OCEAN DRILLING PROGRAM www.oceandrilling.org Scientifi c Application Extended Core Barrel (XCB) (see the XCB tool sheet) (both sys- A core bit (Figs. 1, 2) is a drilling Cutting shoe tems use the same BHA), tool with a hole through the center that removes sediment rock and II. Rotary Core Barrel (RCB) (see allows the core pedestal to pass the RCB tool sheet), and through the bit and into the core III. Advanced Diamond Core Barrel barrel. The Ocean Drilling Program (ADCB) (see the ADCB tool (ODP) employs different coring sheet). systems and bits to obtain continu- ous cores in all types of oceanic Bottom-Hole Assembly sediments and igneous basement. Operation The BHA for each coring system Once a coring system is selected is slightly different, but in gen- based on the expected lithology, eral consists of a primary core bit, the engineer determines which outer core barrel (OCB), short type of core bit to use. As coring sub assembly, and drill collars. conditions change, the coring bit The OCB supports the wireline A can be changed in an attempt retrievable inner core barrel, which to improve the recovery and rate Cutting shoe receives and carries the core. Drill of penetration with that coring collars are heavyweight pipes that system. The type of bit used apply weight to the bit on the depends on the expected lithology bottom of the OCB. The BHA is and past bit performance in the run on the drill pipe string, which area or in a similar lithology. Once is rotated at the surface to drive a bit is removed, it is “graded” the bit (except the Motor-Driven (i.e., it is examined to determine Core Barrel [MDCB]) and advance wear on the cutting structure, the BHA (see the BHA tool sheet gauge, bearings, etc., based on for more information). The MDCB industry standards) to optimize and Pressure Core Sampler (PCS) coring performance. are compatible with the APC/XCB BHA. Coring System Bits B There are three coring systems. Core Bit Types Each uses different bottom-hole Core bits are classifi ed according Figure 1. A. APC/XCB four roller cone assemblies (BHA) and types of core bit with an APC cutting shoe. B. APC/ to the cutting structure and type of bits: XCB four roller cone core bit with the bearings. There are fi ve basic types XCB cutting shoe. I. Advanced Piston Corer (APC) of bits used by ODP based on (see the APC tool sheet) and their function or structure: drag, inner core barrel (e.g., APC [Fig. temperature probes; and log- 1A], XCB [Fig. 1B], MDCB, and ging tools, which can pass PCS tools). The shoes on the inner through the bit throat without core barrel protrude below the requiring a trip to remove the primary roller cone bit and trim bit or drop it; therefore, coring the formation to core size. In con- and logging operations can be trast, the primary core bits in the resumed without a trip. RCB (Figs. 2, 3) and ADCB systems APC/XCB Specifi cations cut away most of the formation Maximum inner core barrel length: to create the core (i.e., there is no 9.5 m shoe). ODP most commonly uses a four roller cone type bit. APC/XCB bit throat diameter: 9.652 cm (3.80 in.) I. APC/XCB System APC shoe throat diameter: Core Bits 6.197 cm (2.440 in.) Tool Operation XCB shoe throat diameter: Figure 2. RCB four roller cone bit. The APC/XCB coring system can 5.872 cm (2.312 in.) use three types of bits for coring APC/XCB Typical Operating scraper, abrasive, roller cone, and soft to fi rm sediments: hammer. Ranges 11⁷/₁₆ in. four roller cone bit with 1) Formation Drag-type bits have a fl at chisel- tungsten carbide chisel teeth like surface to plane away soft (Fig. 1), APC: sediments ranging from soft formations (i.e., clay and chalk). to fi rm silts/carbonates/chalks/ 10¹/₈ in. PDC “anti-whirl” (Fig. 4) clays. Polycrystalline diamond compact bits, which are rarely used, or (PDC) bits use multiple tungsten XCB: fi rm to moderately fi rm lime- ¹ ₈ carbide studs with artifi cial dia- 10 / in. tungsten carbide blade stones, claystones, dolomites, and mond cutting surfaces in a claw- “drag” bits, which are also rarely with limited penetration in chert or like scraping action to remove used. basalt. soft formations (e.g., clay and Design Features and Benefi ts chalk) up to hard claystone and 2) Depth Range 1) Formation Compatibility limestone. APC: limited by piston core barrel Diamond bits use either surface-set APC/XCB bits are compatible with penetration in stiff or laminated or impregnated diamonds to sediments ranging from soft silts, fi rm/soft formations (i.e., to abrade (i.e., sanding-like pro- sands, and sticky clays to moder- “refusal”) and by wireline overpull cess) hard formations like shale ately fi rm limestones, claystones, to retrieve the core barrel (~100– or basalt. and dolomites. 