Morphological Alterations of Granitic Hot Dry Rock (HDR) Due to Short-Term Interactions with Alkaline Salt Solutions and Their Implications on Single-Fracture Permeability

Water–rock interaction (WRI) due to fluid invasion into formation fractures causes potential formation damage during drilling in hot dry rock (HDR) reservoirs. Due to the developed artificial fractures, drilling fluids will contact and interact with HDR reservoir fractures, leading to alterations in fracture characteristics. With the increasing popularity of brine-based drilling and completion fluids, these alterations induced by WRI could be further enhanced, which requires investigations. In this study, short-term WRI experiments between the HDR and three reactive solutions, including pure water and alkaline (pH = 10) 6 wt% NaCl/KCl solutions, under temperatures of 25 °C and 180 °C, were carried out. Rock-surface topography alterations were identified using laser scanning. Morphological alteration mechanisms were revealed through microscopic and mineralogical alteration determination using a field emission scanning electronic microscope. Hydrogeochemical simulations, including reaction kinetics and equilibrium, were conducted to support the mineralogical alterations. Implications of morphological alterations on fracture permeability were demonstrated based on a roughness–permeability model. The results show that interactions with the alkaline NaCl solution cause the smoothening of the rock surface due to a coating effect of secondary silicate precipitations, while interactions with pure water and the alkaline KCl solution result in rock-surface roughening because of feldspar dissolution, differential mineral dissolution and biotite hydrolysis dispersion. The secondary precipitations either cover or fill the pores and cracks, the dissolved feldspar enlarges the pores and cracks, the differential mineral dissolution coarsens the rock surfaces, and the biotite hydrolysis dispersion generates pores and cracks on its surfaces. An increase or a decrease in surface roughness induces a corresponding increase or decrease in fracture permeability, because the roughness augmentation (roughening) enlarges seepage channel spaces while roughness reduction (smoothening) narrows them. This work reveals potential formation damage induced by drilling fluid invasion into fractures within HDR geothermal reservoirs, and provides theoretical insights for mitigating such damage.