to differences in their injection water fraction (IWF).
When sulfate-rich brines mix with barium-rich brines, chemical incompatibility occurs, leading to barite (BaSO₄) precipitation. In this case, the mixing of fluids from Well 1 and Well 2 generates the highest scaling potential. The subsequent addition of fluids from Wells 4 and 5 further increases barite precipitation. In contrast, Wells 3 and 6 do not exhibit incompatibility when commingled with the other produced fluids.
Figure 2: BaSO4 scale risk profile at different manifold mixing points.