If this situation occurs no increase in ROP results from an increase in WOB unless the hydraulic horsepower (HHP) generated by the fluid flowing through the bit is improved . The HHP at the bit is given by:
To increase HHP therefore requires an increase in Pb (smaller nozzles) or Q (faster pump speed or larger liners). This may mean a radical change to other drilling factors (e.g. annular velocity) which may not be beneficial.
B. Type of formation
WOB is often limited in soft formations, where excessive weight will only bury the teeth into the rock and cause increased torque, with no increase in ROP.
C. Hole deviation
In some areas, WOB will produce bending in the drillstring, leading to a crooked hole. The drillstring should be properly stabilised to prevent this happening.
D. Bearing life
The greater the load on the bearings the shorter their operational life. Optimizing ROP will depend on a compromise between WOB and bearing wear.
E. Tooth life
In hard formations, with high compressive strength, excessive WOB will cause the teeth to break. This will become evident when the bit is retrieved. Broken teeth is, for example, a clear sign that a bit with shorter, more closely packed teeth or inserts is required.
Rotary Speed (RPM)
The ROP will also be affected by the rotary speed of the bit and an optimum speed must be determined.
The RPM influences the ROP because the teeth must have time to penetrate and sweep the cuttings into the hole. Figure below shows how ROP varies with RPM for different formations.
The non-linearity in hard formations is due to the time required to break down rocks of higher compressive strength. Experience plays a large part in selecting the correct rotary speed in any given situation.
The RPM applied to a bit will be a function of :
A. Type of bit
In general lower RPMs are used for insert bits than for milled tooth bits. This is to allow the inserts more time to penetrate the formation. The insert crushes a wedge of rock and then forms a crack which loosens the fragment of rock.
B. Type of formation
Harder formations are less easily penetrated and so require low RPM. A high RPM may cause damage to the bit or the drill string.
In order to prevent an influx of formation fluids into the wellbore the hydrostatic mud pressure must be slightly greater than the formation (pore) pressure. This overbalance, or positive pressure differential, forces the liquid portion of the mud (filtrate) into the formation, leaving the solids to form a filter cake on the wall of the borehole. In porous formations this filter cake prevents any further entry of mud into the formation. This overbalance and filter cake also exists at the bottom of the hole where it affects the removal of cuttings.
When a tooth penetrates the surface of the rock the compressive strength of the rock is exceeded and cracks develop, which loosen small fragments or chips from the formation. Between successive teeth the filter cake covers up the cracks and prevents mud pressure being exerted below the chip.
The differential pressure on the chip tends to keep the chip against the formation. This is known as the static chip hold down effect, and leads to lower penetration rates. The amount of plastering which occurs depends on mud properties. To reduce the hold down effect:
- Reduce the positive differential pressure by lowering the mud weight (i.e. reduce the overbalance to the minimum acceptable level to prevent a kick).
- Reduce the solids content of the mud (both clay and drilled solids). Solids removal is essential to increase drilling efficiency.
In less porous formations the effect is not so significant since the filter cake is much thinner. However dynamic chip hold down may occur. This occurs because, when cracks form around the chip mud enters the cracks to equalise the pressure. In doing so, however, a pressure drop is created which tends to fix the chip against the bottom of the hole.
The longer the tooth penetration, the greater the hold down pressure. Both static and dynamic hold down effects cause bit balling
and bottom hole balling. This can be prevented by ensuring correct mud properties (e.g. mud weight and solids content).