Laboratory Study on Precipitation of Calcium Sulphate in Berea Sandstone Cores

Abstract

Abstract Scale deposition is one of the most serious oil field problems that inflictwater injection systems primarily when two incompatible waters are involved.Mixing of these waters could cause precipitation of CaSO4, BaSO4, and/or SrSO4. To generate reaction kinetics data, the rate of calcium sulphate depositionin porous rock was measured while flooding Berea core samples of uniformproperties with super-saturated brine. The brine was formulated by mixing aCa+2-rich solution with a SO4−2-rich solutionat the core inlet. The rate of CaSO4 scale formation was estimatedby monitoring the core effluent's Ca+2 ion concentration. Severalparameters were varied including temperature, pressure, degree of brinesuper-saturation, and flooding velocity. The results indicated increased rate of CaSO4 precipitation athigher temperatures, higher flood velocities, and greater brinesuper-saturation, whereas pressure had a slight effect on CaSO4deposition. The results were utilized to build a general reaction rate equationto predict CaSO4 deposition in Berea sandstone for a giventemperature, brine super-saturation, and flooding velocity. Introduction Scale is an inorganic mineral solid precipitated out of a salt solution(brine). The most common oil field scales are the carbonates and sulphates ofcalcium, barium, and strontium. Scale formation and deposition is a seriousproblem that inflicts oil field water injection systems. Scalelimits, and sometimes blocks, oil and gas production by plugging theoil-producing formation matrix or fractures, especially near the well bore.It can also plug production lines and equipment and impairfluid flow. The consequences could be production-equipment failure, increasedmaintenance cost, and overall decrease in production efficiency. Several factors may lead to scale formation, chief among which is when thesolution becomes super-saturated with the mineral. That is when the mineral'sconcentration exceeds the solubility limit of the mineral in the solution. Suchstate of super-saturation might be brought about by mixing of two, otherwiseunder-saturated solutions with respect to the mineral. Scale can also form whenthe mineral's solubility decreases as a result of changes in solutiontemperature, pressure, pH, and CO2/H2S partialpressures.