Macrodispersion in sand‐shale sequences

Abstract

Macrodispersion in sand‐shale sequences is investigated by a series of numerical tracer tests. Hydraulic conductivity is modeled as a binary, spatially correlated random function. Realizations of the random conductivity field are simulated on a nodal grid discretizing the heterogeneous formation. Corresponding realizations of the random velocity field are obtained by solving the equation for saturated steady state flow. Particle tracking, with flux‐weighted tracer injection and detection, is used to generate experimental residence time distributions (RTDs). Moments of the RTD are used to characterize longitudinal tracer spreading. Results show that macrodispersive transport in sand‐shale sequences cannot be represented by a Fickian model. RTDs display a bimodal structure caused by the fast arrival of particles traveling along preferential sandstone channels and by the much slower arrival of particles following tortuous routes through sandstone and shale. The relative importance of channeling and tortuous flow transport mechanisms is determined by sand‐shale conductivity contrast, shale volume fraction, and conductivity spatial correlation structure. Channeling is promoted by high conductivity contrasts, low shale fractions, and flow parallel to bedding in anisotropic media. Low contrasts, high shale fractions, and flow perpendicular to bedding act to break up channels and to enhance tracer spreading.