It has been established that weight on bit increase the penetration rate. However, excessive weight will damage the bit causing either bit cramming, bit braiding, and bit balling. The weight on bit depends on the  drill collar.The weight on the drill collar depends on the outside diameter and bore diameter. The outside diameter is as close to the hole diameter as possible and the inside diameter is large enough to permit circulating with pressure drop. Drill collars weighs less in mud than in air because of the buoyancy effect of the mud. The heavier the mud the greater is the buoyancy effect of the mud. The heavier the mud the greater is the buoyancy effect and the lighter the apparent weight of the colars. Extra collars are usually employed to offset buoyancy effect. The usual practice is to use 10%-30% excess drill collars weight over the amount needed on the bit to prevent buckling of the drill pipe.

Drill collar weight to the bit may be as little as a few thousand pounds, or in excess of 100,000 Ib, depending on the bit and hole size. Outside diameter of standard drill collars may vary from 3 1/2” to 11”. The weight of drill collars vary from 21 to 306 Ib/ft. As a few as six collars or as many as fourty eight may be used. Resistance to bending is best provided by drill collars of largest possible size that can be safely sown into the hole. Square drill collars are much stiffer than round collars of equal size.

In general, bottom hole drilling increases with rotary speed, provided the hole cleaning of the nozzle is efficient. For increased weight on bit especially in a hard formation, the cutting teeth of the bit will quickly wear out.

The type of bit used on a given formation will affect the rate at which the formation is drilled. A soft formation roller bits have larger, widely spaced teeth with considerable code offset. These impact a drag bit cutting action along with nearly static percussive effects. Thus the drilling action is a complex combination of two mechanisms. Hard rock roller bits have shorter closely spaced teeth with zero-offset. Thus they approach true roll and hence drill by grinding process.
Numerous factors affect bit performance, such as tooth geometry ( tooth type, wedge angle, spacing , row to row patterns etc).
Usage of diamond bits is largely restricted to hard rock areas and/ or deep wells where reduction in trip frequency is a prime objective.

The mud properties that have significant effect on penetration rate are density, solid content, viscosity, water loss, and oil content.

Lightweight mud (<10ppg) permits faster drilling, because the hydrostatic pressure of the mud column is kept  to minimum. When mud density is too high a high differential pressure is created between the mud and the formation and what is known as a chip hold-down down effect is created.

Low solid water where feasible will enable faster penetration ratio than heavy weight mud, provided that bit weight, rotary speed and hydraulics are adequate and constant. Small particles of weighing materials in the mud plugs the fracture where a chip has been sheared from the hole bottom, hence holding the chip in place. Thus fine solids in the mud contribute to the chip hole down effect , inhibiting the removal of formation cuttings and decreasing the penetration rate.

Increase in viscosity mud resistance to flow, cause circulation pressure losses, which lesson bit hydraulic horsepower (BHHP) and make bottom hole cleaning more difficult . Highly viscous or thick muds are likely to hold formation chip on bottom than are low viscosity muds, which reduce penetration rate. Sometimes a quantity of high viscosity mud is circulated through a system for hole cleaning purpose. This is known as high viscosity sweep. It is a good way to clear the hole of unwanted junk before setting casing or running a diamond bit.

This ability of a mud to keep  cuttings from setting when circulation is stopped should also be observed carefully so that a clean hole is maintained is such instances.

The pressure of the mud column may force some of the fluid component of the mud into cavities in the more permeable formation through which a hole may be drilled. This water loss (filtration loss) in beneficial in two ways. The initial loss of fluid from the mud, is called spurt loss”, which can make the formation easier to drill. Overtime, this process of filtration allows for the build up of wall cake, which seals the well bore and prevent the loss of the whole fluid into the formation.
Excessive fluid loss can cause problem, resulting in too much wall cake, which may cause the drill stem to stick to the well bore. Certain shales may cave (heave) or slough , creating future problem of stuck pipe and loss circulation, hence reducing penetration rate.

The addition of oil to water-base muds can affect penetration rate in certain formation. The oil components keeps the hole friction to a minimum. In effect, a reduction in friction increases the bit weight. Oil in the mud serves as a good lubricating agent for the bottom hole assembly and keeps the bit from balling in certain hydrated clay and shales.

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