Optimization of Hydraulics for Polycrystalline Diamond Composite Bits in Gulf Coast Shales With Water-Based Muds
Authors: Radtke Robert P. , Pain David D. 
Published: December 2007

Optimization of Hydraulics for Polycrystalline Diamond Composite Polycrystalline Diamond Composite Bits in Gulf Coast Shales With Water-Based Muds

Summary
Polycrystalline diamond composite (PDC) bits have Polycrystalline diamond composite (PDC) bits have drilled U.S. gulf coast shales economically in oil-based muds since their inception. However, water-based mud drilling was originally a problem because of insufficient hydraulics. With the correct hydraulic parameters, these shales also can be drilled economically with water-based muds. Bit life can be maximized with cutter cooling along with the correct operating parameters-particularly rotary speed. A detailed evaluation of field data has shown that rate of penetration (ROP) is dependent on hydraulics according to three regimes: flow rate, hydraulic horsepower, and mechanical horsepower.

Introduction
Previous investigation has reported the advantages of Previous investigation has reported the advantages of high ROP and long bit life when using oil-based muds with PDC drill bits. However, at least 80% of the footage drilled for oil and gas is with water-based mud. Therefore, to realize the full cost savings potential of PDC bits, designs and operating parameters needed to be PDC bits, designs and operating parameters needed to be developed for effective water-mud drilling.
The primary tangible measurement of drilling efficiency is drilling cost per foot. Doubling the ROP can reduce cost by 50%, while doubling bit life may reduce costs by only 11%. The ROP of PDC bits is governed by rock properties, operating parameters, and bit design. Shales properties, operating parameters, and bit design. Shales were initially difficult to drill with water muds because of relatively low hydraulic energy levels used. Improved bits designed to operate at higher hydraulic energy levels were needed.
This paper presents the hydraulic design criteria developed for economic performance with the PDC in rotary drilling of gulf coast shales.

Background
The mechanical energy available for rotary drilling is only 20 to 30 hp [14.9 to 22.4 kW], while hydraulic energy at the bit can be 200 to 1,500 hhp [149 to 1119 kW]. With the PDC drill bit, fluid flow is needed across the bitto cool the polycrystalline diamond cutters,to clean formation cuttings from the bit face, andto provide hydraulic impact force (IFN) to scour cuttings provide hydraulic impact force (IFN) to scour cuttings differentially stuck to the bottom of the hole (chip hold-down).
Conversely, incorrect design of the hydraulic parameters including flow rate (q), pressure drop ( p), parameters including flow rate (q), pressure drop ( p), impact force and hhp per square inch of bottomhole can result in (1) PDC cutter overheating, resulting in high cutter wear rates and cutter loss caused by attachment braze failure, (2) low ROP caused by bit balling whereby cuttings build up in front of cutters and clog fluid escape passages, and (3) the regrinding of cuttings differentially passages, and (3) the regrinding of cuttings differentially stuck on the hole bottom.
PDC Cutter Temperature PDC Cutter Temperature PDC cutters have high resistance to abrasive wear as PDC cutters have high resistance to abrasive wear as determined in controlled laboratory tests. PDC cutters were shown to be equivalent to the most wear-resistant single crystal West African natural diamond. However, at higher cutter speeds, the frictional heat developed results in a marked reduction in diamond hardness, which results in a corresponding increased wear rate.
At temperatures below 1,350 deg. F [732 deg. C], the primary mode of diamond wear is microchipping. Above 1,350 deg. F [732 deg. C], severe thermal damage occurs that is caused by the stress created by the thermal expansion differential between the diamond and entrapped cobalt metal phase in the PDC microstructure.
The brazing method used to attach the PDC cutter to the carbide stud support has sufficient strength up to 700 deg. F [371 deg. C]. However, above this temperature the strength decreases and the bond may fail.
Analytical studies have indicated that cutters could reach 1,560 deg. F [849 deg. C] and laboratory measurements have confirmed that cutter temperatures did reach 1,110 deg. F [593 deg. C] very quickly." These investigators determined that PDC cutter temperature can be reduced to less than 700 deg. F [371 deg. C] by providing clearance behind the PDC cutter. The clearance would reduce frictional heat generated by the drag of the carbide support on the rock surface.
A composite graph shown in Fig. 1 illustrates the practical consideration of the effect of rotary speed on the practical consideration of the effect of rotary speed on the wear rate and estimated temperature of the gauge cutter. The PDC wear rate has been determined to be exponential with cutter speed.
JPT
P. 1697