Scale Treatment Optimisation in Geothermal Reservoirs

Abstract

Abstract Geothermal power capacity to generate electricity is steadily growing in the last decade from 10.9 GW in 2019 to 15.9 GW in 2020, due mainly that is a sustainable source of energy with low CO2 emissions, where the electricity is generated by heating water underground. There are three types of plants, namely, dry-steam, flash-steam and binary. Dry-steam generators use steam directly from the reservoir; and then directed to a turbine to generate electricity, reservoirs with high enthalpy, >200°C. Flash-steam and binary are more common, flashing consists in the separation of steam from liquid brine through boiling. In binary plants, hot brine is circulated through a heat- exchanger, where heat is transferred to a lower boiling liquid, commonly known as Organic Rankine Cycle (ORC), common in low enthalpy reservoirs, 50-100°C. In flash and binary plants, once the heat from the produced brine is extracted, the cooled brine is reinjected in the reservoir. Due to the dramatic changes in pressure and temperature occurring in water dominated reservoirs, binary and flash cycle geothermal systems results in the precipitation of scale minerals. Scaling may occur at different locations, such as producing wells, surface facilities, reservoir formation and re-injection lines. Scale inhibitors have been commonly used in the oil and gas industry to prevent the mineral deposition. The objective of this paper is to optimize a scale treatment strategy in a geothermal doublet injector-producer layout in a low enthalpy geothermal plant, where the reservoir is assumed to be fractured granitic/crystalline rock, assuming low SI adsorption, due to the nature of the formation and formation brine composition, where the well spacing of 750m and production rate of 50m3/day. The results suggested that the optimum strategy, assuming 20ppm MIC, is to deploy the SI chemical pill in the injector, resulting in over 650 days treatment lifetime, after injecting 12,809Kg of SI for EW well doublet orientation, and 474 days and 5,881Kg for NS. In terms of squeeze treatment, resulted in 410 days treatment lifetime after injecting 27,600kg for EW, and 137 days and 27,115Kg. The results suggest that deployment of the SI slug in the injector is preferable than at the injector. However, compared to continuous injection at 20ppm at 50m3/day, results in 650kg in 650 days, significantly smaller than the other options. Although, it might be an alternative as the whole system will be protected from injection to surface facilities.