Imaging of Fluid Mobility in Fractured Cores Using Time-lapse Positron Emission Tomography

Abstract

Positron emission tomography (PET) continues to have wide-ranging medical application and is based on the detection of gamma radiation emitted from the decay of certain types of radionuclides. Modern PET scanners produce three-dimensional (3D) images of the radiation source, in discrete time steps, using tomography analysis. This paper presents an application of PET for studying fluid mobility in pressurized low-permeability rock samples in the presence of natural fractures. This technique uses a high-resolution PET scanner and image reconstruction based on filtered back-projection. Traditional techniques have been limited to pressure measurement of fracture conductivity and effective permeability, but little is understood about the dynamic flow and velocity profiles within the fracture. The objective of this work was to investigate if it is possible to measure the time-lapse distribution of the fluid flow as a function of the overburden stress. PET imaging was applied to the flow of brine solution, which was tagged with 18F-fluorodeoxyglucose (FDG) positron emitting radionuclide, through nonfractured sandstone and naturally fractured shale cores. A special composite container was manufactured to sustain high-pressure conditions and minimize the absorption of emitted gamma rays. The experimental apparatus is described, and it is demonstrated that the 3D images obtained with a grid resolution of 2 × 2 × 2 mm3 allow clear determination of the fluid flow rate through the core as a function of overburden pressure and time. PET images are direct observations of the radiation source and allow an unambiguous determination of the fluid distribution in the core. The results of this research can be used to validate the numerical modeling of fluid flow through fractured rock matrices, to enable more accurate estimates on the directionality of fractures from the fluid distribution as a function of time, and to obtain more quantitatively sound estimates of fracture connectivity.

Dynamic 3D Imaging of Fluid Mobility in Natural Fractures Using High-resolution Positron Emission Tomography

“…A 3D image can be constructed from a collection of such LORs, measured at many angles, using tomography analysis. Further details on the PET method are presented by Maučec et al (2013aMaučec et al ( , 2013bMaučec et al ( , 2013c and in references therein (e.g., Saha 2010).…”

“…This approach (Maučec 2013b) uses and benefits from a new method and algorithm for the analysis of list mode data from PET experiments, which efficiently achieves the maximum possible spatial resolution (Bickell et al 2012). Traditional PET imaging techniques typically rely on static or quasi-static systems.…”

“…While the confining pressure for the sandstone cores was sufficient at 500 psi (Maučec et al 2013a(Maučec et al , 2013b(Maučec et al , 2013c, for all of the naturally fractured shale core experiments, higher operating pressures and an alternative injection approach were required to ensure reasonable flow through the column. The shale core dimensions were 1-in.…”

“…The experimental setup and the results of the first proof-of-principle measurements of fluid front movement in sandstone and fractured shale cores in time-lapse mode were initially presented by Maučec et al (2013a). However, the results of imaging the fluid mobility were inconclusive in interpreting the low-intensity structures observed within the core; hence, the findings on the imaging of individual core fractures or cracks are elaborated in greater detail in Maučec et al (2013bMaučec et al ( , 2013c. This paper focuses on imaging the naturally fractured shale cores and adds some recent results where PET data were acquired continuously over an extended period of time.…”

Of rats and rocks: using pre-clinical PET imaging facilities in core analysis

“…The number, position and size of PET detector elements, as well as the distance between the object and detectors, strongly influence the spatial resolution of PET imaging [1]. PET is established as a diagnostic tool in medicine, where the radioactively labelled "object" is often a human (in clinical imaging) or animal (in pre-clinical research) but has also recently been demonstrated as a highly useful tool for non-medical purposes: flow visualization in porous rocks [2][3][4][5][6][7][8][9][10][11][12][13][14][15]. For practical purposes the beta plus decay is insensitive to pressure and temperature [16].…”