A Three-Layer Modeling for Cuttings Transport with Coiled Tubing Horizontal Drilling

Abstract

Abstract A new approach of three-layer flow of two-phase (solid-liquid) fluid in annulus, i.e. a stationary bed of drilled cuttings at bottom; a moving bed layer above it; and a heterogeneous suspension layer at the top, is proposed to predict and interpret the cuttings transport mechanism. This new mathematical model is presented to overcome the limitation in the existing two-layer models. This enables efficient prediction of cuttings transport during horizontal drilling using coiled tubing. In particular, the model described in this paper formulates transport process and includes the relevant parameters such as rheological characteristics of the drilling fluid, cuttings size/sphericity/ concentration, wellbore geometry, eccentricity of the coiled tubing, and pumping rate of the drilling fluid. This paper presents the model development, solution, and simulation of the cutting transport process to illustrate the steps involved in determining the effect of each parameter during the drilling operation. Graphical charts and developed correlations between annular flow rate, rheology, wellbore geometry, eccentricity, cuttings size and concentration in each layer, and the carrying capacity are also presented. The simulation results show how to obtain a reasonable pumping velocity of drilling fluid with the possible lowest pressure gradient that might serve as an operational guideline during drilling. Furthermore, the effects of parameters affecting the efficiency of cutting transport are discussed. These results are compared with published experimental data. The observed agreement and discrepancies are discussed, and further improvements on the current model are proposed. Introduction Drilling with coiled tubing is one of the most brilliant features of coiled tubing applications in view of economic benefits. While the technology of coiled tubing horizontal drilling has advanced significantly over the past decade, hole cleaning problems are still troublesome matter in horizontal well and extended reach drilling. This paper focuses on cuttings transport during coiled tubing horizontal drilling. Cuttings transport is more difficult during coiled tubing drilling than conventional horizontal drilling because there is no effect of drill string rotation. Cuttings transport has been a major concern for years in the drilling industry. An investigation by Amoco1 showed that 70% of lost time due to unscheduled events were associated with stuck pipe. A case study by Hopkins2 showed that a third of all stuck pipe problems are due to insufficient wellbore cleaning. Improving the prediction of hydraulic efficiency of drilled cuttings transport in horizontal drilling is very important in order to significantly alleviate costly problems and maximize cost savings. There are numerous mathematical (analytical and numerical) and empirical models for the prediction of hydraulics in cuttings transport mechanism. Common problems with most of the existing cuttings transport models include inaccurate predictions when compared with experimental results, questionable predictions when modeling a wide range of conditions, and discrepancy with other models.3 There seems to be two main reasons for these problems. First, the ambitious attempts in developing comprehensive models, which can handle wide range of conditions (from vertical to horizontal). Azar and Sanchez3 pointed out that researchers have used the same methodology for different physical phenomena that occur under different conditions. The other problem is to apply the improper concepts, simplify too many assumptions or neglect certain observed phenomena. Therefore, a new mathematical model using three-layer flow of solid-liquid mixture concept is proposed to predict and interpret the cuttings transport mechanism. The model is developed based on analyzing forces involved in cuttings transport with the use of applicable basic mathematical principles. The model described in this paper formulates transport process and the effects of parameters upon it. These parameters involve rheological characteristics of the drilling fluid, cuttings size/sphericity/concentration, wellbore geometry, eccentricity of the coiled tubing, and pumping rate of drilling fluid.