Device for rock fracture toughness testing under hydrocarbon reservoir conditions

Abstract

The effect of confinement pressure on fracture toughness may be significant in materials with poor tensile strength, as is the case of most rocks. In fracking wells for hydrocarbon exploitation, rock fracture is induced by a pressurized fluid. Indirect estimates of rock toughness based on injected pressure and volume records are much larger than laboratory-measured toughness values. This article discusses the design, fabrication and use of a linear elastic fracture mechanics method to assess the critical stress intensity factor (KI) of rocks under real well bottom conditions. Apparent Fracture Toughness (KIf) is determined using notched 1.5″ plugs machined from rocks. In this novel experimental set up, KI is applied by means of a hydraulic pressure inside the crack, so that fracturing fluid chemistry can be modified in order to optimize fracability. A secondary hydraulic system ensures variable confinement pressures, up to 80 MPa. Preliminary tests carried out in shale rocks at well bottom pressures show that rock toughness more than doubles those found at atmospheric pressure. It is shown that increasing triaxial pressure confinement allows to accurately model the conditions of rocks in oil & gas reservoir conditions.