David P. Huey
Stress Engineering Services Inc.
Houston
Michael A. Storms
Ocean Drilling Program
Texas A & M University
New and modified wire line-retrievable core barrels have improved coring operations, commonly allowing recovery of full cores in good condition, some at in situ conditions.
The need to analyze cores at downhole conditions prompted the development of a series of pressure core barrels, which can capture cores without disturbing the sample through depressurization.
In upper marine sediments, tungsten carbide insert (TCI) roller cone bits tend to have life expectancies of several hundred rotating hours and thousands of meters of penetration. A bit can drill through hundreds of 10-m core intervals before a pipe trip is required for a bit change.
For time efficiency in coring operations in water depths averaging 3,000-5,000 m, both the Deep Sea Drilling Project (DSDP) and the Ocean Drilling Program (ODP) made a full commitment to wire line coring techniques wherever possible, thereby minimizing the number of pipe trips.
The initial "plain vanilla" rotary core barrel became inadequate for the high quality of cores desired for scientific analysis. The first new core barrel developed by DSDP to satisfy the increased scientific specifications was a wire line-retrievable pressure core barrel.
PRESSURE CORE BARREL
Pressure core barrels were already available in industry at that time, but not in a wire line-retrievable form. DSDP engineers produced a wire line-retrievable pressure core barrel (PCB) made interchangeable with the standard rotary core barrel.
Thus, pressure core sampling was possible in selected intervals in a hole where standard rotary coring was being conducted.
The complex PCB design went through three phases until the relatively reliable PCB-III tool was built. This tool had the capacity for a 7.8-m long core at a maximum pressure of 5,000 psi. The PCB included both a 6-m pressure core zone and a 1.8-m unpressured core zone, an internal temperature probe, and a nitrogen-charged accumulator. The PCB vented inert gas from the accumulator instead of the sample material if the contents of the barrel reached excess pressure, or it maintained pressure on the sample in case of small leaks. High pressure relief safety systems were also built into the tool.
The PCB had several operational shortcomings, however. It required a roller cone bit with the cones set to trim a 2 1/4-in. core rather than the 2 7/16-in. standard for routine rotary coring.
The 38-ft PCB was also too long, presenting a serious handling problem upon retrieval to the deck. The PCB had to be kept cool to avoid dangerously high internal pressures if a large quantity of hydrate material had been captured. For operations in the tropics, a large ice bath had to be installed adjacent to the rig floor. The tool did not include a mechanism to sample gases or fluids or to transfer the solid core to another container without first having the internal pressure bled off.
In the late 1980s, ODP sought to improve the PCB by developing a new and better pressure core barrel. The goal was to take smaller, more easily handled pressure cores. This tool was called the pressure core sampler (PCS) to differentiate it from its predecessor.
PRESSURE CORE SAMPLER
The pressure core sampler (PCS) was designed to be fully compatible with a suite of other ODP wire line coring tools in use, including the piston corer and extended core barrel. The PCS is, in fact, a sophisticated variation of the extended core barrel. The PCS incorporates a cutting shoe which extends ahead of the roller cone bit and performs the initial core trimming operation. The PCS can recover a 1.65-in. core 34-in. long at in situ pressures up to 10,000 psi.
The ball valve actuation system, which had been a touchy all-mechanical system operated by wire line pull in the previous PCB-III, was changed to a much more reliable hydraulic system for the PCS. The PCS actuator is a custom modification of the Hydro-lift system originally designed by Eastman-Christensen and licensed to ODP (Fig. 1) (9898 bytes).
The PCS demonstration Phase 1 produced a downhole tool that was mechanically successful in many deployments but was often poor at trimming and capturing core material. ODP engineering is currently working to improve the tool's core-cutting characteristics. Direct access to the core material under pressure is not possible, but fluids and gases can be extracted with an external, pressurized sampling manifold.
Phase 2, which was the plan to create an on-deck interface for transferring core samples under pressure into a laboratory chamber, was never started because of budget restrictions and other priorities.
