MODIFIED WIRE LINE TOOLS IMPROVE OPEN HOLE LOGGING OPERATIONS

David P. Huey
Stress Engineering Services Inc.
Houston
Michael A. Storms
Ocean Drilling Program
Texas A & M University College
Station, Tex.

Specially developed tools and modified conventional oil field wire line equipment have made running open hole logs easier and more practical.

Open hole logging is an important element in Ocean Drilling Program (ODP) scientific investigations, and the ODP Borehole Research Group provides state-of-the-art logging tools, sometimes even beyond those routinely available to the oil and gas industry. The most advanced tools tend to be in large diameter (3 5/8 in.).

To pass these tools through the ODP drillstring, the flapper-type float valve conventionally installed above the 11 7/16-in. OD x 3.80-in. ID core bit must either be removed or locked open while the logging tool is run. A lockable float valve was developed specifically for this purpose (Fig. 1) (11988 bytes).

The lockable float valve acts as a standard, full-open/full-close, spring-loaded flapper valve during routine coring operations. A wire line core barrel landed in the bottom hole assembly (BHA) holds the flapper open. The flapper then closes as soon as the core barrel is retrieved.

Any logging tool passing below the lockable float valve assembly leaves the flapper locked open. When the logging tools are pulled up, the flapper valve is unlocked and closes. The lockable float valve is completely mechanical.

The unlocking mechanism has dual redundancy so that both triggers must be activated simultaneously to unlock the flapper. This feature prevents inadvertent unlocking while the logging tool is downhole and the logging cable is in the throat of the lockable float valve.

To lock or unlock the flapper, the logging tool suite must have a 3 3/4-in. diameter Straight section at least 9-in. long somewhere on the assembly. For very thin tools (such as temperature tools and borehole televiewers), a special triggering bullnose is usually added to the tool assembly to activate the lockable float valve.

If this setup is not feasible, the lockable float valve can be permanently locked open in preparation for logging with a special one-shot go-devil. If a core barrel or larger diameter logging tool is then subsequently run, the lockable float valve will automatically reset.

LINE SAVER

The lockable float valve is built for reliability, but like all downhole tools, it may fail on occasion. If the flapper unlocks with the logging tool downhole, the fall back procedure is to cut the logging line to remove it from the drillstring so that the drillstring can be tripped.

A pair of go-devil tools is used to prevent losing the logging tool in these situations. The go-devils grab the line leading to the logging tool and cut the line near the bottom so that it may be removed rapidly by the logging winch. Virtually no logging line is sacrificed.

(Cut-and-strip-over operations may also be used but often have complications endangering the logging tool. In any case, cut-and-strip-over procedures generally sacrifice the exposed length of logging cable and are a slow and inefficient use of rig time.)

Other problems can require emergency retrieval of logging tools when they are unable to reenter the pipe from open hole (for example, "bird's nest" snarls in the logging line, broken centralizer arms on the logging tools, or entrained stones or clay in the logging tool).

The line saver tools were developed from minor modifications to an explosive wire line cutter which has been marketed by the Kinley Corp. for decades. Two variations were developed:

A crimper is dropped down the pipe to the bit and firmly captures the logging line without cutting it.
The crimper tool has a stout landing shoulder to prevent it from exiting the bottom of the drillstring. Thus, the crimper forces the logging tool suite to follow the drillstring as it is tripped out of the hole.
A conventional Kinley cutter is dropped after the first tool and cuts the line above the crimp point. The logging cable is then winched back, the drill pipe is tripped out of the hole, and the endangered logging tool suite is found dangling from the crimper tool below the bit.

ODP has saved logging tools worth several millions of dollars using this technique on several occasions.

SIDE-ENVY SUB

Holes drilled with seawater as the primary circulating fluid are often so unstable that open hole logging after drilling is sometimes virtually impossible because of bridges that form when the drillstring is pulled to an upper level to expose open hole.

This situation can be alleviated by exposing only the amount of open hole that the logging tool needs to measure at one time. Starting at total depth and raising first the drillstring and then the logging tool will expose the entire borehole for logging without having to force the lightweight, vulnerable logging tools down through bridges that are usually impassable.

