The influence of mechanical contrast on industrial and natural hydraulic fracturing

Abstract

The heterogeneity of layered systems leads to variation in rock or soil mechanical properties, influencing the resistance to failure. A formation breaks by fracture propagation when the effective stress surpasses the formation strength. The propagation of a fracture through a mechanical interface depends on whether the formation strength of the second formation is overcome. This critical effective stress level could be trespassed by high natural or industrial induced pore pressures. Understanding fracture propagation in multi-layered systems with variable pore pressure regimes has important implications and applications to many industries such as quarrying and hydraulic stimulation.
The slope stability of Westerwald Clay Quarries is influenced by inter-bedding of thin sand layers. Surface and slope fractures within the clay formation originated from a high observed hydrostatic head within the sand layers and a reduced confining stress from mining activities. The slopes of the quarry are key in determining the volume of economically mineable clay, these slopes are in turn controlled by the size/extent of fractures and whether they extend through multiple formations.
This study examines the effect of fracture continuation from sand layers into the stiff Westerwald Clay Formation. The soil parameters (cohesion and friction angle) of the different lithologies within a Westerwald Clay Quarry are determined for slope stability analysis by shearbox testing. Soil classification has been done in terms of plasticity, grain size and mineralogy by Atterberg Limits, Sieving and XRD & XRF respectively.
The results show that fracture initiation within the Westerwald quarries is a combination of mining activities lowering the confining stress and a constant natural hydraulic head. The hydraulic head within the small sand formation lowers slope stability by causing fracture initiation and water infiltration into the clay formation. Slope stabilisation occurs by artificial water pumping or natural water dissipation lowering the hydraulic head. Slope stability is decreased by embankments of low permeability backfill and increased by high permeability backfill.
Inter-bedded systems are common target locations for an unconventional reservoir systems and can be found both within source rocks as well as in conventional geological traps. Improvement in recovery from these tight systems often depends on the extent and continuity of fractures through heterogeneous interfaces.
This study examines the propagation and continuity of the fractures in an artificial heterogeneous layered system. The fractures will be initiated by hydraulic fracturing in a dried layered system via water injection in a triaxial cell. Fracture propagation is analysed through Micro-CT scans. The mechanical properties such as acoustic wave velocities, unconfined & confined compressive strength and tensile strength are all determined for the analysed layered systems.
The results show that hydraulic fractures initiated within the weakest layer are arrested at the interface between a mechanically weak and strong formation, whereas fractures initiated within mechanically stronger layers prograde through the interface. Hydraulic fractures are initiated when local pressure difference at the interface exceeds the formation’s critical tensile stress, the formations critical tensile strength is dependent on the confining pressure.