Evaluation of Scale Inhibitors in Marcellus Waters Containing High Levels of Dissolved Iron
The Marcellus waters of Pennsylvania and West Virginia commonly contain elevated levels of calcium, barium, and iron. Theoretical analyses of these waters indicate a propensity toward the formation of calcium carbonate, barium sulfate, strontium sulfate, and iron-related scales. The high level of dissolved iron commonly present in the water adversely affects the ability of the scale inhibitor to inhibit calcium carbonate scale. In this study, the inhibition performance of two new chemicals and some commercial products was evaluated under static and dynamic test conditions using synthetic Marcellus waters at varying iron concentrations. It was shown that both new chemicals were able to control calcium carbonate scale effectively in the presence of dissolved iron up to 200 ppm, whereas the performance of Polycarboxylic acid, Amino Tri(methylene phosphonic) acid and Carboxymethyl Inulin dropped sharply even in the presence of small amounts of Fe2+ (5 ppm). The inclusion of iron sequestering agents with these chemicals and the effect of iron upon calcium sulfate inhibition were also discussed in this paper.
Mineral scale formation is a problem for oil and gas operations that can result in the deterioration of assets, increased lifting costs and lost production. Common mineral scales such as calcium carbonate, calcium sulfate or barium sulfate can precipitate from produced water and create blockages in perforations, production tubulars, and equipment. The most common method of scale control is the use of low concentrations of specialty chemicals (inhibitors) that catalytically prevent the precipitation of solids. These chemical inhibitors are referred to as “threshold” inhibitors because they prevent scale formation at concentrations that are typically hundreds to thousands of times less than that required with acid or chelate addition.
The Marcellus formation does not produce water naturally. The composition of the water recovered during production is a function of the frac water source and the chemical reaction that occurs with the shale. This reaction takes place during the fracturing process and through contact of the frac water with the fracture face over time. The first water produced from the wells is described as flowback water and is characterized by gradually increasing TDS over time. Eventually the TDS level plateaus. Significant but variable quantities of calcium, barium, strontium and iron are present in the waters recovered from the wells. The amount of calcium present in the waters ranges from approximately10,000 to 25,000 mg/L. The barium present will range from 3,000 mg/L in West Virginia and southwestern Pennsylvania to 17,000 mg/L in northeastern Pennsylvania. Strontium commonly falls in the 3,000 to 6,000 mg/L range, but can run over 10,000 mg/L. Dissolved iron is present in the waters from about 50 to 300 mg/L. Theoretical analyses of these waters indicate a propensity toward the formation of calcium carbonate, barium sulfate, strontium sulfate and iron-related scales. To date, the scale deposits analyzed have been comprised of calcium carbonate, iron-related scales, and sodium chloride. The saturation indices calculated for barium sulfate scales in various parts of the play can be in the hundreds or thousands, thereby potentially overloading the scale inhibitor. At the same time, a high level of dissolved iron (200 ppm) commonly present in the water may be reducing the scale inhibitor performance.
It has been well known for some time that the presence of soluble iron has an adverse effect upon the performance of conventional scale inhibitors against calcium carbonate scale, whereas inhibitors show effective control against barium sulfate scale in the presence of iron1,2,3,4,5,6,7. For example, Graham et al.3 reported that the addition of 10 ppm Fe2+ to tesbrine caused the observed minimum inhibitor concentrations (MIC) to increase more than one hundred fold when tested against calcite scale using the common phosphonate scale inhibitor - diethylenetriamine-penta (methylene phosphonic) acid (DTPMP). An increase in the performance of DTPMP on barium sulfate scale control in the presence of iron was observed by another group7, as anaerobic conditions were strictly maintained in a specially designed test apparatus.