Introduction to Hole Cleaning

Hole cleaning can be a real problem when drilling deviated wells. It has been proved that for wellbores with an inclination angle greater than 30°, the removal of cuttings beds can be a difficult issue. Poor hole cleaning can lead to many drilling problems: 
- Mechanical pipe sticking
- Slow rate of penetration (ROP)
- Excessive torque and drag
- Difficulties to run casing to the planned landing depth
- Difficulties while cementing the well
- Formation fracturing 

Fig 01- Formation of cuttings beds while drilling

I- Hole Cleaning Affecting Factors 

Annular drilling fluid velocity

The flow rate is the dominant factor for an effective cutting removal. Increasing the flow rate leads to better cutting removal but there are other issues which have to be considered when planning to increase the flow rate. Factors which can limit the flow rate increase are: 
- The allowed ECD
- Rig hydraulic power availability
- The resistance of the open hole to the hydraulic erosion

Hole inclination angle

The hole cleaning is difficult when drilling deviated wells. It has been mentioned that the maximum flow rate requirements for cleaning are reached at the angle between 65° to 67°. Sudden shutdown of the mud pumps when drilling through deviated wells of an inclination angle around 25° to 45° can cause cutting sloughing to the bottom and may lead to mechanical pipe sticking.

Drill string rotation

The rotation of the drill string contribute effectively in hole cleaning. The cutting beds on the low side of the hole is removed by the contribution of the mechanical agitation and the exposure to higher drilling fluids velocities when the string moves to the high side of the hole. Hole cleaning can be better when combining the whirling motion of the string and other factors like: mud rheology, flow rate and cutting size. There are some limitations for the whirling motion. When using the down hole motor, during the sliding mode, the string is in stationary which affect the cutting removal. Pipe failures caused by the cyclic stresses due to pipe rotation. Casing wear and mechanical destruction of the open hole are other problems which can be caused by the pipe rotation. 

Pipe eccentricity

The string has the tendency to lay on the low side of the inclined section. Very narrow gap is created below the pipe in the annulus section which leads to extremely low drilling fluid velocity leading to difficulties to transport cuttings to the surface. 

Rate of penetration (ROP)

The amount of drilled cuttings is related to the rate of penetration. Good hole cleaning is ensured by combining and adjusting the rate of penetration, the flow rate and pipe rotation. If these parameters are limited due to any issue, rate of penetration can be reduced to allow better hole cleaning. Reducing the ROP can increase the cost of drilling operations, but avoiding drilling problems such as mechanical pipe sticking make it a good choice to optimize drilling costs. 

Mud Properties

There are two main mud properties which are related directly to the hole cleaning: the viscosity and the density. The density of drilling fluid is adjusted to stabilize the borehole and prevent formation fluids to flow into the well bore. The viscosity is adjusted to suspend the weighting materials and the drilled cuttings.

Cutting Characteristics

The cutting size and shape are related to the bit used during operations. The size, shape and specific gravity of cuttings affect the dynamic behavior when flowing into the drilling fluids. Grinding and breakage by drill string rotating make it impossible to control cuttings shape and size. In directional well drilling, cuttings are more difficult to transport, but with adequate viscosity and pipe rotation, small particles stay in suspension which make them easier to transport. 

II- Hole cleaning sweeps

In vertical wellbores, the high viscosity sweeps provide effective hole cleaning; this type of sweep might not be a good option for deviated wells. The eccentric drill pipe creates narrow annular space where the cutting beds remain due to little or no flow at that region. In deviated wells, cutting beds can be removed by applying a combination of low viscosity and weighted pills. Turbulent flow can be produced by using thin drilling fluids. An effective sweep program can be applied as follows:
- The sweep is pump at regular intervals under normal circulating flow rate
- Rotating the drill string with more than 60 rpm when the sweep is into the annulus
- Use continuous circulation till getting the sweep at the surface.

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