Factors Affecting Hole Cleaning and Cuttings Transport

Cuttings movement is defined by the dominated force which is the resultant of many forces such as gravity, buoyancy, friction, drag and inter-particle contacts. 

Many factors can dictate and define the cuttings flow in the annular space such as: 

- Cuttings slip velocity 
- Annular mud velocity 
- Rotary speed of the drill string 
- Rate of penetration ( drilling rate) 
- Drillstring Eccentricity 
- Rheology of the drilling fluid 
- Cutting Characteristics 

Fig 1 - Forces Acting on Cutting

Cuttings Slip Velocity

The drilled cuttings inside the fluid medium have the tendency to fall down at a certain velocity which is defined as the slip velocity. In order to lift cuttings up to the surface, the drilling fluids velocity in the annular space (Va) has to be greater than the slip velocity of cuttings (Vs). The transport velocity of cuttings is defined as: 

Vt = Va-Vs 

The transport ratio is defined as 

Rt= 1-Vs/Va 

While drilling a vertical well, it is recommended to get Rt at 0.5-0.55 for minimum transport performance. In a vertical annular space, the slip velocity has just one axial component Vs=Vsa, Whereas in deviated wells, the slip velocity has two components Vsa and Vsr which are defined as: 

Vsa = Vs x CosQ 

Vsr = Vs x SinQ 

Where Vsa and Vsr are the axial and radial components respectively. 

The axial component of the slip velocity decreases with the increase of the inclination of the deviated sections and vanishes in horizontal sections meanwhile the radial component reaches the maximum level. 

Fig 2 - Slip Velocity Components

Annular Fluid Velocity

The annular fluid velocity has to be at a sufficient level to transport cuttings up to the surface. The importance of this velocity increases when the radial component is predominant. The cuttings are pushed towards the well lower wall when the radial component increases leading to the cuttings bed formation. 

Increasing flow rate and consequently the annular fluid velocity improves the hole cleaning overall performance, however this factor is limited by: 

- The ECD which has to be at a safe interval in order to avoid inducing down hole losses
- The hydraulic erosion of the open hole 
- The ability of the rig to deliver the necessary hydraulic power 

Drilling Rate

Quantitatively, the drilling rate defines the amount of the cuttings generated during operations. The cuttings concentration inside drilling fluid becomes more important when rate of penetration increases which leads to more challenged hole cleaning and hydraulic requirement. 

Rotary Speed

The impact of this factor depends on the hole size and pipe diameter. The effect of drillstring rotary speed on hole cleaning when drilling vertical section is very limited but it becomes more important with hole inclination increase. The rotating of the drillstring tends to redistribute the flow in the annular space and it generates the mechanical agitation of the cuttings beds leading to better hole cleaning performance. 

Drillstring Eccentricity

The drillstring has the tendency to rest on the lower part of the wellbore due to the gravity force. This behavior of the drillstring creates narrower annular space below the string compared to the upper side above the drillstring. The velocity profile is disturbed by this behavior making the velocity at low level and making transporting cuttings very insufficient. 

Fig 3 - DrillString Eccentricity

Drilling Fluids Properties

The drilling fluid is the medium where the cuttings are transported in to the surface. The mud properties have various impacts on cuttings transport overall performance. 

Increasing the yield point (YP) and plastic viscosity (PV) can improve the hole cleaning by contributing in improving the carrying capacity of the drilled cuttings. However, it is experienced that increasing the viscosity can shift the flow regime from turbulent regime to laminar regime. 

The mud density or drilling fluids weight increase has a positive impact on cuttings transport by contributing in improving the buoyancy force and lowering the slip velocity. In other hand, this factor is limited due to its impact on ROP, ECD and many other factors. 

Cutting Characteristics

The cuttings properties such as size, shape and density contribute in the dynamic behavior of the cuttings in the drilling fluids. These factors cannot be controlled due to many reasons. The generation of the cuttings depends on the type of drilling bits and they change shape and size because of the grinding effect performed by the drillstring. Smaller and grinded cuttings are difficult to transport especially in deviated section. This problem can be improved by increasing viscosity and applying pipe rotation to suspend cuttings in the drilling fluids and make them easier to transport.

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