Experimental Study Of Water-Sandstone/Limestone Reaction Under Triaxial Stress

Hydrochemical variations of solutions in the saturated sandstone and limestone with creeping and shearing deformations under triaxial compression were experimentally investigated, and genetic mechanisms of hydrochemical anomalies related to earthquake were discussed.The time-dependent and stress-dependent experiments were conducted by loading differential stress to the samples, which are consisted of ground water and sandstone or limestone, with the gas medium high temperature and pressure experimental system at room temperature. Then concentrations of F-, Cl-, NO3-, SO42-, Ca2+, Mg2+ and Na+ in the produced solutions under different stresses and time were measured with IC and ICP-OES.The experimental results show that the ion concentrations in the product solutions increase with increasing the time of creeping and the differential stress on the saturated rocks, but the concentration of Mg2+ decreases in the experiments of water-sandstone reaction. Concentrations of F- and Cl- in the produced solutions increase by about 10 and 4 times in the experiments of water-sandstone reaction, and F-, Cl- and Ca2+ in the ones increase about 7, 6 and 5 times in water-limestone reaction, respectively. The hydrochemical variations are mainly caused by the dissolution of minerals and interfusion of fluid inclusions.The experiments indicate that effects of differential stress on water-rock reaction likely result in positive and negative anomaly of hydrochemistry, which is probably attributed to dissolution of minerals and ion exchanging, and releasing of the closed fluid companying with deformation and microcracking of rock samples. Concentrations of F- and Cl- change obviously in the product solutions of sandstone samples, so F- and Cl- may be considered as the sensitive components for monitoring earthquake in the sandstone aquifer, and F-, Cl- and Ca2+ are relatively sensitive in the limestone aquifer.