The pipe can either be pulled all the way to immediately below the seafloor in one trip, followed by the logging tools, or the pipe and logging tools can be moved up alternately in increments.

For removing or adding pipe with the logging tool in the hole, the drillstring must have a side-entry sub. The logging cable enters the side-entry sub at some point below the rig floor.

Conventional side-entry subs available on the market did not have sufficient ID for the ODP logging tools or were not designed with sufficient load capacity for ODP deepwater operations. Therefore, ODP and Stress Engineering Services developed a conical side-entry sub expressly suited for ODP (Fig. 2) (17590 bytes).

This conical side-entry sub was designed without the conventional restriction of a confined cylindrical shape. In ODP operations, the conical side-entry sub does not have to enter the hole or pass through casing or riser pipe.

Thus, the tool was designed to allow both logging line and the logging tools to enter through the side door. This unique feature allows much faster changing of logging tools than with conventional side-entry subs because the sub does not have to be removed from the drillstring for tools to be loaded or unloaded.

This advantage saves marginal time if only a single suite of tools is run, but the time savings become significant if multiple suites are run or if problems require the tools to be removed for calibration or troubleshooting.

Another advantage of this design is the ability to extract the logging tools entirely while the drillstring is downhole and the side-entry sub is located well below the ship's keel. This operation is necessary if the drillstring becomes seriously stuck in the hole during logging operations.

By first removing the logging tools, the driller regains the ability to rotate the string. If further attempts to free the stuck pipe are unsuccessful, the logging cable is then used to convey severing tools into the hole to cut the pipe. Without the conical side-entry sub, such operations would be greatly complicated, and the logging tools would be inevitably lost downhole.

SPECIAL INSTRUMENT SYSTEMS

One of the most novel ODP developments is a system to monitor the core as it enters the core barrel. The sonic core monitoring system is based in part on a prototype system first developed by Reed-Dowdco and later acquired by Diamant Boart Stratabit.

When ODP first showed interest in continuing the development, the sonic core monitor was a low-priority research project at Diamant Board Stratabit.

The sonic core monitoring system is a battery-powered tool that tracks the top of a core as it enters the core barrel and records the increasing core length vs. time in a solid-state memory.

A 200-khz signal is sent down the inside of the core barrel and deflects off a small target riding up on the core. When the core barrel is returned by wire line to the deck, the contents of the memory are transferred to a computer.

The core length-vs.-time record is then compared to the driller's penetration record to determine where in a cored interval a particular piece originally resided. Because most ODP cores are usually not 100% recovered, this technique is the only way of correlating the partial recovery accurately with depth.

Magnetic core orientation is possible with a rotary coring techniques if the incoming hard core is scribed such that later decoding can identify north relative to the scribe marks. The changing azimuth direction of the prime scribing knife is recorded over time by a commercial electronic hole-survey tool.

This technique, already used in oil and gas industry coring and in slim line diamond drilling, is only successful if nearly 100% of the core is recovered.

With significantly less than 100% core recovery in hard rock, there is no way to relate a particular section of scribed core to the survey record, The problem is not knowing when the actual scribed core piece enters the core barrel. The sonic core-monitor system provides this missing piece of data.

The combination of the sonic core monitor, core scribers, magnetic survey tool, and recording systems are called the hard rock orientation system (Fig. 3) (21989 bytes). Each of these components has been tested successfully individually, but the system as a whole has not yet been deployed for orienting hard rock cores.

The sonic core monitor, normally reliable in soft formation coring, has a "blind spot" at the start of each hard rock core. ODP engineers are still attempting to define and correct this problem.

Future plans for the sonic core-monitor system call for a measurement-while-coring mud-pulse telemetry system for real-time monitoring of core entry at the rig floor. The driller may then quickly vary coring and drilling parameters to maximize core recovery and give absolute indication of core jams or core loss from washing or faulty core catchers.

IN SITU TEMPERATURE

In hydraulic piston coring, the core barrel extends 91/2 m ahead of the roller cone bit and is the first tool to reach a geological horizon. Each piston corer penetration occurs minutes or hours before the main borehole is deepened. The hydraulic piston corer presents an opportunity to make certain measurements in never-before-reached geological environments before there is any disturbance from the borehole.

