Impact of secondary coatings on the dissolution rate of K-feldspar: A combined experimental and modeling study

Heat transfer fluid mining represents a thermodynamic perturbation for geothermal reservoirs: The pumping of hot water coupled with the re-injection of colder water at depth favors the dissolution of some rock-forming minerals of the deep reservoir (e.g. feldspars), while promoting the precipitation of secondary phases, resulting in a possible change in the permeability and porosity of the reservoir. Such an impact is even greater when one considers the acid stimulations aimed at increasing the injectivity of the geothermal system. In that respect, no consensus exists in the literature regarding the impact of secondary phases on the dissolution rate of primary phases and therefore, on the sustained modification of pore structure. The present study aimed at shedding new light on these questions. Hydrothermal experiments of K-feldspar alteration were conducted at conditions relevant for the geothermal reservoir of Soultz-sous-Forets (T = 180 degree celsius, acidic pH domain). Measurements of cation release rates were combined with characterizations of secondary coatings (mineralogy, extent of coverage, thickness and porosity) to determine the reactivity of submillimeter K-feldspar powders with and without secondary precipitates. The formation of mu m-thick boehmite coatings on K-feldspar grains was found to result in a modest decrease in its reactivity, which might be better explained by the presence of dissolved Al in the bulk solution. This result was independently confirmed by reactive transport simulations, which revealed that the impact of secondary coatings may become significant only when their thickness exceeds a few tens of microns, or if the dissolution rate of the primary phase is significantly greater (10(6) times) than that of orthoclase. Taken together, this study offers new constraints on the intricate interplay between dissolution and precipitation reactions, of prime importance for modeling more accurately the impact of mass transfer and porosity generation resulting from fluid circulation in geothermal reservoirs.