Understanding the Effects of Leakoff Tests on Wellbore Strength

Leakoff tests (LOTs) are performed to test the strength or pressure containment of the shoe after a cement job to help ensure that the new hole has been securely isolated from what has been cased off. A successful LOT can also be used to calibrate the least principal stress (many times, in the case of a vertical well, the minimum horizontal stress), or for geomechanics modeling. This will require initiating a fracture at the wellbore. Because of the near-wellbore stress concentration, for the purpose of geomechanics calibration, it is preferred to take the leakoff to the far-field stress region. To perform this extended LOT (XLOT), a relatively long fracture has to be created. Though an XLOT is needed for these reasons, some engineers tend to refrain from performing this test for fear that the test may damage the wellbore and consequently cause drilling problems. This paper addresses this issue by investigating the effect of wellbore damage on wellbore "strength" or pressure containment. Various issues are discussed to help engineers determine when it may or may not be a concern. This should give practicing engineers the necessary insight into this complex rock-mechanics issue. The discussions are supported with results from analytical and numerical simulations based on rock-mechanics principles.