Monitoring of Fracture Propagation in Shallow Subsurface
Abstract
Shallow hydraulic fracturing can be used the Army purposes in different ways. Creation of fractured media in a controlled manner is needed for production of electricity from geothermal energy to provide US military installations with clean baseload power. Properly controlled hydraulic fracturing can interfere or destroy the subsurface infrastructure. At the same time, well developed fracture monitoring tools can provide the information on the underground state of stress and precursors of subsurface rock fracturing and failure. We are aiming to explore the propagation of hydraulic fractures in low-permeable near-surface rock representatives. For this purpose, we are going to develop an experimental apparatus that would allow subjecting rock specimens to lateral and axial stresses and saturating them with a pore fluid to simulate the subsurface conditions. We will then inject a viscous fluid inside a borehole drilled in the rock until the material fails and fluiddriven fracture propagates through the specimen. The monitoring of fluid-driven fractures is proposed to be performed with ultrasonic transducer sets that would allow determination of their geometric characteristics, as well as elastic and fracture properties of rock. The ductility of nearsurface rock, presence of in-situ fluid in it, and irregular shapes of near-surface fractures poses multiple challenges in characterizing the fracture geometry and the parameters. Hydraulic fractures are tensile cracks that result from high pressure injection of a viscous fluid into a solid medium. Common applications of hydraulic fracturing include the stimulation of lowpermeable or unconventional reservoirs for oil extraction and enhanced geothermal systems. The presence of an asymmetric stress field near the fracture tip can cause the fracture to curve and this condition is most commonly observed when the hydraulic fracture propagates in the vicinity of a free surface. Ultrasonic measurements are a powerful tool in monitoring and understanding the physics of hydraulic fractures and they can be used at both laboratory and field scales. In order to better understand hydraulic fracture propagation, the fracture is monitored with time lapsed ultrasonic measurements with active sources from both compressional (P-wave) and shear wave (S-wave) transducers. As the hydraulic fracture propagates, it interacts with the ultrasonic waves, and by studying these interactions it is possible to detect and characterize the geometry of the fracture. When the fracture crosses the line of sight of two compressional wave transducers, the Pwaves are dispersed and attenuated and the fracture width can be calculated by studying the effect this has on the signals, diffractions caused by the fracture tip can be used to locate the fracture tip and calculate its radius, and S-waves reflected by the fracture can be used to calculate the fracture depth. Knowledge of the fracture length and depth at each moment of time allows evaluating fracture properties at in-situ (elevated pressure) conditions. Novel injection and monitoring methods are needed for better understanding of fluid dynamics and mechanical behavior of solids and enhanced understanding of the physical processes taking place during deformation and fracture initiation in the near surface. Development of experimental methods can validate constitutive models and allow for studying the effects of structure, geometry, composition, and defects in rock on the damage propagation across a material system. Understanding and prediction of the physical and mechanical properties and behaviors of rock and its interactions with their surrounding environment is of major interest for Earth and Solid mechanics programs at Army Office of Research. The acquired equipment will be used for research and educational purposes to introduce graduate and undergraduate students to the principles of ultrasonic monitoring and fracture propaga
Document Details
- Document Type
- DoD Grant Award
- Publication Date
- Jul 28, 2023
- Source ID
- W911NF2310275
Entities
People
- Roman Y Makhnenko
Organizations
- Army Contracting Command
- United States Army
- University of Illinois Urbana–Champaign