NMR LOG'S FOR FORMATION FLUID PROPERTIES
Medical MRI relies on the ability to link specific medical conditions or organs in the body to different NMR behavior. A similar approach can be used with MRIL tools to study fluids in a thin zone a few inches from the borehole wall. MRIL tools can determine the presence and quantities of different fluids (water, oil, and gas), as well as some of the specific properties of the fluids (for example, viscosity). Both Medical-MRI devices and MRIL logging tools can be run with specific pulse-sequence settings, or “activations,” that enhance their ability to detect particular fluid conditions.
NMR AND MICRO POROSITY
Micro-porosity associated with clays and with some other minerals typically contains water that, from an NMR perspective, appears almost like a solid. Water in such micro-pores has a very rapid “relaxation time.” Because of this rapid relaxation, this water is more difficult to see than, for example, producible water associated with larger pores. Earlier generations of NMR logging tools were unable to see water in these micro-pores, and because this water was associated most often with clays, the porosity measured by these earlier tools was often characterized as being an “effective porosity.” Modern MRIL logging tools can see essentially all the fluids in the pore space, and the porosity measurement made by these tools is thus characterized as being a “total-porosity” measurement. Pore-size information supplied by the modern tools is used to calculate an effective porosity that mimics the porosity measured by the older NMR tools.
CALIBRATION OF NMR TO PETROPHYSICAL PROPERTIES IN LABs (AN ADAVANTAGE)
One of the key features of the MRIL design philosophy is that the NMR measurements of the formation made when the MRIL tool is in the wellbore can be duplicated in the laboratory by NMR measurements made on rock cores recovered from the formation. This ability to make repeatable measurements under very different conditions is what makes it possible for researchers to calibrate the NMR measurements to the petrophysical properties of interest (such as pore size) to the end user of MRIL data.