Scientific Application
The Motor Driven Core Barrel (MDCB) is a wireline-retrievable coring system compatible with the Advanced Piston Corer/Extended Core Barrel (APC/XCB) bottom-hole assembly (BHA). It is designed to improve core recovery in formations that are difficult to APC/XCB core (e.g., hard fractured crystalline rock, interbedded hard/soft formations, and friable conglomerate and reef materials). The MDCB is typically used to recover intermittent short intervals (a few 4.5 m cores) because of the handling time (~1 hr) to prepare the tool for consecutive coring runs.
Tool Operation
The MDCB can be run interchangeably with the APC/XCB coring systems with the addition of a latch sub to the
BHA. The MDCB consists of a motor section, thruster section, inner core barrel section, and a
narrow-kerf core bit. The motor section is powered by the hydraulic force of fluid pumped down the drill string, which causes the motor to rotate. The thruster section uses hydraulic force to provide weight on bit
(WOB) and advance the inner core barrel. The inner core barrel section has a 4.5 m core tube with a
thin-kerf core bit on bottom; the core diameter is 2.25 in. (57 mm).
Design Features 1) Diamond Core Mining Technology
Compared to the
APC/XCB coring system, the MDCB operates at higher revolutions per minute
(rpm) with lower WOB and uses narrow-kerf surface-set diamond-impregnated as well as geoset and tungsten carbide core bits to recover cores from friable, laminated hard/soft, and crystalline formations.
Benefit: The MDCB applies less drilling stress to the formation, which may improve core quality and recovery in friable, fractured, or crystalline formations.
2) Positive Displacement Mud Motor
Downhole rotation and torque are produced by a Baker-Hughes Inteq Mach 1P positive displacement mud motor with a 7:8 lobe rotor/stator power section.
Benefit: The motor rotates the bit; therefore, drill string vibration is eliminated.
3) Thruster Unit
Hydraulic force is translated into WOB, and interchangeable nozzles optimize WOB at various flow rates.
Benefit: The WOB can be controlled to provide a more uniform application of weight to the diamond bit, thereby improving diamond bit life and recovery.
4) Core Barrel Assembly
A modified version of a standard Christensen Mining Products HWD4 inner core barrel provides a nonrotating core tube to receive the core sample into a clear plastic liner.
Benefit: Reduces rotational torque and stress in cores to improve core quality and recovery.
5)
Diamond Core Bits A
3.75 in. (95 mm) outer diameter (OD) core bit is used to trim a 4.5
m long × 2.25 in. (57 mm) diameter core. Optional bit types
include: narrow-kerf diamond-impregnated or surface-set diamond bits
as well as geoset and tungsten carbide core bits. Benefit:
Different
bit cutting structures can be selected for each MDCB run, based on
the anticipated formation, to optimize core quality and recovery. 6)
Interchangeability With
the addition of a latch sub, the MDCB can utilize the same BHA as
the APC and XCB. Benefit:
Eliminates
extra pipe trips to change the BHA. Specifications Tool
size OD: 3¾ in. (95 mm)
Limitations
Diamond coring does not work well in soft or unconsolidated granular formations. The core length is 4.5 m, which requires more wireline and handling time than typical 9.5 m
APC/XCB cores.
(March
2005)
Motor pressure: 1160 psi
Pump rate: 190 gpm
Power: 96 hp
Bit speed: 410 rpm
WOB: 2000–8000 lb
Max. torque: 1250 ft-lb
Bit flow rate: 15–35 gpm
Efficiency: 83%
Core diameter: 2.25 in. (57 mm)
Core length: 4.5 m