Geothermal wells represent an important source of energy in many parts of the world, yet they receive little attention in the mainstream literature. Annual worldwide production of geothermally-derived electrical power is some 57000 GWh from 8900MW of installed capacity and Philippines is the second largest producer with almost 2000MW of capacity.
In general, geothermal wells produce steam, generated by the contact of water with hot metamorphic rock, and this is used to drive turbines for electricity generation. Geothermal fields usually comprise both production, and injection, wells and, like oil and gas wells, these can suffer from production (or injection) impairment. The most common forms of impairment in geothermal wells are mineral scales dissolved in the hot water and then deposited as a result of thermal disequilibrium or phase changes. Many mineral species can be present, depending on rock composition and injection water quality, but one of the more prevalent is silica (SiO2). Normally, such mineral deposits are removed, periodically, by acidising with combinations of hydrochloric and hydrofluoric acids.
This paper describes work performed on geothermal wells in Philippines in early 2006. Laboratory testing demonstrated that acid formulations with 3–6% HF, as had been used historically, were capable of dissolving only a fraction of the scale deposits, even with a large stoichiometric excess of acid. Surprisingly, increasing the HF concentration to 9%, in a specially buffered acid system, resulted in total scale dissolution, in laboratory tests. Furthermore, the high silica content of the scale, absence of problematic minerals in the formation and fractured nature of the reservoir, persuaded us to eliminate the HCl preflush on these treatments, a novel concept that had never been tried in Philippines.
The application of this system in the field posed some special technical and HSE challenges. However, all the treatments were carried out safely and successfully, without incident, and with excellent results as briefly described in this paper.
Geothermal energy represents a major resource in Philippines, a country with an expanding population and increasing urbanization that, unfortunately, has only found relatively small amounts of hydrocarbons, to date. Thus, electricity generation from geothermal sources is an attractive alternative and the country has been exploiting this resource, in this way, for the past 30 years or so. In 2005, almost 20% of Philippines annual electricity budget was provided from geothermal sources.
Because of the mechanisms of steam production, mentioned above, geothermal wells experience a variety of problems related to the deposition of mineral scales inside wellbore tubulars, behind slotted or preperforated production liners or within the natural fractures that act as conduits for production or injection water. The composition of such scales can vary significantly, depending on the specific minerals contained in the metamorphic rocks and the exact composition, therefore, of the water that has traversed the rock system. In the case of injection wells, the scales formed are dependant, to a great extent, on the source of the injection water which may come either from surface sources or may consist of mixtures of produced water and condensed steam that are then re-injected.
In this particular project, all candidate wells were injectors and all had experienced dramatic decreases in injectivity due to the build-up of some, as yet unidentified scale. Subsequent X-Ray Diffraction analysis of scale samples recovered from the wells showed that the offending mineral was almost exclusively quartz, present as very hard deposits inside the pre-perforated production liner 2. It was suspected that this quartz scale was also present in the annulus between the uncemented, preperforated pipe and the formation and also in the formation fractures.