Removal of Anhydrite and Mg-Silicate Scales from Production Wells Using Chemical Agents at the Mori Geothermal Field in Hokkaido, Japan: An Application of Chemical Well Stimulation

Abstract

Well stimulation, which enhances permeability, is an important technique in the creation of Engineered Geothermal Systems (EGS). In addition to physical stimulation such as hydrofracturing, chemical stimulation has been used at several EGS fields. In addition to conventional mineral acids, chelating agents and alkaline solutions have been studied and employed to dissolve calcium and silica minerals without significant casing corrosion. We tested the notion that this chemical stimulation technique was applicable to the removal of anhydrite and Mg-silicate deposits as a replacement for costly mechanical workovers.

We conducted two scale removal operations using chelating and alkaline agents at the Mori geothermal field. Two distinct scale minerals, anhydrite and Mg-silicate, were observed in two different production wells. The latter scale consists of crystalline and amorphous structures. We confirmed that the chelating and alkaline solutions not only dissolved anhydrite and Mg-silicate scales but caused them to spall into particles and powders in the laboratory prior to the stimulation on site. The spalled scale fragments can be readily ejected from the wellbore during production.

We injected a chelating agent and alkaline solutions into the wells in July and November, 2008. Combined with successive production, the majority of scale was removed and the treated wells showed improved productivity.