Hydraulic Fracture of Opalinus Shale Under Uniaxial Stress: Experiment Design and Preliminary Results
- O. A. AlDajani (Massachusetts Institute of Technology) | J. T. Germaine (Tufts University) | H. H. Einstein (Massachusetts Institute of Technology)
- Document ID
- American Rock Mechanics Association
- 52nd U.S. Rock Mechanics/Geomechanics Symposium, 17-20 June, Seattle, Washington
- Publication Date
- Document Type
- Conference Paper
- 2018. American Rock Mechanics Association
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- 22 since 2007
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ABSTRACT: The interaction of hydraulically induced fractures with pre-existing fractures in shales is of interest to the petroleum industry. Laboratory experiments can help to understand the fracture initiation, propagation, and coalescence behavior. A fundamental first step is to investigate the fracture interaction of a pressurized flaw (artificial crack) with a non-pressurized flaw where the specimens are subjected to a constant uniaxial stress.
This paper describes the hydraulic fracture experiment design, which allows us to pressurize an individual flaw, monitor the internal flaw pressure throughout pressurization and fracturing, and visually capture the underlying fracture mechanisms. The experiments are performed on prismatic Opalinus Shale specimens with two pre-existing flaws of various geometries. After subjecting the specimens to a constant uniaxial stress, one of the flaws is pressurized until a hydraulic fracture initiates and propagates. The interaction of the hydraulic fracture with the non-pressurized flaw is observed.
Three flaw geometries are investigated in this study: a single vertical flaw, double flaws with a step offset of 30°, and double flaws with a step offset of 60°. The first geometry was tested as a proof of concept for the experimental setup and showed the basic fracture initiation and propagation behavior. The second and third geometries capture the interaction of the hydraulic fracture with the non-pressurized flaw. Although there is only a 30° difference in the stepped angles between the two flaws, the fracture behavior is drastically different.
The hydraulic fracture stimulation technique, used to enhance production from conventional reservoirs and extract trapped hydrocarbons from unconventional resources, has been developed and used for decades. However, the exact geometry of the produced hydraulic fractures remains not well known. The aim of this study is to visually capture and analyze the initiation, propagation, and interaction of hydraulic fractures with pre-existing fractures and bedding planes, and to better understand the underlying mechanisms involved.
Extensive work has been done at MIT to study fracture initiation, -propagation, and coalescence (Reyes, 1991, Bobet, 1997, Wong, 2008, Miller, 2008, Gonçalves da Silva, 2009, Morgan, 2015, Gonçalves da Silva, 2016, AlDajani, 2017). Most of these studies were done on prismatic specimens with two pre-existing artificial fractures (flaws) without the influence of hydraulic pressure (Figure 1 – a). Specimens were subjected to uniaxial compressive loading, and fracture initiation and propagation mechanisms (tensile and shear) were captured using a high-speed camera and a high-resolution camera while simultaneously measuring the stress-strain behavior. The experiments were conducted on different materials: gypsum (artificial material), marble (metamorphic rock), granite (igneous rock), and shale (sedimentary rock).
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