Cuttings-Lag Distribution in Directional Wells Affects Depth Resolution of Mud Logging
Authors: Wilson Adam 
Published: October 2018

This article, written by Special Publications Editor Adam Wilson, contains highlights of paper SPE 189615, “Modeling of Cuttings-Lag Distribution in Directional Drilling To Evaluate Depth Resolution of Mud Logging,” by Shigemi Naganawa, SPE, The University of Tokyo, and Manabu Suzuki, Kenji Ikeda, Norihito Inada, and Ryosuke Sato, Japan Oil, Gas, and Metals National Corporation, prepared for the 2018 IADC/SPE Drilling Technology Conference and Exhibition, Fort Worth, Texas, USA, 6–8 March. The paper has not been peer reviewed.
Dispersion of cuttings-transport velocity limits the depth resolution of mud logging. This paper presents an approach to modeling the cuttings-lag-depth distribution caused by the dispersion of cuttings-transport velocity in directional drilling. The presented approach has the ability to evaluate quantitatively the uncertainty in the depth resolution of mud logging that is crucial for improving the lateral quality of reservoir characterization, which can be beneficial in shale oil and gas projects.
Introduction
Drill-cuttings analysis in mud logging provides actual information in real time about the formation being drilled. However, cuttings moving up the wellbore annulus generally are dispersed because of the dependence of cuttings-transport velocity on particle size (e.g., small particles are easily transported, and large particles may be transported at a small fraction of the drilling-fluid velocity) and the existence of radial distribution in drilling-fluid velocity. This dispersion of cuttings-transport velocity limits the depth resolution of mud logging. Collecting and analyzing small cuttings is the best technique to minimize the dispersion of cuttings-lag time or lag depth. At the same time, the dispersion of cuttings-transport velocity because of the radial distribution of drilling-fluid velocity is independent of particle size and cannot be eliminated from the collected cuttings.
In this attempt to model the cuttings-lag-depth distribution caused by the dispersion of cuttings-transport velocity in directional drilling, the depth resolution of mud logging in directional and horizontal wells is evaluated on the basis of cuttings-lag experiments and cuttings-lag-depth simulations using a developed model. The approach of cuttings-lag calculation is based on a previously developed complete physical model of transient cuttings-transport behavior in directional drilling. The lag distribution is modeled using the log-normal 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.
Average-Cuttings-Lag-Depth Equation
Lag time is defined as the time required for drilling fluid, gas, or cuttings traveling from the bottomhole to the surface through the wellbore annulus. Lag depth is defined as the formation depth in which the cuttings sampled or collected at the surface originally existed. The drilling depth increases depending on the rate of penetration when the cuttings drilled at the lag depth are transported to the surface. The complete paper provides equations representing the relationship between current bit depth and lag depth and the lag-depth equation for consideration of cuttings-transport-velocity variation.