Improved Formation Evaluaton Using Azimuthal Porosity Data While Drilling

Abstract

Abstract Measurement of formation porosity and lithology in azimuthal quadrants around the borehole is now available. This new information is provided by a nuclear tool that makes azimuthal density, photoelectric factor and neutron porosity measurements while drilling. In addition, an ultrasonic sensor provides a tool standoff measurement in each quadrant. While rotating, the quadrants are defined by a magnetometer and oriented with the gravity vector, so that bottom, left, right and top quadrants are identified. The tool can be run with several different stabilizer sizes or without a stabilizer, giving the driller more latitude in configuring the bottomhole assembly. The azimuthal capability allows the measurement of porosity with an unstabilized or "slick" tool without degradation of the measurement. This is accomplished by computing the porosity from the bottom quadrant where there is little tool standoff in deviated or horizontal wells. Utilizing bottom quadrant porosities also results in improved measurement accuracy in cases where borehole conditions are poor due to enlargement or washouts. When the tool is stabilized, the quadrant porosity and lithology measurements result in improved geosteering as well as providing a quantitative measure of formation heterogeneity. At bed boundaries, comparison of top and bottom logs in real time results in better bed boundary detection and confirmation of the tool location within the pay zone, When the tool is between boundaries, an heterogeneity indicator can be computed from the quadrant density, lithology and neutron porosity logs to better evaluate complex formations. The measurement of tool standoff per quadrant provides information on borehole size, shape and rugosity. The data can be used to indicate borehole stability on subsequent bit trips. The paper describes the method of the measurement, hardware implementation and log examples illustrating the tool features.