About the use of scale inhibitors in oil fields

The scale deposit in the process of oil production comes from the precipitation and accumulation of insoluble crystals (salts) from a mixture of incompatible aqueous phases in oil processing systems

There are different factors that can cause the solution equilibrium of the water to be disturbed and lead to scale formation, such as:

Pressure and temperature changes: As the reservoir fluids are pumped from the ground, the pressure and temperature decrease, which can affect the solubility and saturation of certain salts in the water. For example, calcium carbonate and iron sulfide become less soluble as the pressure drops, while barium sulfate and strontium sulfate become less soluble as the temperature drops.

• Dissolved gases: The presence of gases such as carbon dioxide, hydrogen sulfide, and methane can also influence the solubility and saturation of salts in the water.    For example, carbon dioxide can form carbonic acid in the water, which can dissolve calcium carbonate.    When the carbon dioxide is released from the water to the hydrocarbon phase, the pH of the water increases and calcium carbonate precipitates.    Similarly, hydrogen sulfide can react with iron ions to form iron sulfide scale.

• Incompatibility between mixing waters: When different sources of water mix in the reservoir or in the wellbore, such as formation water, injection water, seawater, or freshwater, they may have different chemical compositions and ionic strengths that can cause precipitation of salts.    For example, when seawater mixes with formation water that contains high concentrations of barium or strontium ions, barium sulfate or strontium sulfate scale can form.

Scale deposits are the most common and most troublesome damage problems in the oil field and can occur in both production and injection wells.  They can reduce or block the fluid flow through pipelines, valves, pumps, and other equipment, resulting in decreased production rates, increased operational costs, corrosion problems, and environmental impacts.   Therefore, it is important to prevent or remove scale deposits using various methods such as chemical inhibitors, mechanical removal, or combined treatments.

The functions of scale inhibitors in oil fields are to prevent or reduce the formation of scale deposits that can cause flow assurance issues by damaging the formation or plugging production tubing.  Scale inhibitors are chemicals that are continuously pumped through the oil processing system to increase the solubility of inorganic salts and prevent particles from depositing on surfaces.

Scale inhibitors can help maintain the efficiency and safety of the oil production and processing systems by:

• Preventing restriction or blockage of fluid flow through pipelines, valves, pumps, and other equipment, resulting in decreased production rates, increased operational costs, corrosion problems, and environmental impacts.

• Preventing damage of vessels, downhole completion equipment, safety valves, and gas-lift mandrels, compromising the well integrity and safety.

• Reducing pump wear, fouling, and plugging problems due to high molecular weight polymers.

• Decreasing the potential for under deposit corrosion by interfering with the performance of conventional corrosion inhibitors.

• Improving the recovery and quality of oil and gas products by reducing the formation porosity and permeability near the wellbore.

The use of scale inhibitors in oil fields varies depending on the type and severity of the scaling problem, the characteristics of the reservoir fluid and water, the operational conditions and constraints, and the environmental regulations and standards.  Some of the factors that affect the selection and application of scale inhibitors are:

• The type and concentration of the scaling ions and minerals in the water

• The compatibility of the scale inhibitor with the reservoir fluid, water, and other chemicals

• The stability and performance of the scale inhibitor under different temperature, pressure, pH, salinity, and shear conditions

• The retention and adsorption of the scale inhibitor on the rock surface and its impact on permeability and wettability

• The dosage and frequency of the scale inhibitor injection and its impact on cost and efficiency

• The environmental impact and toxicity of the scale inhibitor and its degradation products.
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