Rapid, High-Temperature, Field Test Method for Evaluation of Geothermal Calcium Carbonate Scale Inhibitors

Abstract

Asperger, R.G.*; SPE, Phillips Petroleum Co. Summary A new test method is described that allows the rapid field testing of calcium carbonate scale inhibitors at 500 degrees F [260 degrees C]. The method evolved from use of a full-flow test loop on a well with a mass flow rate of about 1 X 106 lbm/hr [126 kg/s]. It is a simple, effective way to evaluate the effectiveness of inhibitors under field conditions. Five commercial formulations were chosen for field evaluation on the basis of nonflowing, laboratory screening tests at 500 degrees F [260 degrees C]. Four of these formulations from different suppliers controlled calcium carbonate scale deposition as measured by the test method. Two of these could dislodge recently deposited scale that had not age-hardened. Performance-profile diagrams, which were measured for these four effective Performance-profile diagrams, which were measured for these four effective inhibitors, show the concentration interrelationship between brine calcium and inhibitor concentrations at which the formulations will and will not stop scale formation in the test apparatus. With these diagrams, one formulation was chosen for testing on the full-flow brine line. The composition was tested for 6 weeks and showed a dramatic decrease in the scaling occurring at the flow-control valve. This scaling was about to force a shutdown of a major, long-term flow test being done for reservoir economic evaluations. The inhibitor stopped the scaling, and the test was performed without interruption. performed without interruption. Introduction Calcium carbonate scale is often a severe operational problem in geothermal brine-handling equipment. Deposition problem in geothermal brine-handling equipment. Deposition of this scale is rapid whenever the solution loses CO2 gas, which allows the brine to become supersaturated in calcium carbonate compared with its soluble precursor calcium bicarbonate. Deposition of the carbonate solid on the surfaces of process equipment has been prevented at low temperatures with organic polymers and phosphonic acid materials. Essentially no work on calcium carbonate scale inhibitors at 500 degrees F [260 degrees C] has been reported previously. Statement of Problem This work was designed to find a rapid procedure for evaluating high-temperature anticarbonate chemicals. Although laboratory tests could be used successfully to screen ineffective materials from those of questionable utility, field evaluation was necessary to make a final determination of effectiveness. A new, rapid, useful field test method was needed to compare inhibitor candidates quickly in flowing brines at about 500 degrees F [260 degrees C]. Previous Studies. Vetter et al. studied scaling and Previous Studies. Vetter et al. studied scaling and scale prevention methods in geothermal brines. They evaluated organic scale inhibitors at about 350 degrees F [177 degrees C] and found several commercial compounds that prevent scale deposition on the aboveground piping of prevent scale deposition on the aboveground piping of geothermal wells at that temperature. Harrar et al. also studied scale prevention in geothermal brines by chemical additives, seeding, and other approaches. However, their work was limited to temperatures not exceeding 260 degrees F [127 degrees C]. Lindenmuth et al. studied several commercial antiscaling agents in the 300 to 340 degrees F [149 to 171 degrees C] temperature range. These investigators observed that many antiscalants that are effective at low temperatures become ineffective as the brine temperature increases. Study Methods. Because our work was carried out at brine temperatures of about 500 degrees F [260 degrees C], we confirmed that many of the low-temperature antiscalants were indeed ineffective at this high temperature, as demonstrated by our laboratory bottle tests. About 40 commercial antiscalant formulations were evaluated in the laboratory. The most effective five were tested in a field test loop with the new, accelerated test reported here. Four were found to be effective inhibitors under test conditions. One was chosen for on-line testing of its ability to stop scale in the producing well's master control valve. It did work, eliminating further scale deposition and preventing an inevitable, premature shutdown of the deliverability test then under way in 1979. SPEPE p. 359