FLUID COMPRESSIBILITY EFFECTS DURING HYDRAULIC FRACTURE

Hydraulic fracturing is integral for hydrocarbon recovery. The fracture results when internal pore pressure is increased above a critical value, the breakdown pressure. While this pressure is important for fracking operation designs, it is hard to predict because it is controlled by several factors, including fluid viscosity and flow rate. Here, we focused on compressibility of the fracturing fluid used during hydraulic fracturing experiments under triaxial stress conditions. The samples are hollow cylinder specimens of PMMA and Solnhofen limestone, a finely grained (<3 um grain), low permeability (<10 nD) carbonate (with ~4 wt% organic matter), with modest elastic moduli (E~40 GPA, ν~0.28). In some aspects the limestone is an analog to a typical shale sample. We use water and argon as pore fluids to provide contrasting viscosity and compressibility values. We also recorded acoustic emissions using 8 piezoelectric sensors. After the experiment, we mined-back (core) the fracture and measured its permeability at reservoir overburden stress conditions. We also scanned the fracture surfaces to determine morphology.

Breakdown pressures were larger for argon than those for water at various pressurization rates. Fractures created with gas as pore fluid were more complex. The fractal dimension of the surfaces formed with argon were lower, indicating a rougher surface. More importantly, the fracture generated by gas had a permeability about 10 times higher than that formed with water fracture. The higher gas fracture permeability could owe to a rougher surface or to the surrounding microcracks generated as out-of-plane crack branches. We explain our experimental results with analytical models of linear elastic fracture mechanics (LEFM). Analytical models suggest that initial fractures occurring with compressible fluids tend to stabilize. Hence, formation and extension of additional fractures may occur, leading to a more complex morphology. Conversely, fractures formed by incompressible fluids remain critically stressed as they extend, thus producing a simple bi-wing fracture. This study suggests a tie between breakdown pressure value, fracture complexity, its resulting surface morphology, and permeability; these parameters might be optimized by engineering the pore fluid properties.