52nd U.S. Rock Mechanics/Geomechanics Symposium,
2018. American Rock Mechanics Association
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ABSTRACT: In northern North America, a huge amount of tight gas is trapped in relatively impermeable rock formations at great depths. Physical fracturing of these formations by fluid injection, i.e., hydraulic fracturing, could enhance the overall permeability of these formations, and thus improve tight gas extraction. One of the outstanding issues in hydraulic fracturing in tight formation is to determine the magnitude of the injection pressure for initiating cracks in intact formation, i.e., criterion for crack initiation in intact rock. This study extends Griffith theory to determine the crack initiation criterion in brittle rocks containing minute pores of different shapes under biaxial compression and pore pressure elevation. Analytical results reveal that the pore aspect ratio has a significant impact on the crack initiation near the pore tip. In addition, the combined effect of applied total stresses and internal pore pressure dictates if the initial cracks near the tip would become stable or grow by tensile or shear fracturing. These results have practical implications on permeability enhancement in tight gas shale by hydraulic fracturing.
The gas production rates from tight gas shale formations are extremely low due to its ultra-low permeability in a range of 10-19 m2 (Aguilera, 2014). Multiple hydraulic fracture treatments in horizontal wellbores are becoming a viable technology to economically develop unconventional resources in shale reservoirs. Hydraulically induced fractures radiate outward from the wellbore intersecting the nearby existing natural fractures. The resulting fracture network increases the drainage area and reduces the drainage paths within the stimulated rock volume (SRV). The total production and its rates are dependent on the extent of SRV and the fracture spacing. If the fracture spacings are large, the rates are still low. The production rate will be much enhanced if cracking could be induced in the intact tight shale inside SRV.
Micro cracks and fractures are detected in triaxial compression tests on intact rock specimens in laboratory and underground excavations in intact rock formation in the field. Griffith theory (1920, 1924) provided a starting framework to explain the cracking process from existing flaws, defects or pores in intact rocks. However, there is no or limiting studies on crack initiation and propagation in intact rock under pore pressure elevation in hydraulic fracturing treatment.
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