Question

Two hydraulic fracturing tests were conducted on a marble rock mass. First test was conducted at a depth of 400 m and the second test was conducted at a depth of 800 m. Breakdown pressures, PB were measured to be 12 MPa from first test and 20 MPa from the second test. Also, Shut-in pressures, Ps were measured to be 7 MPa and 14 MPa from the first and second test, respectively. Knowing that the tensile strength of the rock is σt = 10 MPa and the principal stresses are on vertical and horizontal directions, solve:
1) Estimate the ratio between the mean of the horizontal stresses and the vertical stress for the two tests.
2) Check whether vertical fractures would be produced from both tests.

Answer 1