(a) Sandstone without mud (water pressure 3 MPa + confining pressure 6 MPa)
(b) Sandstone with mud (water pressure 3 MPa + confining pressure 6 MPa)
(c) Sandstone without mud (water pressure 4 MPa + confining pressure 6 MPa)
(d) Sandstone with mud (water pressure 4 MPa + confining pressure 6 MPa)
(e) Sandstone without mud (water pressure 3 MPa + confining pressure 15 MPa)
(f) Sandstone with mud (water pressure 3 MPa + confining pressure 15 MPa)
(g) Sandstone without mud (water pressure 4 MPa + confining pressure 15 MPa)
(h) Sandstone with mud (water pressure 4 MPa + confining pressure 15 MPa)
Figure 5: Gradual-cracking process and permeability behavior considering different confining pressure and water pressure.