Real-time permeability evolution of limestone under high temperature and triaxial stresses

Abstract

In deep underground geological disposal of nuclear waste, geothermal energy development, underground coal gasification, geothermal resources development and other underground high temperature projects, they are all closely related to the permeability and thermal expansion characteristics of rocks in high temperature high-pressure real-time state. In this experiment, the real-time evolution of permeability of limestone under high-temperature and triaxial stresses condition was studied using the HTHP rock mechanics triaxial test system and compared with the permeability of limestone after high temperature cooling treatment. The test results show that: the effect of temperature on limestone permeability facilitation under high temperature cooling condition is significant, and the permeability increases by 1–2 orders of magnitude relative to the initial permeability for every 100 °C increase in the preheat temperature after 300 °C. Under high-temperature and high-pressure in real time condition, the difference values of permeability at different temperature points relative to room temperature always do not more than one order of magnitude. During the heating process at 500 °C, the permeability curves of the triaxial pressures range from 15 to 35 MPa are very different. Under the pressure condition of 15 MPa, the permeability curve increases monotonically and rapidly with the increasing temperature; When the pressure reaches 25 MPa pressure, there is a peak value area in the permeability curve; the permeability decreases monotonically with temperature when the pressure up to 35 MPa. Compared with the only thermal effect, the thermal- mechanical coupling effect has a significant inhibitory effect on limestone permeability development, and the permeability disparity between the two types of heat treatment rocks become more and more significant with increasing temperature and hydrostatic pressure.