Modelling of Time-Dependent Wellbore Collapse in Hard Brittle Shale Formation under Underbalanced Drilling Condition

Table 3

Strength changes of shale under drilling fluid immersion.

Sample	Immersion time (h)	Confining pressure (MPa)	Compressive strength (MPa)	Cohesion (MPa)	Internal friction angle (°)
1	0	0	30.80	9.83	24.89
2		50	153.46
3	6	0	23.79	7.86	23.09
4		50	138.30
5	12	0	21.48	7.32	21.42
6		50	129.00