Experimental study on hydraulic fracturing of soft rocks: Influence of fluid rheology and confining stress

This paper presents an experimental study on hydraulic fracturing of unconsolidated rocks focusing on mechanisms of fracture initiation and propagation using different injection fluids at various confining stresses. Thus, a serial of parameter study including confining stress, type of injection fluid and geometry of injection section was carried out to explore the dependency of fracturing on these variables. Results showed strong influence of the confining stress and fluid rheology on the fracturing behaviour of the compacted sand. Injecting viscous Newtonian fluids and cross-linked gel yielded mainly infiltration associated with well bore expansion. Clear fractures were induced when injecting the cross-linked gel containing quartz powder and the bentonite slurry. When adding quartz powder and bentonite to the injection fluid, clear fractures could be created.This implies that the fluid rheology and the fluid additives have strong influence on the fracturing behaviour of material. Range of the applied confining stresses was between 0.5 and 20 MPa. At high confining stress, the ratio of fracture initiation pressure to confining pressure was around 2 while at lower confining stress the ratio was much higher averaging around 10. This would imply that it is not much easier to create fractures in shallow well bores since the injection pressure should reach 10 times the confining stress.