Scaling is the deposition of mineral solids on surfaces as ions become less soluble and the solution supersaturated. Knowing the different types of scale that can form as well as understanding how to predict and prevent scale can considerably improve equipment life and prevent costly reduction to efficiency in the production of oil and gas.
Scaling must be taken into account in the production of oil and gas as it can reduce permeability near well bore and frack locations as well as reduce diameter of pipes from excess buildup. In extreme cases, refracking or pipe replacement may be required for production to continue.
This can result in significant costs and drastically reduce the profitability of a well.
Water produced from oil and gas extraction is typically considered brine due to the high content of inorganic components such as salts. Brine water is highly susceptible to scaling as temperature, pH, and other water properties change throughout production. Scale also occurs as outside water is introduced through water floods and other processes that may not be compatible with formation water.
Corrosion of equipment and pipes can produce components that will interact with the produced water and create scale as well. These scale mechanisms, known as self-scaling, incompatible water scaling, and scaling due to suspended solids, can and do occur simultaneously. Scaling must be taken into consideration to maintain profitable oil and gas production.
Improve equipment life and increase production
When the temperature and pressure is reduced from production, self-scaling typically occurs. The reduced temperature and pressure can decrease solubility, causing over-saturation of minerals that result in scale formation. This scale can coat perforations, casing, production tubulars, valves, pumps, and down-hole completion equipment. If not removed or treated, the scale can limit and even halt production.
The most common self-scaling minerals include, but are not limited to: calcite, gypsum, and halite. Calcite and halite are both acid soluble and can be easily treated by decreasing pH. Gypsum, on the other hand, is not acid soluble and must be pre-treated with a sulfate converter for successful acid treatment to occur.
Scaling from Incompatible Water Mixing
Mixing waters for purposes such as water flood, fracking, or disposal can introduce components not originally found in the formation creating highly insoluble minerals. These minerals include, but are not limited to: barite, celestite, and iron based minerals. These highly insoluble minerals are not readily affected by reducing pH through acid treatment and typically must be physically removed if no chemical was used to prevent scale formation.
Total Suspended Solids (TSS)
TSS can form through other types of deposition such as by-products of bacterial activity, acid gas corrosion of metallic components, and the mixing of incompatible waters. TSS minerals can form deposits and be just as detrimental to the reservoir and equipment as scaling minerals. Biocides, corrosion inhibitors, and avoiding the mixing of incompatible waters are the best way to avoid increase in TSS mineral deposits.
Water Analysis and Scale Prediction
Knowledge obtained through water analysis is helpful when designing a production chemical program as well as determining feasibility of produced water for reuse or disposal. Typical lab analysis of produced water for mineral scale program can be found in the table below.
The measurement of cations and anions helps determine the types of minerals that may form. TDS, TSS, and pH help determine how much of each may form. Knowledge of bacterial colony size helps determine if biocides are needed to prevent by-product scale and if the chance of well fouling may occur. ATP testing may be used in place of SRB and IPB testing, if total bacteria population is desired.
TDS and calcium levels drastically affect the solubility of scale as well as corrosion inhibitors. By analyzing the composition of water, predictions can be made to determine which minerals may form, and in what quantity. These predictions can also determine if two waters are incompatible and help avoid the formation of barite, celestite, and other scale that are more difficult to treat.
Tests can also be run to simulate treatment of water in order to determine efficiency of a process on the specific water submitted. This allows for more informed decisions on treatment types needed if produced water reuse is desired. Langlier index and STIFF diagrams are included in analysis report to show visual representations of scale prediction and water comparison.
Predicting mineral scale by better understanding water composition provides producers with an improved ability to make appropriate decisions on which scale inhibitors to use and in what quantities. Predictions of scale from the mixing of two waters also provides the information necessary in deciding treatment steps to ensure scale and other forms of deposits do not form upon mixing.
The table below shows various types of mineral scale and deposits along with typical properties and solubility tendencies.
All laboratory analysis and scale prediction options discussed in this document can be performed by Zedi, as well as sample collection using the appropriate API and ASTM methods. If you have any questions or would like an example report, feel free to contact us.