Hydraulic fracturing and horizontal drilling are keys to unlocking unconventional hydrocarbon resources. The efficiency and success of hydraulic fracturing require solid understanding of fundamental physics involved with injection so controlling factors can be identified and optimized for completion and operation designs.
This paper describes and quantifies critical geomechanical, geological, and engineering variables for fracture initiation and propagation in horizontal wells. Formation breakdown pressure and net pressure are evaluated and compared with varying rock stresses, laminations, and perforations. Recognizing each unconventional formation has its own geomechanical and geological characteristics, this study highlights the importance of adapting engineering designs to accommodate the formation difference.
Through analytical, numerical, and case studies, this paper finds the following:
Significant longitudinal fractures can develop in a normal in-situ stress regime when the stress perturbations around perforations are sufficiently large. This magnifies fracture tortuosity and pressure loss near the wellbore. The extent of stress perturbations depends on wellbore orientation relative to the in-situ stress direction, perforation spacing and orientation, rock expansion and pore pressure change with fluid injection.
Pumping pressure is dominated by lamination properties such as permeability and strength. Higher net pressure is observed with less permeable and weaker laminations. This indicates that shear slippage along weak lamination interfaces does not help propagate hydraulic fractures. Instead, it dissipates energy and elevates injection pressure, in some cases well above the overburden.
Perforation patterns play important roles in formation breakdown and fracture geometry near wellbore. Due to stress interference from the competing neighbors, traditional spiral and dense perforations can result in much higher breakdown pressure and more fracture complexity, compared to more planar and sparse configurations.
For the case studied, a single long perforation design helped injection escape the near wellbore stress concentration and maximize stimulation efficiency.
These findings help advance the fundamental understanding of the physics involved in hydraulic fracturing processes. Results and learnings can be applied to optimize completion and operation designs, minimize horsepower requirements, and improve stimulation efficiency.