Numerical Modelling of Cuttings Transport in Horizontal Wells Using Conventional Drilling Fluids

Abstract

Abstract A one-dimensional transient mechanistic model of cuttings transport with conventional (incompressible) drilling fluids in horizontal wells has been developed. The model is solved numerically to predict cuttings bed height as a function of drilling fluid flow rate and rheological characteristics (n, K), drilling rates, wellbore geometry and drill pipe eccentricity. The results of the sensitivity analysis showing the effects of various drilling operational parameters on the efficiency of solids transport are presented. The model developed in this study can be used to develop computer programs for practical design purposes to determine optimum drilling fluid rheology (n, K) and flow rates required for drilling horizontal wells. Introduction Poor hole cleaning can lead to pipe sticking, higher drag and torque, slower rate of penetration, formation fractures and wellbore steering problems(1). It has been estimated that one third of stuck pipe problems are due to inadequate hole cleaning(2). Optimization of drilling hydraulics design for trouble free drilling, therefore, requires a good understanding of the mechanics of cuttings transport. Experimental studies have shown that hole cleaning is affected by many parameters, such as well geometry (diameter, inclination, eccentricity), cuttings characteristics (size, porosity of bed), drilling fluid properties (rheology, density, drag coefficient) and drilling operational parameters (drilling rate, drilling fluid circulation rate)(3–7). Based on the results from experimental studies, empirical models were developed(8–13) and rules of thumb for field practices were also suggested(14–16). Cuttings transport has also been investigated extensively by using analytical models. Two-layer(1,17–22) and three-layer(23,24) mechanistic models of cuttings transport were developed. Most of the existing models do not consider the slip velocity between solids and drilling fluids. In some cases, the authors simply neglected the slip velocity and treated the flow of the liquid-solids mixture as homogeneous flow(20,21,23,24), while others simply assumed the slip velocity is equal to the solids terminal settling velocity(19). Steady-state transport of cuttings is the major assumption inherent in all of the previous cuttings transport models. Therefore, the transient nature of the cuttings transport cannot be studied adequately by using these models. As the drilling goes on, cuttings are continuously deposited along the horizontal wellbore. There is a critical limit for cuttings bed height above which drilling operation exhibits a high risk of operational integrity, such as risk of lost circulation due to high ECD, increased probability of pipe sticking, high drag and torque. The common practice is to stop drilling occasionally and clean the borehole by using viscous pills, pipe rotation and reciprocation, and drilling fluid circulation. It is very critical to be able to predict after how many hours of operation drilling should be stopped and remedial wellbore cleaning actions be resumed. A transient cuttings transport model is, therefore, needed for better design of drilling hydraulics; in particular for drilling long horizontal sections of the wellbore and extended reach (ERD) wells. In this study, a 1D transient, mechanistic model of cuttings transport with conventional (incompressible) drilling fluids in horizontal wells has been developed. The prediction of cuttings bed as a function of various drilling operational parameters is presented. A detailed description of t