Medical MRI and MRI Logging:
Magnetic resonance imaging (MRI) is one of the most valuable clinical diagnostic tools in health care today. With a patient placed in the whole-body compartment of an MRI system, magnetic resonance signals from hydrogen nuclei at specific locations in the body can be detected and used to construct an image of the interior structure of the body. These images may reveal physical abnormalities and thereby aid in the diagnosis of injury and disease. Magnetic Resonance Imaging Logging (MRIL®), introduced by NUMAR in 1991,1 takes the medical MRI or laboratory NMR equipment and turns it inside-out. So, rather than placing the subject at the center of the instrument, the instrument itself is placed, in a wellbore, at the center of the formation to be analyzed.
Comparison of the MRIL Tool to Other Logging Tools
Because only fluids are visible to MRI, the porosity measured by an MRIL tool contains no contribution from the matrix materials and does not need to be calibrated to formation lithology. This response characteristic makes an MRIL tool fundamentally different from conventional logging tools. The conventional neutron, bulk-density, and acoustic-travel-time porosity-logging tools are influenced by all components of a reservoir rock. Because reservoir rocks typically have more rock framework than fluid filled space, these conventional tools tend to be much more sensitive to the matrix materials than to the pore fluids. The conventional resistivity-logging tools, while being extremely sensitive to the fluid-filled space and traditionally being used to estimate the amount of water present in reservoir rocks, cannot be regarded as true fluid-logging devices. These tools are strongly influenced by the presence of conductive minerals and, for the responses of these tools to be properly interpreted, a detailed knowledge of the properties of both the formation and the water in the pore space is required. MRIL tools can provide three types of information, each of which make these tools unique among logging devices:
• information about the quantities of the fluids in the rock
• information about the properties of these fluids
• information about the sizes of the pores that contain these fluids
MRIL/NMR TOOL PRINCIPLE:
At the center of an MRIL tool, a permanent magnet produces a magnetic field that magnetizes formation materials. An antenna surrounding this magnet transmits into the formation precisely timed bursts of radio-frequency energy in the form of an oscillating magnetic field. Between these pulses, the antenna is used to listen for the decaying “echo” signal from those hydrogen protons that are in resonance with the field from the permanent magnet.
Because a linear relationship exists between the proton resonance frequency and the strength of the permanent magnetic field, the frequency of the transmitted and received energy can be tuned to investigate cylindrical regions at different diameters around an MRIL tool. This tuning of an MRI probe to be sensitive to a specific frequency allows MRI instruments to image narrow slices of either a hospital patient or a rock formation. Fig. 1.2 illustrates the “cylinders of investigation” for the MRIL-Prime tool, which was introduced in 1998. The diameter and thickness of each thin cylindrical region are selected by simply specifying the central frequency and bandwidth to which the MRIL transmitter and receiver are tuned. The diameter of the cylinder is temperature-dependent, but typically is approximately 14 to 16 in.