Experimental Study of the Effect of Formation Water Salinity on Creep Laws of the Hard Brittle Shale

Hard brittle clay shale has micro-fractured bedding and tends to cause side-wall collapse. Particularly when being invaded by foreign fluids,the rock has obvious rheological effects. Taking hard brittle shale as the object of study, uniaxial creep experiments on water saturation samples of formations with different degrees of mineralization were conducted. Research results showed that under the same externally applied loads, the creep deformation degree of the formation water saturated rock samples with a salinity of 6.0×104 mg/L was the least (0.001-0.014) and the creep deformation degree of the formation water saturated rock samples with a salinity of 8.0×104 mg/L was a little greater (0.005-0.024), while the creep deformation degree of the formation water saturated rock sample with a salinity of 4.0×104 mg/L was the greatest (0.010-0.030).Learning from the modeling ideas of classic element combination models, the creep model reflecting three creep stages was established.Three important parameters (instantaneous elastic modulus, viscous coefficient, accelerated creep viscous coefficient) in the model tended to decrease obviously. Test results showed that the relation between rock mineralization and creep deformation degree was quadratic non-linear, and that too high or too low salinity would have the greatest influence on the mechanical properties of rocks. The study suggested that the influences of drilling fluid salinity on borehole wall stability should be considered in drilling operations.