Mineral Scale
What is Mineral Scale?
Water is a universal solvent, and water flowing in rivers, streams, oceans, and underground carries various amounts of dissolved solids. In the petroleum industry specifically, vast amounts of mineral scales are carried in the water that is used during production operations for the purposes of heating, cooling, cutting, drilling, washing, diluting, etc. Continuous changes in the operating parameters like pressure, temperature, and pH levels, can cause mineral scales to deposit on the internal surfaces of producing equipment and facilities. Total Dissolved Solids (TDS) is a measurement of the amount of dissolved solids present in one liter of fluid.
As an example, the City of Calgary drinking water TDS ranged from 141-274 mg/L when combining Bearspaw and Glenmore averages. On the other end of the spectrum, there are produced waters in Alberta basins that have been measured at over 200,000 mg/L.
Causes of Scale Precipitation and Deposition
Mineral scale deposition can be quick and severe, even if the issue is believed to be minor. Scale affects fluid flow, impairs equipment, decreases efficiency, increases operating costs, and can require expensive maintenance. Some scale treatments are simple, and others require extensive maintenance to repair. The key is prevention.
Prevalent types of scale in the WCSB
The Western Canada Sedimentary Basin is vast. Scale issues and scale types differ as you move from Saskatchewan through Alberta and into northeast British Columbia, but shared across these areas is high TDS water. High TDS does not always mean a scale issue because specific chemistry of those solids matter more than the amount of solids themselves, however, high TDS water may contain calcium, barium, sulfate, bicarbonate, iron, etc. Let’s dive into a few of common mineral scales we see in the WCSB. Calcium Carbonate, Calcium Sulfate, and Barium Sulfate. What drives their specific formation, precipitation, and deposition?
Calcium Carbonate Scale (CaCO3)
Why? A mixing of two different sources of water occurs. One source rich in Calcium, the other rich in Carbonates. A common source of water rich in Calcium is water that has dissolved calcium containing geological features. Calcium based brines can also be lost into formation during drilling operations. Carbonate ions themselves are relatively rare in produced water, but bicarbonates are present over a wide pH range. When a change occurs, bicarbonates can covert into carbonate, thus driving the creation of CaCO3. On top of that, approximately 45% of the world’s petroleum reserves occur in carbonate reservoirs.
Factors influencing CaCO3
pH Level – The higher the PH, the more likely the CACO3 precipitation will occur.
Temperature – CaCO3 is an inverse solubility material. Increasing the temperature will promote CaCO3 formation. This is why you see larger scale buildup in your home kettle. The same principle applies in oil and gas production equipment like heat exchangers, heaters, high pressure, and high temp environments.
Pressure – Reduction is pressure favors a precipitation reaction as CO2 is lost from the solution. With the loss of CO2, the PH of the solution increases. Pressure reductions commonly occur across chokes, valves, pumps, downstream of the pipeline, and at separators.
Calcium Sulfate Scale (CaSO4)
Why? Calcium ions are abundantly found in most formation waters, while the sulfate ions are coming from reservoir rocks. When water contacts anhydrite rock it will dissolve rock into the water. This releases calcium and sulfate ions.
Factors influencing (CaSO4)
pH Level – Unlike carbonate ions, sulfate ions are not strongly controlled by pH. Sulfate stays relatively stable.
Temperature - Generally, CaSO4 is an inverse solubility solution. Increasing temperature will promote CaSO4 formation.
Pressure - Pressure drops are the primary cause of CaSO4 precipitation. The greatest pressure change is the entry point of the fluid into the wellbore from the formation; however, pressure drops can also occur at valves, pumps, and downstream in the pipelines.
Barium Sulfate Scale (BaSO4)
Why? A mixing of two different sources of water occurs. One source rich in calcium ion, the other rich in sulfate ions. Sounds the same as CaCO4, right?The difference between this water and the water that forms CaSO4 is that barium ions are present in one of the waters. The sulfate prefers to bond with the barium instead of the calcium. Once formed, BaSO₄ precipitates rapidly, is very difficult to treat chemically, and once formed typically requires mechanical removal or complete replacement of the affected operations equipment.
Factors influencing BaSO4
Temperature – Precipitation is most affected by temperature. Hotter water tends to hold more dissolved BaSO4. However, this should not be confused with being highly soluble. BaSO4 is very difficult to dissolve at any temperature.
Pressure – One of the primary causes of BaSO4 deposition. Pressure drops at chokes and valves induce turbulence in the water and that helps to overcome supersaturation effects and initiate precipitation.
Testing and Mitigation
There isn’t a “one size fits all” approach to scale. It takes testing to fully understand the root of the scale problem. You’ve seen above a few different types of scale, how they form, and how they’re treated. Your best plan of attack is to always test your waters to ensure you know what type of scale is possible to form. From those tests, Flowet can help you determine what specific Scale Inhibitor you require. Flowet is here to support you by providing solutions that work for your application — not solutions that simply check a box.