EFFECT OF MAGNETIC MINERALS ON NMR RELAXATION IN SHALE

Interpretation of nuclear magnetic resonance (NMR) logs, which are used to assess fluid saturation and mobility, from microporous shale reservoirs is a challenge for unconventional oil and gas exploration because it was developed for the characterization of macroporous sandstone and carbonate reservoirs and is not always accurate in shale. The partitioning of transverse relaxation time (T₂) distribution into irreducible and mobile fluids using a T₂-cutoff value is standard practice. However, in shale this approach does not always yield accurate results due to high clay contents and magnetic minerals that can strongly influence the NMR response. To effectively characterize the NMR response of shale, the effects of clay and magnetic minerals common in shale need to be considered.

To begin addressing this issue, NMR experiments were designed to explore the effect of a range of clay minerals (illite, glauconite, celadonite, chamosite, montmorillonite, kaolinite), as well as pyrite, on the NMR response. These experiments were performed using pure minerals, as well as mixtures of these minerals saturated with brine. Slurries of brine and sediment were compacted in a centrifuge at pressures ranging from 40 to 600 psi. A standard CPMG pulse sequence was performed on all samples at ten echo spacings ranging from 0.15 to 1.50 ms. Results of NMR analysis confirm that the concentrations of magnetic or paramagnetic components of shale contribute to the transverse relaxation signature of the slurries investigated, and future research will focus on how these effects impact the determination of fluid saturation and mobility in shale reservoirs.