A Study Of The Fouling Characteristics Of Prairie Geothermal Waters

Abstract

Abstract The aim of this paper is to assess the importance of fouling of heat exchanger surfaces in the recovery of heat from low-temperature geothermal brines. Samples of brines associated with four western Canadian gas and oil well operations were re-circulated through a test heat exchanger loop over periods of 11 to 30 days. Over-all heat transfer coefficient, pressure drop and brine composition were continuously monitored. The decrease in heat transfer coefficient and the increase in pressure drop which occurred during this time indicated fouling of the heat exchange surface. Typically, the over-all heat transfer coefficient decreased by 2% to 9% over 15 days; however, for one brine sample the drop was as high as 34%. Analyses of deposits found on the heat exchanger tube indicated that corrosion was playing an important role in the fouling process. Introduction Geothermal brines from the Canadian prairies are potential sources of low-grade energy, obtainable as they are at well-head temperatures which are typically in the vicinity of 60 °C. However, the brine produced in geothermal wells is usually very saline with dissolved solids concentrations up to 30%. The heat recovery from such brines is often adversely affected by the scale deposited on the heat transfer surfaces. Deposits may originate as soluble species in the geothermal water, in which case they are usually composed of CaCO3 ? silica or CaSO4, that precipitate due to local supersaturation. Cooling and de-pressurization of the brines will promote deposition of normal solubility salts and carbonates respectively. Alternately deposits may be associated with corrosion processes in the piping system, in which case they may consist largely of iron oxides, silicates and sulphates, or copper or nickel compounds. Thus, the nature of the brine, and its temperature and pressure, as well as the piping materials, will influence the mechanisms of deposit formation and the deposit composition. The work of Bott and Gudmundsson(1,2) deals with deposition from Icelandic brines entering the heat exchanger at about 80 °C to 90 °C and where silica is the primary species in the deposit. Chan and co-workers(3,4) have carried out higher temperature tests using artificial brines with inlet temperatures of about 165 °C, again with silica as the predominant species in the deposit. Lombard(5) studied fouling of California brines at a heat exchanger inlet temperature of about 177 °C, and found deposits which were mainly compounds of iron, sodium, copper and silicon. Presumably, corrosion processes were involved. Materials selection and treatment methods to minimize corrosion and scaling of some U.S. geothermal brines have been described(6). The geothermal brines of interest in the Canadian prairies have been described by Vigrass, Vandenberghe and Postlethwaitea(7,8). Those brines contain roughly 60 000 mg CI/I and are relatively low in silica. Some corrosion testing has been done(9), but no heat exchanger fouling studies have been reported. There is a need for such data because fouling will impact\ on the economics of the heat recovery process through its effects on heat transfer and pressure drop.