Giovanni Grasselli

University of Toronto

Rock fabric and not principal stress dictates SRV: the story of how a discounted data point still plagues our industry

Hydraulic fractures should open against the least stress. First published in 1957 by Hubbert and Willis, this theory was experimentally validated by injecting a plaster slurry into “ordinary gelatine” (homogeneous medium) under different principal stress orientation and observing the final geometry of the set plaster. All tests confirmed the hypothesis, except for one test that "shows a horizontal fracture (that was tested) in stratified gelatin". This result was discounted because of the pre-existing "weakness due to bubbles between two gelatin layers”. Our hydraulic fracturing experiments, in highly laminated reservoir rocks, consistently reproduced this discounted behaviour, demonstrating that the presence of bedding planes can influence the development of the stimulated rock volume (SRV) more than principal stress. Understanding the dominant driver of fracture geometry, including the characterization of bedding planes, could allow for the development of an improved risk mitigation workflow and production optimization.