HYDRAULIC PISTON CORER
In 1978, DSDP engineers combined the principles of oceanographic piston coring with the benefits of deep-penetration rotary coring and wire line core barrel retrieval. The result was the successful original hydraulic piston corer
(Fig. 2). The hydraulic piston corer (HPC) could obtain a 2 1/2-in. diameter x 4.5-m long core in a clear butyrate liner which would house the core, virtually undisturbed, through the entire lab analysis process. Later improvements lengthened recovery to 9.5 m, incorporated azimuth orientation measurement for paleomagnetic analysis, and made the HPC compatible with the extended core barrel system in a common bottom hole assembly (BHA).
The HPC can acquire undisturbed cores to depths as great as 300 m below the seafloor, Stored energy in the form of compressed water in the drillstring is released suddenly by preselected shear pins to enable the core barrel to penetrate the formation very rapidly. This 1.5-3 sec stroke effectively decouples the motion of the core barrel from the heave of the ship.
ADVANCED PISTON CORER
The original HPC tool was replaced by the advanced piston corer (APC) which uses the 3.80-in. ID of a special seal-bore drill collar as the cylinder for its drive piston. The increased piston area and greater structural cross section from this design change have produced a tool capable of more than 25,000 lb of thrust. The APC can be extracted from a sticky formation with up to 100,000-lb overpull without risk of damage. The current version of the APC retains full magnetic orientation capability, and the core barrel can be safely drilled over for retrieval if it remains stuck in the formation beyond the 100,000-lb overpull limit. Typical core recovery approaches 100% in normal APC operations (Fig. 3) (8766 bytes).
EXTENDED CORE BARREL
- Almost all DSDP and ODP drill sites are cored initially using the hydraulic piston corer and a roller cone bit with a nominal 3/4-in. opening between the cones. At the refusal depth of the piston corer, rotary coring can only be continued if the core barrel used is capable of further trimming the core to a nominal 2 1/2.-in. diameter.
- This pretrimming action by a pilot drag bit operating ahead of the roller cones has been thought for years to be advantageous in producing cores with minimum mechanical and hydraulic disturbance.
The XCB coring system was originally developed in the early 1980s by DSDP engineers and redesigned for greater strength and reliability through several design changes by ODP (Fig. 4) (10827 bytes), Much developmental emphasis was placed on cutting shoes. The current tool can use either hardened steel, saw-tooth cutting shoes, or shoes with massive tungsten carbide inserts capable of penetrating crystalline rock for short distances.
A significant advantage of a wire line-retrievable XCB is that the core trimming cutting structure on the cutting shoe can be examined, and if necessary replaced, after each 9.5-m core interval. The core barrel locks into the BHA and rotates with the roller cone bit. A spring-loaded extension system allows the cutting shoe to extend 6-14 in. ahead of the bit but retract if hard ground is encountered. The core is captured in a nonrotating, dear butyrate core liner.
Circulation of the drilling fluid to the roller cone bit is partially tapped by a passive flow divider to provide a small amount of flow to the extended cutting shoe for cooling and cuttings removal. This approach works well in sandy or indurated formations but is inadequate if the formation is dominated by sticky clays which tend to clog the cutting shoe jet orifices, blocking flow.
To address this problem, engineers from ODP and Stress Engineering Services Inc. developed the XCB flow control system which incorporates a downhole anticlog valve (Fig. 5) (10155 bytes). The still-developmental valve acts as an active flow divider that varies flow split between roller cone bit and cutting shoe according to back pressure conditions. When the system senses incipient clogging in the cutting shoe, the valve immediately provides higher pressure (and more flow) to that path in an attempt to wash out the clogging material. A sustained clogging condition in the cutting shoe is signaled to the driller by a nonlinear, diagnostic increase in system back pressure.
MOTOR-DRIVEN CORE BARREL
ODP needed a coring tool capable of penetrating massive or highly fractured crystalline rock (typically basalt) and capturing cores with minimal disturbance from hard/soft sequences typically characterized by chert/chalk interbeds.
