Numerical Modelling of Cuttings Transport With Foam in Vertical Wells
Authors: Li Y. , Kuru E. 
Published: March 2010

Abstract
In this study, a 1D unsteady-state two-phase mechanistic model of cuttings transport with foam in vertical wells has been developed. The model is solved numerically to predict the optimum foam flow rate and rheological properties to maximize the cuttings transport efficiency in vertical wells. Comparisons of model predictions with the field test data have shown that model predictions are in close agreement with the field test results.
The numerical solution allows analyzing the effects of borehole geometry, drilling rate, foam rheological properties, gas and liquid flowing rates, and reservoir fluid influx on the cuttings transport efficiency when drilling vertical wells. Results of sensitivity analysis study are presented.

Introduction
The Alberta Energy and Utilities Board defines underbalanced drilling (UBD) as, "when the hydrostatic head of a drilling fluid is intentionally designed to be lower than the pressure of the formation being drilled(1)." The benefits of UBD include increased productivity by reducing formation damage, increased rate of drilling bit penetration, minimization or elimination of lost circulation, improved formation evaluation while drilling, reduction or elimination of differential pipe sticking, reduced stimulation requirements, and earlier production.
Four drilling methods are referred to as UBD:gas drilling- air, nitrogen, and some hydrocarbon gases can be used as a circulating medium;flow drilling-fluid with density below the formation's hydrostatic gradient is used as circulating fluid;gas/ liquid drilling-a gas-liquid two-phase drilling fluid is used in the drilling operation; and,foam drilling-stable foams are used as cuttings transport media.
Because of its variable density and good cuttings transport ability, foam is often used in UBD operations. The use of foam as a drilling fluid in many drilling operations and the results from various field cases have been well documented in the literature(2–5). Field application of the foamed drilling fluids is very often complicated because of the difficulties encountered in controlling fluid hydraulic properties. In general, compared to conventional (incompressible) drilling fluids, much less is known about the hydraulic and rheological properties of foamed drilling fluids, and even less is known about their cuttings transport capabilities. The complex flow mechanism involving in compressible drilling fluid circulation makes the determination of the optimum combination of liquid and gas injection rates very difficult. Other questions also exist, such as how to predict the bottom-hole pressure and how to combine different controllable variables in order to obtain optimum cuttings transport performance and bit hydraulics.
In order to realistically simulate the hydraulics of foam drilling, factors including foam rheological properties, drag coefficient of cuttings in foam, formation fluid influx, drillpipe eccentricity, and drilling rate need to be considered in any modelling study. An example of such a modelling study is presented in this article.

Literature Review
The following section provides a review of literature on cuttings transport in general, the rheology of foam, and of cuttings transport with foam.
Cuttings Transport
Hole cleaning (cuttings transport) is one of the major factors affecting cost, time, and quality of directional, horizontal, extended reach, and multilateral oil/gas wells.