Scale Treatment Optimization in Geothermal Wells

Geothermal energy refers to the heat stored in the subsurface that can be extracted by producing the hotfluids (water and/or steam) in contact with the hot formation. A major issue that may restrict the extractionof geothermal energy is precipitation of mineral scales which can occur within the reservoir, inside thewellbore, or surface facilities. The objective of this paper is to find the most efficient scale treatment strategyto prevent mineral scaling. Continuous injection of chemical scale inhibitor (SI) downhole in the production well, is the mostcommon method to prevent mineral scale in geothermal plants. This method although effective does notprotect the near-wellbore area, where the highest pressure drop is expected. To address this issue, twomethods will be studied, bullheading the production well with SI, commonly known as squeeze treatment,and injecting SI in the injection well. Optimum designs for both methods were identified consideringdifferent levels of SI adsorption, and also permeability variation in fractured and non-fractured formations. As expected, the volume of SI required in continuous injection in producer was lower than the othertwo methods. However, in cases where the highest risk of precipitation is in the near-wellbore area or itis below the continuous injection point, it is necessary to apply one of the suggested methods. While thesqueeze treatment protects only the formation around the producer well, treatments deployed in injectorwells will protect the whole system and this extra protection may offset the extra volume of chemicalnecessary. The application of SI in injector well was studied in both continuous and batch mode withdifferent injection frequencies. It was shown in continuous injection that even though less SI volume is used,the SI breakthrough time in producer can be so long that a series of squeeze treatments might be requiredto protect the well. The simulation results showed that in high adsorption formations, squeeze treatment ismore efficient than deploying SI in the injector well. However, in cases of low adsorption and fracturedreservoirs, the scenario commonly found in geothermal plants, SI injection at the injector is more optimal. Two different scale treatment methodologies were studied in geothermal wells, including squeezetreatment in producer and SI injection in the injector and the results were compared with the continuous SIinjection in producer, which is the most current treatment in geothermal wells. It was illustrated in fracturedgeothermal reservoirs with relatively low levels of adsorption, that SI injection in the injector is the mostoptimum treatment that can effectively protect the whole plant from scaling.