The motor-driven core barrel (MDCB) was developed to meet these two needs. The motor-driven core barrel uses high-speed, narrow-kerf, slim-hole diamond coring technology to cut cores in hard rock. In conjunction with the APC and XCB coring systems, it can extend many single bit holes from mudline to basement with the best possible coring tool deployed at each stratum. No pipe trip for a different roller cone bit is required, and open hole logging can follow the end of coring or be run at intermediate stages.
The MDCB, originally dubbed the Navidrill core barrel (NCB), evolved through three stages of development. The first two prototype designs were developed in conjunction with Eastman-Christensen using its 3 3/4-in. positive displacement Mach I and III mud motors, known commercially as Navidrills. The NCB1 and NCB2 models were partially functional tools that performed adequately in lab tests. However, both tools had undesirable positive hydraulic feedback characteristics that tended to drive the system towards weight-on-bit overload and stall when difficult-to-core formations were encountered.
The third generation was renamed the more-generic MDCB, even though it was based on the 3 3/4-in. Mach 1P "drain hole" mud motor from Christensen. Using extensive computer performance modeling in the design process, ODP and Stress Engineering Services built a new, reliable tool with negative hydraulic feedback and a built-in tendency to avoid stall conditions.
The current MDCB is freefall deployable and wire line retrievable. It extends ahead of a stationary roller cone bit to cut a 2 1/4-in. diameter x 4.5-m long core. The extending portion of the tool is a standard (HWD4) core barrel, used in the mining industry. The tool uses any of a variety of diamond cutting shoes (off-the-shelf 3 3/4-in. mining industry core bits).
The Mach 1P motor provides up to about 400 rpm and 1,200 ft-lb of torque to drive the core barrel. Weight on bit is achieved primarily by hydraulic thrust developed in a special telescoping section of the tool, which also provides for thrust damping to prevent lunging of the core barrel when it breaks through hard layers while coring hard/soft interbeds. Motor speed is controlled directly from the deck by flow rate. Thrust (weight on bit) is controlled by selecting in advance the best combination of back pressure nozzles to provide the desired thrust at the anticipated flow rate.
Because the pressure drop across the stationary mud motor varies with reaction torque, the system is self modulating. That is, weight on bit is reduced in inverse proportion to increased reaction torque which, in turn, inhibits stall. When the full 4.5-m stroke is achieved, a small valve dumps most of the flow to the motor and signals the driller with a sudden pressure drop of 700-800 psi (Fig. 6) (8934 bytes).
CORE CATCHES
Core catchers, like core barrels, must be customized for the materials to be captured; there is no universal core catcher design.
Core catchers for retaining hard rock and indurated sediments require two special features: maximum ruggedness and the ability to close securely, even if a large section of core spans the catcher zone when coring is stopped.
ODP therefore developed several versions of spring-loaded, finger core catchers with these features The fin(Fig. 7).gers can fully close or wedge the rock in place with a partial closure. Breaking the core to dislodge it from the formation, a common requirement in diamond coring, is virtually never required in ODP coring operations because the high vibration from the roller cone bits helps separate the cores. Collet-type core catchers are not normally used because the roller cone bits do not trim cores with the same diameter control as diamond bits do. Furthermore, common basket-type core catchers are virtually never rugged enough for deep ocean operations.
Core catchers for soft sediments must present zero resistance to the incoming core material to avoid core disturbance. The catchers must then close liquid-tight to prevent loss of slurry materials in the core during wire line recovery. ODP developed two flapper-type core catchers with these characteristics and B(Fig. 8)oth in(Fig. 9).volved difficult and expensive machining operations because of their complex geometries. Costs for both types were reduced by about a factor of 20 using steel investment casting methods.
CORING MUD MOTOR
One of the most difficult deepwater drilling assignments for ODP has been starting holes oh exposed hard rock (basalt) on mid-ocean ridges where little or no sediment cover exists to stabilize the BHA during spud-in. A major difficulty is achieving enough weight on a nominal 10.5-in. roller cone bit to penetrate basalt. The 5,000-15,000 lb weight required to start the hole requires moving the neutral point from 50 to 150 ft above the seafloor. The result is a highly unstable, laterally unsupported, near-vertical BHA that is unacceptably susceptible to connection failure if rotated. One way to drill without rotating the vulnerable portion of the drillstring at or below the neutral point is to use large diameter mud motors immediately above the bit. Unfortunately, with a standard mud motor this setup would eliminate the possibility of coring.