300 mbsf). XCB: limited by core shoe survival Benefi t: One bit can core a wide Roller cone bits rotate cone-shaped in hard formations (typically used range of sediments without a rollers encrusted with teeth to from APC refusal depth to ~500 trip to change bits. remove soft to hard formations mbsf). through a combination of scrap- 2) Large Bit Throat ing and crushing processes. 3) Mean Recovery The APC/XCB bit throat has a Hammer bits use percussion to APC: ~100%. 3.80-in. opening. crush the hard rock around the XCB: ~55%–75% in sediments and core. Benefi ts: The APC/XCB bit is com- ~15%–35% in basalt. Not recom- Smaller bits called “shoes” are patible with the PCS and MDCB mended for penetration in basalt screwed onto the bottom of the coring shoes; water samplers; other than to sample basement. Hycalog Threads cannot be used in cal bit release to permit wireline igneous rocks. The logging. Flow Port diameter of RCB to Nozzle Benefi t: WSTP and DVTP tools can roller cone bits is 9⁷/₈ Stablizer Pads be run through RCB bits so no in., and the diameter TCI Wear Buttons Cone Arm time is lost for bit trips. on Shirt Tail of PDC bits is 10¹/₈ Nozzle in. Core Guide 2) Rugged Design There are four classes TCI Cutters The four cone-arm segments are of 9⁷/₈ in. RCB four Armored Recessed welded together in the throat area Grease Reservoir Jet Nozzles roller cone bits (Fig. and are armored (faced with hard Throat 5) with different Roller Cones nickel-chrome) on the leading cutting structures edge and upper arm areas. designed for differ- ent formation types: Benefi t: Reduces the chance of cat- astrophic cone-arm failure while Figure 3. Schematic of the four roller cone core bit C-3 bits have long drilling with heavy weight on bit used by the RCB coring system. extension chisel in hard and abrasive formations. teeth and are used in 4) Rate of Core Recovery soft sediments, 3) Ductile Teeth Depends on water depth (i.e., C-4 bits have medium extension The bit teeth are made with a wireline time) and formation. Pen- chisel teeth and are used in fi rm fi ner-grained tungsten carbide to etration rate usually slows with to hard sediments (e.g., dolo- increase tooth ductility, and the depth. mite and mudstone) and limited teeth are pressed into smaller holes APC: ~28.5–9.5 m/hr. upper basement, using greater force. XCB: ~19.0–4.5 m/hr. C-7 bits have short extension coni- Benefi ts: Reduces tooth breakage cal teeth for coring/drilling hard 5) Rate of Penetration and loss due to rough drilling in sediment and igneous base- Depends on rock properties. Pen- ment, and etration rate usually slows with C-9 bits have short exten- depth. sion hemispherical teeth APC: averages ~70–30 m/hr. for coring/drilling very XCB: averages ~30–12 m/hr. hard igneous basement. Limitation Design Features and Recovery is poor in laminated fi rm/ Benefi ts soft (e.g., chert/clay) or granular Flow sediments (e.g., sand, fractured 1) Tool Compatibility Ports Diamonds rock, or rubble). Nozzle RCB bits have a 2.312-in. Flow opening through the bit Channels II. RCB Core Bits throat that enables the Tool Operation Water Sampler Tempera- The RCB coring system typically ture Probe (WSTP) and the uses roller cone type bits (Fig. 2) Davis-Villinger Tempera- PDC Cutters for fi rm sediments to very hard ture Probe (DVTP) to be PDC Core Bit Surface Set Diamond igneous rocks. PDC RCB bits (Fig. run through the bit in fi rm Core Bit formations without hard Figure 4. Side and bottom view schematics 4) have been used occasionally in of PDC and diamond bits. The PDC bit is used fi rm sediments, as they core faster layers. The RCB bit must be in friable lithologies (e.g., limestone or clay- and can last longer under the right dropped with the mechani- stone). The surface set diamond bit is used conditions; however, PDC bits with sediments. fractured rock and reduces tooth 7) Bit Deplugger losses at high temperatures. A bit deplugger can be run in an attempt to remove 4) Recessed and Armored coring debris that can become Lubrication System jammed in the throat of the C-3 Long C-4 Medium The cone bearing has a pressure- bit. The 2.25-in. deplugger Extension Tooth Extension Tooth (soft formations) (medium firm formations) compensated grease reservoir that nose extends through the bit is recessed and covered with a throat and bit cone core trim- steel plate.