PHYSICAL BACKGROUND TO THE BREAKDOWN PRESSURE IN HYDRAULIC FRACTURING TECTONIC STRESS MEASUREMENTS

A new theory is presented to interpret the breakdown pressure which is the wellbore pressure at fracture initiation in hydraulic fracturing tectonic stress measurements. The theory is able to explain the effects of wellbore diameter and pressurization rate on the breakdown pressure. It was shown that these effects were explainable in a simple way by taking into account the effective stress distribution beyond a wellbore surface and by applying a newly-constructed fracture criterion. The criterion assumes the fraction initiation occurs when the maximum tensile effective stress reaches the tensile strength of a rock at a point that is not on the wellbore surface but is inside the rock. The distance between that point and the point of stress concentration is a material constant. In order to verify the present theory, laboratory hydraulic fracturing experiments were performed on cubical rock specimens under uniaxial compression. Results show that experimental breakdown pressures decrease with the increasing wellbore diameter and increase with the pressurization rate. In all cases, the experimental results agree perfectly well with the prediction based on the present theory contrary to the prediction based on the conventional theory.