Paramagnetic nanoparticles for direct pore size determination from NMR

Core sampling and analysis are currently required to characterize pore sizes in drilling operations, however, this is both expensive and time consuming. NMR techniques coupled with nanoparticles have the potential to reduce these costs.

Characterization of pore size distributions from NMR data relies on measurement of the longitudinal or transverse relaxation time (T1 or T2, respectively), which is a function of the ratio of pore volume to pore surface area. The exact relationship between T1 or T2 and volume-to-surface-area ratio is a function of concentration of paramagnetic ions on the pore walls (Kleinberg, 1999). Since parmagnetic ion concentration is usually not known, determining volume-to-surface-area ratio from NMR data requires calibration with porosimetry measurements, typically either from mercury injection capillary pressure or nitrogen gas adsorption.

Paramagnetic iron-cored nanoparticles may be coated with different chemicals to promote or inhibit their adhering to different surfaces. It is therefore possible to engineer nanoparticles that will adhere to mineral grain surfaces on pore walls. Once coated with a known concentration paramagnetic nanoparticles, the pore surfaces will exhibit a predictable T1 or T2 response, which will allow for direct determination of volume-to-surface-area ratio from NMR measurements. This will ultimately lead to development of a process by which pore size and surface area may be determined directly from downhole NMR measurements without the need for core sampling and analysis.

(a) Naturally occurring paramagnetic ions like Fe3+ or Mn2+ create small magnetic fields of unknown spatial distribution inside pores during NMR measurements. (b) A layer of iron-cored nanoparticles creates a field of known strength and spatial distribution within a pore during an NMR measurement.

PIs: Hugh Daigle and Steve Bryant
PhD student: Chunxiao Zhu