Experimental Study On Thermal Cracking Of Saturated Rock

Thermal cracking of rocks can increase the length and density of the fractures and results in a well-connected fracture network in the rock,which will increase the permeability of the rock and improve its transport property.In this paper,heating experiments under different conditions were carried out on dried and saturated rocks and their physical properties(permeability and porosity)were measured.Microstructure characteristics,mineral compositions and acoustic emission characteristics of the rock were observed using scanning electron microscopy(SEM),X-ray diffraction(XRD)and acoustic emission system.The results show that the permeability of the rock gradually increases with increased temperature.The effect of thermal cracking by heating wholly is better than that of heating locally.The larger heating rate and the longer heating time are,the greater increments on rock permeability have.After heating treatment at same temperature,the permeability of the saturated rock has a better enhancement than that of the dried rock.Among these,the rock saturated with KCl has the largest increment in the permeability;In addition,the rapid cooling(cold water cooling)results in a larger permeability increment than natural cooling due to the greater thermal stress gradient.Mineral analyses before and after heating treatment show that some minerals are decomposed at high temperature and chemical solvent reacts with some mineral components,which causes the destruction of mineral crystal structure and improves rock's permeability.After heating treatment at a high temperature,the number and density of micro-crack increase obviously,more small cracks connect each other and form a well-connected fracture network in the rock,which significantly improves the permeability of the rock.Acoustic emission results show that there are some thermal cracking events for each heating temperature.The higher the heating rate is,the denser the acoustic emission signals is and the more obvious the thermal cracking effect of the rock will be.Based on above experimental studies,a permeability model for saturated rocks with heating treatment was established from the theories of thermoelasticity and fracturing mechanics,and parameter sensitivity were studied.The results show that thermal stress gradient,fluid pressure gradient,thermal expansion coefficient,initial permeability and rock stress intensity are the main factors affecting thermal cracking.Finally,the technical limitation of thermal enhancing permeability for rocks was established combining with experimental data and model prediction data.The study provides a reference for practical engineering application.