in. bottom-hole assembly (BHA) and requires some (~20 m) lateral support (i.e.,
deployment in an existing hole) to commence coring. The ADCB relies on rotation
of the drill string to advance the hole while the 7
in. drill bit trims the core sample.Scientific Application
The Advanced Diamond Core Barrel (ADCB) coring system may be used to attempt to recover continuous core samples from firm to well lithified sedimentary or igneous formations when Advanced Piston Coring, Extended Core Barrel, and Rotary Core Barrel (APC/XCB/RCB) coring techniques are ineffective. The ADCB provides a crucial alternative technique using diamond coring technology to attempt to improve recovery of formations that are difficult to core with conventional rotary coring tools.
Tool Operation
The
ADCB uses a 6
in. bottom-hole assembly (BHA) and requires some (~20 m) lateral support (i.e.,
deployment in an existing hole) to commence coring. The ADCB relies on rotation
of the drill string to advance the hole while the 7
in. drill bit trims the core sample.
Design Features
1) Improved Core Quality
The ADCB uses a diamond mining-style bit to trim the core and incorporates a pressure indicator to monitor core jams.
Benefit: May improve core recovery in hard formations, interbedded firm and hard formations, and poorly consolidated formations that are difficult to recover with the RCB and XCB.
2) Improved Hole Stability
The
ADCB’s 7
in. mining-style thin-kerf diamond bit produces a smaller and smoother borehole
wall than rotary drilling, and the 6
in. BHA provides a "packed hole" with a narrow annulus.
Benefit: Hole stability is improved in hard formations, which requires less time for reaming and hole cleaning and reduces stuck pipe problems.
3) Low Fluid Invasion of Core
The ADCB diamond bit creates a fine rock powder (i.e., not rock chips or cuttings); therefore, hole cleaning requires less fluid velocity, circulating rates are lower, and the core is not directly exposed to high-pressure fluid from the bit jets.
Benefit: Reduces fluid invasion (i.e., "core flushing") and improves core quality in porous and water sensitive formations.
4) Improved Log Quality
The
ADCB diamond bit produces a smooth 7
in. hole rather than the larger and more rugose borehole walls typical of rotary
coring.
Benefit: Electric log quality is improved by better pad contact and smaller hole diameter (7
in.).
ADCB Specifications
Core Size
The PQ3 mining-style bit produces cores with a 3.27 in. (83 mm) diameter when optional split steel or lexan liners are used. The PQ bit, which does not use liners, has a core diameter of 3.345 in. (85 mm).
Core Length
Typically 15 ft (4.75 m) to reduce core weight and problems with core jamming in the core barrel. A 30 ft (9.5 m) version is also available.
Equipment
The inner core barrel has a positive indicator latch to confirm inner core barrel latch-in.
Diamond stabilizers on the outer core barrel reduce core barrel vibration.
Bit types include diamond impregnated for hard rock, polycrystalline diamond compact (PDC) for friable rock, and surface-set diamond bits for sedimentary formations.
Typical Operating Range
Formation
Designed for hard or well-cemented sediments and igneous formations.
Recovery
May be higher (relative to XCB and RCB) in formations that are difficult to core, such as fractured hard rock or rubble and interbedded hard and soft formations.
Limitations
Does not recover soft sediments and granular formations (such as sand, gravel, or coral debris).
Cannot be used for bare rock spud.
The ADCB requires good weight-on-bit control and use of the active heave compensator.
The inner core barrel and core may be pulled before coring the full 4.75 m length if the core jams.
Depth range is limited by the number of 6
(May 2002)