Research on mass transfer mechanisms due to short-term water-rock interactions between granite cuttings and alkaline NaCl solution and their patterns

Abstract

Water-rock interactions induced by working fluids in hot dry rock (HDR) reservoirs lead to reservoir damages through mass transfer. Therefore, investigating the mass transfer mechanisms due to the interactions between working fluids and reservoir rocks contributes to minimizing HDR reservoir damages. Drilling fluid is an important working fluid, yet the interactions between drilling fluids and HDR reservoir rocks lack understandings. Here, interaction experiments were conducted using a high-temperature and high-pressure flow reactor under temperatures of 180 °C and 240 °C, a pressure of 24 MPa, and a flow rate of 0.05 mL/min for 7 days. Pure water and an alkaline NaCl solution, which is referred to a HDR reservoir drilling fluid, were prepared to interact with granite cuttings from the HDR reservoir at Qiabuqia site in Gonghe Basin, Qinghai Province, China. Mineral characteristics, mass and chemical changes in the cuttings, solution element content concentrations, micro-morphology and element contents on the cuttings surface, as well as the size of suspending fines in the solutions were determined to reveal the mass transfer mechanisms. The results indicate that the mass transfer mechanisms include mineral reactions and fines migration, and fines migration is the dominating mechanism. The mineral reactions, including feldspar dissolution, quartz dissolution and feldspar alteration, are strong at the front section, while are weak at the end section. At the middle section, the released element contents by mineral reactions form various precipitates, mainly including aluminosilicate, silicate and silica. Strong mineral reactions at the front section lead to fines migration, while the precipitation of Na-bearing minerals at the middle section inhibits fines migration through a coating effect. Suspending fines are identified in the solutions, with a diameter of 100–300 nm, and settled fines are observed at the end section, with a diameter of hundreds of nanometers to a few microns. Simulations of reaction equilibrium show that the precipitation of quartz and biotite increases with a higher NaCl concentration and a lower temperature, while albite changes from dissolution to precipitation as NaCl concentration rises from 6 wt% to 12 wt%. The formation of halite by simulations supports the coating effect. This study provides theoretical basis to minimize HDR reservoir damage induced by working fluids.