Depth Filtration of Clay in Rock Cores Observed by One-Dimensional 1H NMR Imaging

A previous study (M. A. Horsfield, E. J. Fordham, C. Hall, and L. D. Hall, J. Magn. Reson. 81, 593 (1989)) used one-dimensional NMR imaging to measure water/clay volume ratios in a colloidal filter cake; proton spin-lattice relaxation time ( T 1) was used as the indicator of moisture content within the cake. The present Note reports the application of that technique to depth filtration of clay within natural sedimentary rocks. A region of the filter medium exposed to pressurized clay suspension is found to exhibit significant depression in T 1 on a timescale of less than 15 min; little subsequent change is observed over 12 h. The depth of penetration was up to 3 cm in an oolitic limestone of exceptionally high permeability (> 10 Darcy), but less than 1 cm for a sandstone with a permeability more typical of natural sedimentary rocks.