Transportation of cutting particles is known as a mechanism by which vital factors of drilling should effectively be employed [7]. In the solid-liquid two-phase flow during the transport process the drill-mud is utilized as a carrier for the solid-phase of rocks that are drilled by the tool-bit.
A substance is termed non-Newtonian when its flow curve is nonlinear.
Alternatively, its flow curve may be linear, but it does not pass through the origin.
This happens when its viscosity is not constant at a given temperature and pressure and it exhibits non-equal normal stress in a simple shearing flow. The value of the viscosity depends upon the flow conditions, such as flow geometry, shear rate or stress developed within the fluid, time of shearing, kinematic history of the sample.
Under appropriate conditions, some materials can exhibit a blend of solid and fluid- like responses. Though somewhat arbitrarily, it is customary to classify the non- Newtonian fluid behavior into three general categories [13] as follows:
1. Purely viscous, time-independent, or GNF (Generalized Newtonian Fluids), where the applied rate of shear is dependent only on the current value of the shear stress or vice versa.
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2. Time-dependent systems in which the relation between the shear stress and the shear rate depend upon the duration of shearing with respect to the previous kinematic history.
3. Visco-elastic fluids. Those exhibiting combined characteristics of both an elastic solid and a viscous fluid, and showing partial elastic and recoil recovery after deformation.
Drilling mud is non-Newtonian fluid that exhibits Thixotropy behavior, in which it displays a decrease in viscosity over time at a constant shear rate [14]. Most of the drilling fluids are non Newtonian fluids, with viscosity decreasing as shear rate increases [15]. This is similar behavior to the Pseudoplastic or shear thinning fluids.
At both adiabatic and non-adiabatic conditions, a two-phase flow system can be a very complex physical process. This is because such systems combine the characteristics of deformable interface, conduit geometry, flow direction, and, in some cases, the compressibility of one of the phases. In addition to inertia, viscous and pressure forces, the two-phase flow systems are also affected by the interfacial tension forces, as well as the characteristics of the phases, the exchange of mass, momentum, and energy between the phases [16]. The ability of drill-fluids to suspend and transport the drilled solids out of the wellbore is the critical target to gain a successful well drilling operation. For further expansion to the production and refinery stages, proper transport and thereby successful drilling demand an adequate drilling plan. The problem of well-bores cleaning has been recognized as a serious problem in drilling fields as long as wells have been drilled. Therefore it is necessary to identify where the critical spots are with regard to the wellbore cleaning.
Many parameters are found to affect hole cleaning operation. These may generally be categorized into major three groups as follows:
The first group: parameters which are related to the carrier fluid, such as fluid density, fluid viscosity and fluid flow rate.
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The second group: solid cutting parameters include cuttings density, cutting shape and size and cutting concentration.
The third group: operational parameters which may be related to geometric features or other effects. This group contains inclination, pipe rotation and pipe positioning in the hole (concentric / eccentric).
Over the previous three decades, many researchers have attempted to clarify some of ambiguities related to the matter of transport. Various studies were conducted to investigate and hopefully improve the mechanism of drilled cuttings transport. Much difficult and painstaking work was directed at trying to obtain a realistic understanding of the phenomena. It was noticed that most of the previous studies were focused upon only a few parameters while neglecting various others. This approach is a common strategy, frequently used to reduce the level of complexity of the problem.
Notice that, simplifying of such problem should be done via rational assumptions to avoid distortion. An extensive survey was carried out on the available sources, such as research centers, universities, journals and conference proceedings, in addition to some private communications. Research efforts can be classified into three categories:
(a) experimental investigation, (b) mathematical modeling, and (c) computational fluid dynamics or, (CFD) simulations.
Researchers working within the previously- mentioned three groups of parameters to investigate their influence and their interaction through diverse conditions of drilling practices. These efforts facilitate drilling operations and help overcome barriers involved in the directional drilling. Nearly all Former studies were excessively focused upon the transport problems in vertical wellbores. Unfortunately, there still is an absence of some the basic data required to fully evaluate the present field practices in the directional drilling operation.
The collected reviews were subdivided into three subsections. The first part reviews the experimental works. The second part reviews the mathematical and mechanistic modeling, and the third part reviewed the studies on CFD simulation. At
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the end of the chapter, the private communications, conclusions and comments are presented.