CHEMICAL-SHIFT IMAGING OF PARTICLE FILTRATION IN SANDSTONE CORES

Recent developments have led to increased interest in the application of borehole nuclear magnetic resonance (NMR) as a probe of petrophysic al properties. Of particular importance in this connection is the meas urement of the longitudinal relaxation time, T-1. As T-1 is controlled by the pore surface area, its value may be strongly influenced by the invasion of submicron-sized clay particles found in drilling muds. We have studied this effect by the application of phase encode magnetic resonance imaging (MRI) techniques. The extent to which T-1 values are affected by particulate invasion is found to depend strongly on the m ud characteristics. With thinned spud muds there if a region deep with in the core where T-1 values are significantly reduced due to an initi al spurt of clay particles. In better formulated muds this effect is g reatly reduced.