Modeling of Cuttings Lag Distribution in Directional Drilling to Evaluate Depth Resolution of Mud Logging

Abstract

Abstract Dispersion of cuttings transport velocity limits the depth resolution in mud logging. An attempt to model the cuttings lag depth distribution caused by the dispersion of cuttings transport velocity in directional drilling is presented. The depth resolution of mud logging in directional and horizontal wells is evaluated based on the cuttings lag experiments and lag depth simulations using the developed model. The approach of cuttings lag calculation is based on the previously developed complete physical model of transient cuttings transport behavior in directional drilling. The lag distribution is modeled using the lognormal distribution probability density function. The parameters of the distribution function are determined by lag time measurement experiments for various hole inclination angles using a large-scale cuttings transport flow loop apparatus. The implementation of lag depth calculation can be generally achieved by convolving the lag distribution functions determined for each hole section with different inclination angles. In this study, however, the linear combination of distribution functions is mathematically considered for simplicity. Lag depth simulation studies are presented for a realistic model directional well based on a field data with consideration of variation in rate of penetration. The significant finding is that cuttings sampled at the surface can be contaminated by cuttings originated from other unintended depths to the extent being not negligible compared to the typical sampling interval of 30 ft or 10 m. This tendency of smearing in the formations and depths from which the sampled cuttings are originated would be significant if the high inclination or horizontal hole section exceeds a certain length depending on the rate of penetration. The presented approach has the ability to quantitatively evaluate the uncertainty in the depth resolution of mud logging that is crucial for improving the lateral quality of reservoir characterization which can be beneficial such as in shale oil and gas projects.