Experimental studies in hydraulic fracture growth : fundamental insights and validation experiments for geomechanical models
Creators: name: Al Tammar, Murtadha Jawad
name: 0000-0002-6566-4815
Publisher: name: The University of Texas at Austin
Year: 2019
Subjects: Hydraulic fracturing
Fracturing experiments
Heterogeneity
Porous materials
Layering
Geological layers
Heterogeneous specimens
Heterogeneous materials
Fracture propagation
Crack growth
Kinking
Digital image correlation
DIC
Crack detection
Specimen saturation
Breakdown pressure
Cyclic fluid injection
Cyclic pressurization
Static fatigue
Gas fracturing
Nitrogen fracturing
Injection rate
Constant pressure injection
Pore pressure

description: Novel experimental capabilities to study hydraulic fracturing in the laboratory are developed and utilized in this research. Fracturing experiments are conducted using two-dimensional (2-D) test specimens that are made from synthetic, porous materials with well-characterized properties. Fracture growth during the experiments is captured with clear, high resolution images and subsequent image processing using Digital Image Correlation (DIC) analyses. First, we investigated the problem of a hydraulic fracture induced in a soft layer bounded by harder layers. The experiments reveal a clear tendency for induced fractures to avoid harder bounding layers. This is seen as fracture deflection or kinking away from the harder layers, fracture curving between the harder bounding layers, and fracture tilt from the maximum far-field stress direction. In addition, when a fracture is induced in a relatively thin layer, the fracture avoids the harder bounding layers by initiating and propagating parallel to the bounding interfaces. Fracture propagation parallel to the bounding layers is also observed in relatively wide layers when the far-field stress is isotropic or very low. Complex fracture trajectories are induced in layered specimens when the far-field differential stress is low or intermediate. In a second set of experiments, we used homogeneous specimens with multiple fluid injection ports. It is clearly shown that injection-induced stresses can appreciably affect hydraulic fracture trajectories and fracturing pressures. We show that induced hydraulic fractures, under our laboratory conditions, are attracted to regions of high pore pressure. Induced fractures tend to propagate towards neighboring high pore pressure injection ports. The recorded breakdown pressure in the fracturing experiments decreases significantly as the number of neighboring injectors increases. The influence of an adjacent fluid injection source on the hydraulic fracture trajectory can be minimized or suppressed when the applied far-field differential stress is relatively high. Preferential fracture growth due to changes in pore pressure in field applications as compared to our laboratory observations is also discussed. In a third set of experiments, we show that the breakdown pressure of test specimens can be reduced markedly with low injection rates, cyclic borehole pressurization, and/or constant pressure injection. This is largely related to the extent of pressurized region around the borehole caused by fluid leakoff in dry specimens and possible specimen weakening by fluid contact. The breakdown pressure can also be reduced by notching the specimen borehole when the injection fluid is allowed to flow and leak off along the borehole notch. In a fourth set of experiments, we compared fracture growth induced by a viscous liquid and a gas which are glycerin and nitrogen, respectively. The experiments show that fractures propagate through test specimens in a gradual manner when induced by glycerin at various injection rates. By contrast, nitrogen injection induces fractures that grow much more rapidly, which we attribute to its compressible nature and ultralow viscosity. The breakdown pressure is also shown to be markedly lower for nitrogen fractures compared to glycerin fractures. Moreover, an experimental evidence of fluid lag when fractures are induced with viscous fluids is demonstrated. Lastly, experiments were conducted to examine the behavior of an induced hydraulic fracture as it approaches a cemented natural fracture. We show a tendency for the induced hydraulic fracture to cross thick natural fractures filled with softer materials than the host rock and to be diverted by thick natural fractures with harder filling materials. The induced hydraulic fracture also tends to cross hard natural fractures when the natural fractures are relatively thin. In addition, the induced hydraulic fracture from the injection port is shown to be diverted by a thin, hard natural fracture that is placed relatively close to the injection port but crosses the same natural fracture when placed farther away from the injection port. These observations, and numerous others, documented in this dissertation provide fundamental insights on various aspects of hydraulic fracture propagation. Our extensive set of laboratory observations are also very useful in validating numerical hydraulic fracturing simulators due to the small-scale, 2-D nature, and characterized properties of the test specimens used in the experiments.