Modeling the Viscoelastic Behavior of Quartz and Clay Minerals in Shale by Nanoindentation Creep Tests

Table 2

Fitting parameters obtained by different viscoelastic models of typical creep curves at 30 mN.

Model	Three element Voigt				Burgers					Two dashpot kelvin
Parameter	E1	E2	η1	R^2	E1	E2	η1	η2	R^2	E1	E2	E3	η1	η2	R^2
	(GPa)	(GPa)	(GPa·s)		(GPa)	(GPa)	(GPa·s)	(GPa·s)		(GPa)	(GPa)	(GPa)	(GPa·s)	(GPa·s)
Quartz	88	402	98812	0.989	97	1887	127207	797	0.996	1924	322	93	624	106087	0.998
Clay	11	132	5035	0.942	12	162	25112	948	0.984	161	139	11	329	8853	0.995