Question

Three samples of the same depleted reservoir formation were analyzed using conventional tri-axial cell and the results of axial and radial stresses at the point of failure are reported below:
Sample
Sample         Radial Stress (MPa)           Axial Stress (MPa)

1                        0.358                                            8

2                        1.052                                           10

3                        1.692                                           12
(a) Using graphical representation of Mohr circles determine the value of cohesion and internal friction angle of this formation. You must draw the circles and failure criteria into exact scale and read the values from your plot.
(b) If the confining pressure applied at a certain depth is 2.2 MPa, determine the minimum vertical stress which is required to cause failure of the formation according to Mohr-Coulomb failure criterion. Use both graphical approach and calculations and compare the results.

Answer 1