Drilling Plastic Formations Using Highly Polished PDC Cutters

Abstract

Highly plastic and over-pressured formations are troublesome for both roller cone and PDC bits. Thus far, attempts to increase penetration rates in these formations have centered around re-designing the bit or modifying the cutting structure. These efforts have produced only moderate improvements. This paper presents both laboratory and field data to illustrate the benefits of applying a mirror polished surface to the face of PDC cutters in drilling stressed formations. These cutters are similar to traditional PDC cutters, with the exception of the reflective mirror finish, applied to the diamond table surfaces prior to their installation in the bit. Results of tests conducted in a single point cutter apparatus and a full-scale drilling simulator will be presented and discussed. Field results will be presented that demonstrate the effectiveness of polished cutters, in both water and oil-based muds. Increases in penetration rates of 300400% have been observed in the Wilcox formation and other highly pressured shales. Typically, the beneficial effects of polished cutters have been realized at depths greater than 7000 ft, and with mud weights exceeding 12 ppg. Introduction Historically, drill bits employing polycrystalline diamond to medium-hard formations, with low abrasivity. Nearly 70% of the formations drilled with PDC bits are shales, but successful applications in carbonates, evaporites, mud and siltstones, and sandstones are also common. Extensive studies performed in laboratory wellbore simulators have also provided important data for the proper geological applications and drilling practices that optimize drilling with PDC bits. Even though a PDC bit is deemed appropriate to the targeted formation, efficient, and thus economic, drilling performance depends on an optimum combination of bit design, operational practices, mud chemistry and hydraulics. Formations that can be successfully drilled increase steadily as knowledge is advanced in all these areas. Paradoxically, PDC bits occasionally perform unsuccessfully in formations they would seem to be ideally suited to drill. The U.S. Gulf Coast is one region where this is known to occur, even though the formations are not exceptionally strong or abrasive. There, the typical problem is the severe drop in the rate of penetration (ROP) experienced when drilling deep shale, especially with heavily weighted muds. These types of rock should be well suited to the shear cutting action of PDC bits, but surprisingly, PDC, as well as roller cone bits, have proven to be less than effective in drilling these formations. Increasing weight-on-bit early in the drilling cycle to reach an acceptable ROP is often unsuccessful, resulting in the bit being pulled prematurely. Examination of the bit often reveals no sign of balling or cutter damage, despite the application of heavier weight. P. 277

Drilling More Stable Wells Faster and Cheaper with PDC Bits and Water Based Muds

“…The overall performance improvements that can be gained in the field by using aggressive PDC bits, polished cutters, ROP enhancers and combinations of the above have been reported before (4,5,7,8,9) . A Eugene Island well is a particularly good example.…”

“…The first and most common occurs when the chips do not separate from the individual PDC cutters and accumulate as a large mass of material (3,4,5) . The second, and only recently understood mode, is found when drilling with weighted WBM at high bottomhole pressures.…”

“…The high pressure single point cutter drilling simulator used to produce the results reported here has been described previously (4) . Briefly, the test device rotates a single cutter under mud pressure at a constant RPM while the cutter advances axially at a constant rate and the axial and tangential loads are monitored.…”

Using Advanced Drilling Technology to Enable Well Factory Concept in the Marcellus Shale

“…The latest technology 16mm cutters with a high-gloss polished finish were used. Lab tests by the service provider have shown that the polished cutters increase bit aggressiveness in shale applications as well as aid the hydraulic cleaning of the cutting structure (Smith, et al 1995). This promotes tangible gains in penetration rates without any adverse affects on primary bit stability.…”