MAJOR FACTORS AFFECTING DRILLING PENETRATION RATES (ROP)

The Drilling Penetration Rate (ROP)

In the drilling industry, the rate of penetration (ROP), also known as penetration rate or drill rate, is the speed at which a drill bit breaks the rock under it to deepen the borehole. It is normally measured in feet per minute or meters per hour.

Generally, ROP increases in fast drilling formation such as sandstone (positive drill break) and decreases in slow drilling formations such as shale (reverse break). ROP decreases in shale due to diagenesis and overburden stresses. Over pressured zones can give twice of ROP as expected which is an indicative of a well kick. Drillers need to stop and do the bottoms up.

The penetration rate that can be achieved by a bit has an inverse effect on the drilling cost per foot. The main factors that affect penetration rate are

• bit type
• formation properties
• drilling fluid properties
• bit weight and rotary speed
• bit hydraulics

BIT TYPE

• Penetration rate is highest when using bits with long teeth and large cone offset angle. These bits are practical only in soft formations. The lowest cost per foot drilled usually is obtained by using the longest teeth that are consistent with bearing life at optimum bit operating conditions.

• Drag bits such as diamond and PDC drill bits are designed to obtain a given penetration rate by the selection of the number and size of the diamonds or the PDC blanks. The penetration rate of PDC bits also depends on the back rake angle and the exposure of the blanks.

FORMATION CHARACTERISTICS

• The ultimate compressive strength of the rock is the most important rock property that affect penetration rate. The higher the compressive strength the lower the penetration rate.

• The mineral composition of the rock has some effect on penetration rate. Rocks that contain abrasive minerals can cause rapid dulling of the bit teeth. Rocks that contain gummy clays can cause the bit to ball up and drill inefficiently.

• The permeability of rock also affects penetration rate. In permeable rock, the drilling fluid filtrate can move into the rock ahead of the bit and equalize the pressure differential acting on the chips formed beneath each tooth and increase the penetration rate.

DRILLING FLUID PROPERTIES

Drilling fluid properties that affect penetration rate are

• density
• rheological properties
• filtration properties and
• solids content and size distribution

Density

• An increase in the drilling fluid density would decrease the penetration rate.
  Increasing the drilling fluid density would cause an increase in the bottom hole hydrostatic pressure beneath the bit and thus an increase in the differential pressure (overbalance) between the bore hole and the formation pore pressure.

• An increase in the differential pressure would increase the strength of the rock and, therefore, decrease the penetration. Also, a higher differential pressure prevents the ejection of the crushed fragments of rock formed beneath the teeth of the bit resulting in a lower penetration rate.

• The effect of differential pressure on penetration rate in Berea sandstone is shown below. Note that the effect of overbalance on penetration rate is more pronounced at low values of overbalance than at high values of overbalance. If overbalance is quite large, additional increase in overbalance has little effect on penetration rate.

The effect of differential pressure on penetration rate in Berea sandstone

Mud Rheology

• Penetration rate tends to decrease with increasing viscosity and solids content and tends to increase with increasing filtration rate.

• Increasing the viscosity increases the frictional losses in the drillstring and thus decrease the hydraulic energy available at the bit for cleaning the bottom of the hole.

Solid Content

• The solid content of the mud controls the pressure differential across the zone of crushed rock beneath the bit. Increasing the solid content decreases the filtration rate and therefore increases the differential pressure. As mentioned earlier, an increase in the differential pressure results in a decrease in the penetration rate.

Effect Of Differential Pressure on Penetration Rate

Many studies have been conducted on the effect of differential pressure (overbalance) on the penetration rate. Bourgoyne and Young observed that the relation between differential pressure and penetration rate can be represented by a straight line on a semi log paper as shown below. The equation for the straight line is given by

Terminating A Bit Run

The decision to terminate a bit run is not always simple in drilling operations. A bit should be pulled out if the bearings are worn out or the cutters have worn out to the point that it is no longer economical to continue drilling with the bit.

Badly worn bearings can be detected by monitoring the rotary table torque. When bearings are worn, one or more of the bit cones will lock and cause a sudden increase in the rotary torque needed to
rotate the bit. When this happens the bit should be pulled out. When the penetration rate decreases rapidly as bit wear progresses, it may be advisable to pull the bit before it is completely worn.

If the lithology is somewhat uniform, the total cost can be minimized by minimizing the cost of each bit run. In this case, the best time to terminate the bit run can be determined by keeping a current estimate of the cost-per-foot for the bit run, assuming that the bit would be pulled at the current depth. Even if significant bit life remains, the bit should be pulled when the computed cost-per-foot begins to increase.

However, if the lithology is not uniform, this procedure will not always result in the minimum total well cost. In this case, an effective criterion for determining optimum bit life can be established only after enough wells are drilled in the area to define the lithologic variations.