Abstract

Recent developments have led to increased interest in the application of borehole nuclear magnetic resonance (NMR) as a probe of petrophysical properties. Of particular importance in this connection is the measurement of the longitudinal relaxation time, T1. As T1 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 T1 values are affected by particulate invasion is found to depend strongly on the mud characteristics. With thinned spud muds there is a region deep within the core where T1 values are significantly reduced due to an initial spurt of clay particles. In better formulated muds this effect is greatly reduced.