Abstract

Hole cleaning is still among the most important problems to handle in drilling operations and one of the most studied phenomena within the petroleum industry.Insufficient hole cleansing can lead to stuck-pipe, excessive pump pressures and equivalent circulating density (ECD), pack-offs, lost circulation, etc. all of which again can lead to loss ofsections or complete wells. Due to the number of parameters influencing the hole cleaning operation and the complex mechanisms involved, the phenomenon has not yet been fully understood.

The issue of transportation of cuttings along with drilling fluids is essentially a two-phase flow.Several other industries are dealing with similar issues when transporting solids using air (fluid) as the transporting medium.Although the transporting mediums are different, flow regimes and basic transporting mechanisms similar to hole cleaning can be observed also in pneumatic conveying of particulate materials.While horizontal and vertical pneumatic transport are quite often used, very rarely people try to use inclined pneumatic conveying for the very reason that it involves much higher pressure drop as compared to both horizontal & vertical conveying and is associated with the problems of particles dropping out of suspension unless a very high fluid velocity is used.Through an extensive laboratory study a simple model[1,2,3] has been developed for pressure drop calculation in pneumatic conveying based on classical Darcy's equation with some modifications.The predicted pressure values match well with the test data.

This article has tried to look at the possibility of a unified approach for two-phase flow. An attempt has been made to relate the knowledge developed in the field of two-phase pneumatic conveying systems to the challenges associated with the hole cleaning mechanism.The paper also describes some of the work that has been performed for studying hole cleaning and the associated pressure loss prediction.

Introduction

Pneumatic conveying is a two-phase material transportation process in which particulate materials are moved over horizontal and vertical distances within a piping system with the help of compressed air stream as the carrying medium. Using either positive or negative pressure of air or other gases, the material to be transported is forced through pipes and finally separated from the carrier air and deposited at the desired destination.It has wide range of applications, with examples ranging from domestic vacuum cleaners to the transport of some powder materials over several kilometers.Virtually all powders and granular materials can be transported using this method.Klinzing et.al[4] lists more than 270 different products which have been successfully conveyed pneumatically.It has been reported [5] that some plants have transport distance of more than 40 km, material flow rate of few hundreds tons per hour and solid loading ratio (the mass flow rate ratio of solid to air) of more than 500.

In the oilfield industry, the hole cleaning operation can also be considered to be a two-phase flow, where the solid phase (drilled cuttings) is removed from the wellbore using a drilling fluid.The solids can be transported out of holes being several kilometers long having various cross sectional areas depending on hole, drill pipe and casing sizes.The amount of solids to be transported out of the hole can be up to 30 tons per hour.The type of material to be transported out of hole can also vary quite a bit both in terms of shape and consistency, which also affects the transport rates significantly.Although air drilling exists for some top hole land drilling operations, most drilling operations are conducted using a drilling fluid.Unlike in air drilling or powder transport, the transporting fluid is designed to fulfill several tasks in addition to cleaning the hole, including maintaining well control, hole stability and transmission of information to and from the downhole tools.Therefore, engineering a drilling fluid can vary from designing a fluid with an almost Newtonian behaviour to highly shear thinning fluids like the oil based drilling fluids.Furthermore, the fluids can have a significant variation in degree of viscoelasticity.All these rheological factors along with drilling fluid wetted cuttings surface properties will in turn affect the cuttings transporting capabilities for the fluid.As the cuttings are transported up annulus between the drill string and the wellbore wall, the total pressure loss in this enclosed circulating loop can be very high.

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