Using Coiled Tubing to Remove Silicate Scale from Geothermal Wells

Abstract

Geothermal power plants use heat from the Earth's interior to produce one of the cleanest sources of electric power currently available, but the production of geothermal fluids can be costly if large quantities of silica scale are deposited on production tubulars. This widespread problem significantly reduces the ID of the tubulars, thereby reducing the production rate of the well. Previously, scale removal techniques involved the use of a drilling rig and a combination of such mechanical tools as drill bits and mills. Rig-based methods, however, are expensive and time-consuming. This paper discusses the design, use, and evaluation of an alternative technique that includes a coiled tubing unit (CTU). When compared to traditional rig-based scale removal techniques, coiled tubing methods offer several economic, environmental, and safety benefits. Specifically, the use of coiled tubing reduces job time because coiled tubing is faster to rig up/down and pull out of the well. From an environmental standpoint, well effluent and debris can be flowed through the flowline to the existing processing facilities; pits are unnecessary. Coiled tubing also provides a greater degree of safety and well control when operations are being performed on a live well. In the case histories provided in this paper, three methods were used to remove scale. For the first method, the cleaning energy was applied by highly focused polymerized fluid jets, which were specially designed for the job by a computer program. This hydraulic technique can be performed on a live well, which results in additional savings over traditional methods. The steam production during cleaning creates the required annular velocities to remove debris from the well without the need for energized treatment fluids. The second method consisted of a high-speed coiled tubing motor in combination with a flat-bottom junk mill. Although both mechanical techniques have been performed with varying degrees of success, both increased the geothermal production rates of the wells treated. The third method involved the use of a silicate scale remover that was used between the slotted liner and openhole section. It was also used to treat the well when either of the first two techniques were slowed or stopped by particularly hard scale deposits.

Coiled Tubing Injection String for Geothermal Wells

“…However, these wells encourage the accumulation of solid substances which can restrict production. 2,3 Moreover, geothermal wells produce high percentages of corrosive gasses, such as H 2 S, and chlorine, that can be detrimental to completion materials as well as the overall infrastructure of the well.…”