ODP and Eastman-Christensen produced a 9 1/2-in. positive displacement mud motor with a hollow rotor especially for this assignment. The hole in the rotor accommodates a custom slim-line, wire line-retrievable mining core barrel.
To have a nonrotating core barrel housing, the Christensen Mach 1 mud motor was modified to operate upside-down compared to conventional applications (Fig. 10) (9825 bytes). The rotor is connected to the drillstring above and held stationary while the housing and stator are connected to the bit and rotated.
The system can cut a 2 1/4-in. diameter core up to 30 ft long. The bit rotates at 90-120 rpm with up to 6,000 ft-lb torque. This motor also has a lock-out system rated at 20,000 ft-lb torque. The lock-out system fixes the rotor and stator so that the entire assembly can be rotated as a unit when necessary, such as for activating drilling jars or engaging J-tools. On the down side, this motor system must use a minimum 10 1/2-in. OD x 2.2-in. ID bit. Unfortunately, this unfavorable kerf-to-core ratio made this a poor performer in fractured hard rock coring assignments.
DIAMOND CORING SYSTEM
ODP's premier engineering development task (still under development) has been the diamond coring system (DCS). The diamond coring system attempts to use slim-line, narrow-kerf, high-speed diamond coring in difficult assignments where high percentage recovery is traditionally impossible for standard floating offshore coring technologies. The system involves hanging a modified 4,500-m mining coring rig in the derrick of the drillship. The platform is compensated by the primary heave compensator.
A 3 1/2-in. OD coring tubing string is deployed through the stationary 5 1/2-in. standard drillstring hanging from the platform. The standard drillstring acts much like a slim riser by confining the slender tubing string and providing lateral stability. A 3.96-in. diamond core bit is rotated at 60-540 rpm by an electric top drive on the platform. Wire line-retrievable core barrels, based on a modified Longyear HQ-3 standard, are deployed through the tubing string. These core barrels capture cores 2.2 in. diameter x 5 or 10 ft long. The system is a more sophisticated scale-up of several "piggyback" coring concepts developed for geotechnical and core sampling ships in Europe, most notably the Norwegian vessel, Bucentaur, which frequently uses a similar system at water depths up to 600 m.
The most difficult components of the system are seafloor interface and secondary heave compensation. A ballasted guidebase on the seafloor provides reentry capability, casing hangoff points, and sufficient weight (50,000 lb in water). The guidebase allows pulling up to 40,000 lb on the 5 1/2-in. drill pipe to maintain the entire string in tension.
A special 30-ft tapered flex joint provides a transition between the guidebase and drillstring and also provides a design weak point for emergency breakaway. Secondary heave compensation of the tubing string is necessary because the primary compensator is not 100% efficient. Its natural inefficiencies, acceptable when drilling with large diameter roller cone bits, produce output variations which exceed the target weight-on-bit tolerance range required for the narrow kerf diamond core bits (500-1,000 lb) in all but unusually calm sea conditions.
This system has its own mud pumps, hydraulic power and control system, and wire line winch for recovering core barrels. The sophisticated control system produces secondary heave compensation by modulating the motion of the dual-feed cylinders to compensate for residual platform motion relative to the seafloor. The built-in feed control system allows gentle touchdown when a hole is spudded or reentered with a fresh bit (Fig. 11) (15270 bytes)(Fig. 12) (17469 bytes).
Three ODP legs were designated engineering legs, in which the primary emphasis was field testing developmental drilling and coring hardware and techniques. Two of these legs were devoted exclusively to shakedown testing of the diamond coring system and the third used over half its time testing that system.
Copyright 1995 Oil & Gas Journal. All Rights Reserved.
