A Computer Simulation Model of a Horizontal Wellbore Cleaning Using a Transverse Axial-travelling Dynamic-pressure Jets System
Authors: Mutua D.K. , Rockwell M.C. , Changirwa R.M. 
Published: March 2010

Abstract
The upsurge in horizontal wellbore drilling technology in the present decade underscores its importance in radically increasing the productivity of hydrocarbons. Secure horizontal wellbore drilling cannot be divorced from efficient wellbore cleaning. However, conventional horizontal wellbore cleaning, which involves a reduction of the cuttings concentration in the wellbore annulus by the circulation of drilling fluid using controlled longitudinal pressure drop, suffers from the lack of independent and controlled transverse dynamic pressure drop. For a broader approach to remedy this impediment, this paper presents a computer simulation model for enhancing horizontal wellbore cleaning by inducing controlled dynamic suspension of cuttings in the wellbore annulus. The process involves a transverse axialtravelling dynamic-pressure jets system (TAD-Jets system) in collaboration with conventional axial laminar flow in multiple variant conduits, orbitally-cascaded and embedded in the drillstring annulus. The simulated results were found to be very promising.

Introduction
The process of cleaning a horizontal wellbore is carried out under varied conditions of uncertainty. Conventional wellbore cleaning involves a number of input variables, whose estimated probability distribution and interrelationships have not yet been fully derived mathematically or otherwise. The first attempt to establish a single multivariable mathematical relationship among major input variables involved in horizontal wellbore cleaning, using a transverse axial-travelling dynamic-pressure jets system, was described in the paper by Mutua and Changirwa(1). The present study is a computer simulation model of their previous work. It is geared to evaluating and establishing stochastically the optimum operational conditions of TAD-Jets system.

Formulation of Simulation Model
The development of the simulation model herein is based on the best available knowledge of the actual horizontal wellbore cleaning and TAD-Jets system. Figure 1 shows that the TAD-Jets system is an integral part of the drilling string. It's function is to dynamically suspend the solids in the annulus.
To clarify the mechanism of the TAD-Jets system function, a section of the wellbore annulus is illustrated by Figure 3. In the system approach of problem solving employed in this paper, a conscious attempt has been made to understand the relationship between various input parameters of the system. The objectives and pertinent constraint: and restraints which govern the solution of horizontal wellbore cleaning have been identified and classified into major processes and forces (Table 1). The model formulation consists of a set of equations describing (1) two-phase heterogenous axial flow in the horizontal wellbore eccentric annuli, (2) two-phase heterogenous transverse flow in the horizontal wellbore eccentric annuli, (3) material balance of cuttings in the eccentric annuli.
Assumptions: Cuttings
The cuttings (drill cuttings in Figures 1 and 2) compact bed forms in wellbore sections where the drilling fluid upward radial flow velocity Vr is less than the cuttings slip velocity Vs (Figure 1). The cuttings-drilling fluid mixture is not sticky and does not block the transverse jet nozzles. The cuttings do not react with the drilling fluid.
Transverse Jets Flow
Transverse jets flow is turbulent and localised in a given well- bore section and does not affect the neighbouring sections substantially.