Performing experiments in the laboratory that mimic conditions in the field is challenging. In an attempt to understand hydraulic fracture in the field, and provide laboratory flow results for model verification, an effort to duplicate the typical fracture pattern for long horizontal wells has been made. The typical “disks on a string” fracture formation is caused by properly orienting the long horizontal well such that it is parallel to the minimum principal stress direction, then fracturing the rock. In order to replicate this feature in the laboratory with a traditional cylindrical specimen the test must be performed under extensile stress conditions and the specimen must have been cored parallel to bedding in order to avoid failure along a bedding plane, and replicate bedding orientation in the field. Testing has shown that it is possible to form failure features of this type in the laboratory. A novel method for jacketing is employed to allow fluid to flow out of the fracture and leave the specimen without risking the integrity of the jacket; this allows proppant to be injected into the fracture, simulating loss of fracturing fluids to the formation, and allowing a solid proppant pack to be developed.
The decrease in the price of oil increases the need for more efficient extraction of oil and gas from source rocks exploited by hydraulic fracturing. With the decline in oil and gas prices comes the need for more efficient and effective extraction from these reservoirs to maintain economic viability. This means that research into efficient extraction of oil and gas is more important than ever.
To this end, Sandia has endeavored to replicate aspects of in situ hydrofracturing in the lab, and prop said fractures. Proppant location data is coupled with the injection parameters and fluid rheology. This data is then used to inform flow simulations, which will then be used to understand relationships between injection parameters, proppant placement and fracture conductivity; thereby more effectively predicting proppant location in a fracture. This paper focuses on the methodology for generating and propping properly oriented fractures which are clearly visible using micro computed tomography (µCT). The modeling efforts will be presented separately in a future publication.
Generation of hydraulic fractures is not a new concept; the mechanics of hydraulic fracturing were described by Hubbert and Willis in 1957. Since then hydraulic fracturing has been used in determining in-situ stress conditions from borehole fracturing (ex. Kehle 1964, Haimson and Fairhurst 1969). Laboratory scale hydraulic fracture experiments have been taking place since the 1960s or earlier (ex. Pegler 1967, Zoback et al. 1977, Warpinski et al. 1982). These early studies focused on measuring the minimum principal stress magnitude and direction in the earth using hydraulic fracturing (ex. Kehle 1964, Haimson and Fairhurst 1969). Subsequently the interest shifted to transmission of fluids in the subsurface, and the interaction of hydraulic and natural fractures (ex. Zoback et al. 1977). Since then the focus has been on interaction with natural fractures, generation of fluid flow paths, and recently propping of hydraulic fractures (Wen et al. 2006, Fredd et al. 2000, Alramahi and Sundberg 2012). However, recent work has moved away from laboratory testing under in situ conditions towards validation of numerical simulations, which have so far focused on simplified geometry like flow between parallel plates (e.g. Morris and Chugunov, 2014, Lane and Thompson, 2010). This is because it has been shown that fractures can be generated, and proppant can be pumped into the fracture, but fracture geometry is usually simplified.