Scaling Assessment, Inhibition and Monitoring of Geothermal Wells

Abstract

The formation of inorganic, sparingly soluble salts from aqueous brines during geothermal energy production, is known as ‘scale’ and is one of the major flow assurance problems. Scale forms and deposits under supersaturated conditions, wherever the mixing of the incompatible types of water, formation water from the bottom hole and the injected seawater, takes place. Or when temperature or pressure changes are severe enough to produce a supersaturated solution. The deposited scale adheres on the surfaces of the producing well tubing and on parts of water handling equipment, where it builds up in time and leads to problems in reservoirs, pumps, valves and topside facilities. The rapid increase of the mineral deposits leads to inevitable damage of the equipment parts. As a consequence, suspension of production is necessary for the recovery or replacement of damaged parts. Steel pipes are known to be susceptible for scale deposition. The formation of scaling on the inner surface of casings depends on the scaling tendency of the water and the surface chemistry and morphology of the casing. Many mitigation strategies have been proposed: coatings, scale inhibitors, acids washes etc. The scaling tendency can only be lowered by a chemical intervention on the production fluids: reduce the supersaturation of the salts in the water by inhibitors that form chemical complexes with the scaling ions. We show that the amount of scaling that is deposited on a substrate depends on three parameters: • Surface energy of the casing, compared to the one of the scaling crystals and the water environment: the more the surface energy resembles the scaling crystals, the more scale will be deposited and stick to the surface, because of the high interfacial forces. • Roughness of the surface: higher roughness leads to lower the scale deposition. The lowest adhesion force is found when the roughness is high, the interfacial tension is low, and the radius of the roughness is low. This case is for the adhesion between scale and surface. So if scale is formed in the bulk solution, the adhesion is low for a rough surface. However, a different process will occur when the scale is growing on a rough surface. There, we have seen that scaling is favored when the roughness is high. • The bulk modulus of the surface: a lower bulk modulus results in lower forces that are needed to remove the scale crystals from the surface. These parameters were used for the development of a single parameter that can describe the scaling tendency of a surface. A more long term solution for scaling prevention is the implementation of a casing material that is intrinsically better resistant against scaling. A new class of pipes that may show intrinsically improved scaling performances is glass fiber reinforced composites. The inner surface of the pipes consist of the polymer matrix of epoxy or polyethylene and will have a different surface energy than steel pipes. Several sets of experiments were done on steel and polymer matrix casings under static scaling conditions. The scale that formed was a combination of heterogeneous nucleated on the surface and homogeneous nucleated in the bulk solution. The latter scale was removed by rinsing the samples and an adhesive tape after drying to only assess the scale that was grown on the surface. Moreover, we have shown that the measurement of the light reflection from the scaled surface is a good way of assessing the amount of scaling. With the results collected in this study, it is possible: to predict the scaling tendency under specific conditions, to measure the scale to validate the predictions and to assist the development of material for pipelines to lower scaling risks.

AAPG Datapages/Search and Discovery Article #90345 © 2018 AAPG European Region, Geothermal Cross Over Technology Workshop, Part II, Utrecht, The Netherlands, April 17-18, 2018