In 1980, Deep Sea Drilling Project (DSDP) engineers in conjunction with scientists and technicians at Woods Hole Oceanographic Institute developed a battery-powered temperature instrument which could be housed in a pressure chamber built into the 9/16-in. wall of a special piston corer cutting shoe (Fig. 4) (16993 bytes).

The tiny instrument, called a heat-flow probe, was a dedicated data logger used to record the temperature of a thermistor located less than 1 in. from the end of the cutting shoe. The electronics and small battery pack are housed in a two-part, 3 1/2-in. OD advanced piston-corer cutting shoe in a pressure-tight cavity. A computer interface allows programming and data retrieval.

A larger, upgraded version of the heat-flow probe produced by Adara of British Columbia has more battery life, advanced programming options, and better software for data analysis.

REENTRY CONE SEAL

An ODP borehole with a reentry cone installation and one or more casing strings cemented in place over a section of stable open hole is a "window" into the largely unexplored geological world below the seafloor.

The value of the borehole as a site for geologic observation increases over time as the downhole environment returns to natural conditions and borehole disturbances from drilling diminish.

ODP's reentry cone seal (called a "cork") isolates the 11 3/4-in. casing string and reentry cone to prevent flow of seawater in or out. The cork assembly houses a battery-powered data logger attached to a multisensor thermistor string.

The string is installed in the borehole with the thermistors spanning the open hole section. It records pressure in the borehole and at the seafloor. A Teflon tube can be installed to enable fluid sampling from a selected depth below the cork.

The cork mechanical assembly can only be installed or removed by a drillship, but the data logger, thermistor string, and fluid sampling tube can be removed or replaced from an oceanographic research vessel.

Access to download the data logger, recover fluid samples, or change batteries is also possible from a research submersible or remotely operated vehicle (ROV). Six cork systems have been installed worldwide.

The data logger was developed in conjunction with the Pacific Geoscience Centre, Victoria, B.C. The thermistor string was developed by the University of Miami, and the ROV-interface system was produced in conjunction with Lehigh University, Bethlehem, Pa.

TECHNOLOGY TRANSFER

The technology transfer from DSDP and ODP to industry or other research organizations has been spotty.

On one hand, neither DSDP nor ODP has attempted to patent any new inventions.

On the other hand, DSDP and ODP have never discouraged technology sharing with industry and have themselves benefitted many times from industrial assistance and cooperation in the pursuit of new engineering developments for the advancement of ocean science research.

In the few cases in which outside organizations sought specific information or assistance regarding a tool or technique, reasonable efforts were normally made to accommodate the request.

A formal technology-transfer mechanism or policy has yet to be established by ODP, however. Past requests for detailed technological information have been handled on a case-by-case, time-available basis, and this approach is likely to continue.

At present the National Science Foundation (NSF) does not mandate nor specifically fund ODP for the purposes of expediting technology transfer to industry.

Over the course of DSDP and especially in the final months of the project (198384), the technology developed was partially documented in a series of 23 technical reports produced by Scripps Institution of Oceanography on behalf of the NSF and circulated to many libraries worldwide.

These reports included accounts of individual engineering developments for particular downhole tools (hydraulic piston corer and wire line pressure core barrel), general descriptions of engineering problems addressed over a long period (dynamic positioning and measured stresses in very long drillstrings), and detailed accounts of actual drilling and coring operations at sea for every DSDP scientific leg.

These reports are still available from the National Technical Information Service, U.S. Department of Commerce, Springfield, Va.

The emphasis on engineering development during the ODP years was and remains greater than that during the earlier DSDP efforts. Virtually all of the tools, techniques, and systems developed during DSDP were improved upon or replaced by new developments by ODP. Consequently, much of the information in the DSDP technical reports no longer represents the current state of the art, yet the information has value as background.

ODP engineers have embarked on a limited program to document recent engineering developments formally. These reports are produced and circulated as ODP technical notes, although the reports also include science-related narratives on non-technological subjects. (See bibliography in Part 1, OGJ, Jan. 16.)

Engineering accounts of drilling and coring operations and general equipment development for each ODP leg have been written but not yet cleared for external circulation or publication.