Investigation of radial invasion of mud filtrate in porous media

This study explores the rate of drilling mud filtrate invasion. Resistivity probes inserted into 1 inch by 1 foot slabs of Berea Sandstone monitored the advance of the saltwater filtrate leaking outward from a central borehole. The slabs were first saturated with brine and flooded to residual water saturation with oil. Attapulgite muds with varying filtration rates were then circulated past the sandface and both invasion fronts and dynamic filtration rate data were collected. Variations in permeability, wellbore pressure, and in-situ oil viscosity were investigated. Analysis of the data showed that dynamic filtration rate is clearly the most important factor in determining invasion rate. A simple, semiempirical, mathematical model is developed to predict invasion rate given wellbore radius, approximate porosity, and dynamic filtration data. One immediate use for improved filtration data is in determining where to locate "measurement while drilling" (MWD) logging tools on the drillstring so that they can read substantially uninvaded formations, that is, before mud filtrate "has had a chance to invade the zone of interest. Results of this study indicate that with normal wellbore sizes, the logging tool may need to reach the zone of interest within 15-40 minutes of the time the bit reaches the formation.