Cost Per Foot Equation

The cost per foot equation is used for the comparison of alternative equipment, chemicals, and procedures for the drilling of a formation or an interval. The comparisons are often called break-even calculations and are usually between drill bit types or manufacturers; however, any of the variables may be compared.

Bit + Tools + Mud + [ Drill Time + Trip + Lost ] [ Rig + Support + Tool Rental] C "" Drill Rate * Drill Time

C = Cost per foot for the interval of concern; $/ft

Bit = Cost of delivered bit at the drill site; $

Tools = Cost of tools or repairs to tools; $

Mud = Cost of mud to drill the interval; $

Drill Time = Time required to drill the interval or bit life; hr

Trip = Time to pull and rim a bit; hr

Lost = Time chargeable to non-drilling task; hr

Rig = Contract rental rate of a rig; $/hr

Support = Third party contractors rates; $/hr

Tool Rental = Rental of tools; $/hr

Drill Rate = Average drilling rate over the interval; ft/hr

In a comparison of drill bits, the drilling rate and life of the proposed bit will always be in question. The usual procedure is to compute the cost per foot with the data from a standard bit with the proposed bit; and, then construct a chart of required drilling rate and bit life for the proposed bit. The following example illustrates the method.

A study of bit data from a data base predicts that the expected values from a bit run with a tooth bit are as follows.

Tool = Stabilizer $250

Drill Time = Bit life; 33 hr

Lost = Surveying after every trip; 0.3 hr

Support = Contractor and Supervisors; $250/hr

Tool Rental = Large drill collars; 9 $/hr additional

Drill rate = 13 Mir


C = 84.72 $/ft Cost per foot for the data base tooth bit. Cost of PDC bit

The costs for a proposed PDC bit which is to be run on a bottom hole motor are the following.


= $9,283


= (1) Bottom hole motor repair charge; $1,894

(2) Two stabilizers; 2 * $250 = $500


= $2,500 ($500 additional required by the BHM)

Drill Time

= Bit life; Unknown; hr


= 7.5 hr (.5 additional horn:)


= Surveying after every trip; 0.3 hr


= 550 $/hr


= Contractor and Supervisors; $250/hr

Tool Rental

= Bottom hole motor; 200 $/hr

Drill rate

= Unknown; ft/hr

_ 9283 + (1894 + 500) + 2500 + [Drill Time + 7.5 + .3][550 + 250 + 200] 84-72 ~ Drill Rate * Drill Time

Reduction of the equation yields,

A break-even chart of required drill rates versus required drill time (bit life) may be prepared with the equation.

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