Eggbeater PDC Drillbit Design Eliminates Balling in Water-Based Drilling Fluids
- D.H. Zijsling (Nederlandse Aardolie Mij. B.V.) | Roland Illerhaus (Hughes Christensen Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- December 1993
- Document Type
- Journal Paper
- 246 - 252
- 1993. Society of Petroleum Engineers
- 1.10 Drilling Equipment, 1.11 Drilling Fluids and Materials, 5.1.2 Faults and Fracture Characterisation, 6.5.3 Waste Management, 1.11.4 Solids Control, 1.7.7 Cuttings Transport, 1.5 Drill Bits, 1.6 Drilling Operations, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.2.3 Rock properties, 1.5.1 Bit Design, 4.3.1 Hydrates, 4.3.4 Scale, 5.3.4 Integration of geomechanics in models
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This paper describes a novel polycrystalline-diamond-compact (PDC) -bit concept based on insights into PDC-bit cutting mechanism and rock behavior during drilling. The design comprises a hydraulic layout that optimizes bit cleaning and cuttings removal in soft and sticky formations. Significant improvements in performance have been achieved in Cretaceous and Triassic formations drilled with water-based muds.
Since the introduction of PDC bits in 1973, a new generation of bits has become available for drilling oil and gas wells. Since then, a large variety of PDC-bit designs have been developed for specific applications, allowing a wide range of formations to be drilled economically. Optimum PDC-bit performance is usually achieved in an oil-based-mud environment. Comparable performance with regard to rate of penetration (ROP) and bit life is largely unattainable in water-based muds. This difference in performance capability is most pronounced in soft or sticky formations and is attributed to accumulation of drilled cuttings at the bit face, resulting in bit balling and reducing drilling performance dramatically in water-based muds. Use of oil-based muds, however, is subject to rigorous statutory regulations being developed and implemented to mitigate pollution from their use. Consequently, oil-based muds are replaced with water-based muds where possible. This trend increases drilling company's concern over the inferior performance of PDC bits in water-based muds compared with that in oil-based muds. In view of these developments, Nederlandse Aardolie Mij. B. V. (NAM) has taken a proactive approach toward minimizing environmental impact of the use of oil-based muds in their drilling operations. Alternatives to oil-based muds are actively sought, and significant efforts have been made to provide rigs with cuttings cleaning systems that meet the current regulations (i.e., a maximum 10 wt% oil retention on cuttings) if technical reasons dictate the use of oil-based muds. In addition, improved PDC-bit designs are being investigated and field tested to optimize their drilling performance in water-based muds. This paper examines the PDC-bit cutting process in soft or sticky formations, and describes associated balling mechanisms. On the basis of these insights, a new "eggbeater" bit design concept was developed and is presented along with field test results of the eggbeater bit in NAM's drilling operations.
Bit balling is closely related to the size and the properties of the cuttings produced by the cutting elements on a bit. These cuttings characteristics can be determined from insights into the PDC-bit cutting process in low-permeability formations, such as chalk, marl, claystone, and shale. PDC bits primarily use a shearing action in a narrow zone to the front of the cutter for cutting (Fig. 1). During rock shearing, dilatancy occurs because microcracks in the formation are opened as the rock reaches its failure strength. This increases the rock PV. Because of the high shear rate during cutting and the low permeability of the formations, pore-fluid and mud-filtrate invasion into the shear zone is limited, and the pore pressure in the rock passing through the shear zone drops. In extreme cases, the pore pressure in the rock decreases to the vapor pressure of the pore fluid. This suggests that chips initially are subjected to a high differential pressure [as high as the bottomhole-pressure (BHP)], which decreases gradually because of mud-filtrate invasion. This high differential pressure provides considerable cuttings strength and lifetime so that long, continuous cuttings can be produced. Laboratory drilling tests in soft or sticky formations revealed that the cuttings produced by PDC bits look like stacked chips. However, such cuttings disintegrate totally into rock flour shortly after test completion. This indicates that shearing is a continuous process and that the cuttings strength is provided by differential pressure rather than by cohesion. The disintegration rate under downhole conditions is considerably slower than that observed during laboratory testing because after test completion the release of pressures to simulate downhole conditions reduces differential pressure across the cuttings. This increases permeability drastically in the cuttings, which accelerates fluid invasion.
The PDC-bit cutting mechanism in soft or sticky rock, described above, implies that the effective contact stress at the cuttings/PDC-bit surface interface initially can be as high as the BHP. As a result, a frictional force develops at this interface at the base of the cutting and this force increases as the size of the contact area increases. When the compressive force required to overcome this friction exceeds the cuttings strength, instability occurs, and subsequently, chips are generated and stacked up (Fig. 2). This instability also occurs when cuttings removal is hampered by the bit body (e.g., small waterway size). Cuttings accumulate when the cuttings production rate exceeds the erosion rate of the balled-up cuttings caused by the drilling fluid. This balling mechanism may occur in both water- and oil-based muds and occurs almost instantaneously after cuttings have formed. This phenomenon is "primary" bit balling. If primary bit balling does not occur, the cuttings are surrounded by mud and transported to the annulus. However, when drilling with water-based mud, cuttings have a strong tendency to suck in filtrate from the mud and thus equilibrate ft pressure differential across the cuttings. Consequently, such cuttings can easily accumulate or become stuck on the bit surface, ultimately balling up on the bit. If the cuttings contain clay fractions that are sensitive to water, this tendency is increased significantly. The release of formation BHP and destruction of the original bonding between the rock grains caused by the cutting process activates the full swelling potential of the cuttings. This swelling potential, which is based on hydration of the clay fraction, may be substantially larger than that associated with the pressure equilibration process. The balling mechanism associated with pressure equilibration and swelling does not occur instantaneously because it is governed by fluid invasion; thus, it is called "secondary" bit balling. With oil-based muds, secondary bit balling (i.e., that caused by the suction effect) is much less likely to occur than with water-based muds because oil invasion does not activate the swelling potential of the clay fraction of the cuttings.
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