Initiation pressure, location and orientation of hydraulic fracture

The Hubbert and Willis (1957) [1] fracture criterion has been widely adopted to predict longitudinal fracture initiation in vertical or horizontal wellbores. Current transverse fracture criteria predict fracture by comparing the axial stress in the wellbore wall with the tensile strength of rock. It is shown in this paper that the axial stress is not a good predictor of transverse fracture initiation, because it remains constant during hydraulic pressurization. The magnitude of the most tensile principal stress in the wellbore wall should always be compared with the tensile strength of the rock to predict hydraulic fracture initiation based on elastic theory. Special cases have been found in which the most tensile principal stress reaches the tensile strength of the rock simultaneously at all point on the circumference of the wellbore. It is proposed that transverse fractures are initiated in those cases. (C) 2011 Elsevier Ltd. All rights reserved.