LOGGING

ODP Logging Services

ODP Logging Services provides downhole logging operations, as well as logging data processing, distribution, and database services for ODP. ODP Logging Services is managed by the Borehole Research Group of LDEO, but also includes logging groups in the United Kingdom, France, Germany, and Japan. ODP Logging Services is responsible for (1) shipboard logging operations and staffing, (2) shore-based log analysis, (3) log database development and management, (4) data publication and distribution, and (5) engineering development.

Situations to Avoid while Logging

Holes containing bridges and ledges can pose extreme risk of loss to logging tools. Numerous scenarios to be avoided are detailed in the Logging Manual (Borehole Research Group, 2004) and therefore will not be listed here. Use of the CSES can greatly assist logging operations in difficult holes; therefore, its use should be thoroughly considered.

High-Temperature Logging Precautions

There are several procedures that should be followed before, during, and after logging operations in high-temperature environments. Prior to the leg, the Logging Staff Scientist should make arrangements with the logging subcontractor and Logging Engineer to have the capability of measuring in situ borehole fluid temperatures during all tool deployments. Discussions prior to the leg should also include the availability of high-temperature wireline cable and cable heads. This will ensure that high-temperature logging operations can be carried out during the leg and that borehole temperatures will be monitored closely, thus avoiding potential damage to the tool strings.

The Logging Staff Scientist and the Operations Manager should plan to perform several hours of hole circulation procedures before any tool deployment if the temperatures exceed the safe operational limits of the tool strings. In some cases where there is a quick thermal rebound, the deployment of the sidewall entry sub (CSES) might be necessary for avoiding tool damage and saving time if more hole circulation is needed once the logging operations have already begun. The Logging Staff Scientist, Operations Manager, and Co-Chief Scientists should also discuss time estimates, potential benefits, and procedures for such deployment.

Cautionary measures should be taken at the time of retrieving a tool string, as hot fluids may spray a large area of the rig floor. Significant amounts of H2S and hot fluids may also concentrate along joints in large tool strings; therefore, protective clothing and eyewear should be used when dismantling the tools. If a memory tool that uses lithium batteries has been deployed in these environments, extreme caution will be needed before dismantling the tool, as exploding batteries can be extremely harmful.

After logging operations in a high-temperature environment have been completed, the Logging Staff Scientist and Logging Engineer should conduct a careful inspection of the wireline cable and tools in order to assess any potential damage from prolonged exposure to H2S. At this time, it may be necessary to discard sections of the cable that show signs of corrosion due to exposure to high concentrations of H2S.

Logging-While-Drilling Precautions

Drilling operations with logging-while-drilling (LWD) and measurement-while-drilling (MWD) collars can proceed by following standard drilling guidelines under most circumstances. Because the physical nature of these tools is vastly different than that of a standard drill collar, special care and attention must be paid to key drilling procedures in order to avoid a stuck or lost drill pipe situation. LWD and MWD collars deployed in ODP operations are typically 6 inches in diameter, whereas drill collars are >8 inches. This difference in size creates two problems: (1) a stabilizer on the density neutron tool must be used to ensure constant contact with the borehole wall and (2) the interface between the drill collars and LWD/MWD collars is not as strong as a drill collar to drill collar connection. Caution must be used when spudding into a hardened or crusty substrate to avoid excessively loading the drill pipe and possibly causing a weak-link failure between the drill collars and LWD/MWD collars. Caution must continue to be used until the LWD/MWD collars have penetrated below the seafloor.

Jars can be used to provide assistance in freeing a stuck BHA. In past ODP experience, however, jars were frequently jammed by cuttings or were located below the stuck point, where they could not be operated. In addition, jars further weaken the BHA and the sediments may leak, providing undesirable pathways for downgoing circulated seawater.

In the event an LWD/MWD collar becomes irretrievably stuck, the stored data and radioactive sources must be retrieved using the wireline LWD Inductive Coupling (LINC) tool. The LINC retrieval operation consumes approximately the same amount of time as one standard wireline run.

Fluid Pumping Strategies

Drilling muds such as sepiolite are used to stabilize the borehole prior to logging. Fluids are never pumped while a tool is in the open hole or while a logging tool is in the BHA. If drilling in unconsolidated materials and the LWD/MWD collars become lodged, make all attempts using standard drilling techniques to free the tool. If this is unsuccessful, allow the formation to relax by not pumping for 10–15 min and then apply overpull. This scenario occurred during Leg 174, where the LWD collar was considered hopelessly stuck, yet it was eventually recovered after pumping ceased.

Overpressure

LWD/MWD tools are most often used where difficult formations are expected and hole stability is a significant concern. Environments such as convergent margins and, in particular, décollements pose a serious risk to high-dollar drilling equipment. More conservative drilling techniques must be used to prevent a LWD/MWD assembly from becoming lodged in a zone of overpressure where hole instability is a possibility.

Recovery Attempts and Tool Abandonment

If a tool is lost downhole, a reasonable effort must be made to recover it to satisfy our obligations to the environment, Schlumberger, and the insurance provider. The recovery effort should follow accepted practices and include multiple recovery attempts if technically feasible. The shore-based ODP Logging Services representative must be notified of the stuck or lost tool situation by the Logging Staff Scientist or the Operations Manager.

If all reasonable efforts have been made to recover a stuck or lost tool without success, then the decision to abandon the tool must be made collectively by the Logging Staff Scientist, Operations Manager, OIM, Co-Chief Scientists, Staff Scientist, and the Schlumberger engineer. In the event of loss involving a radioactive source, the tool and hole must be filled with cement, plugged, and abandoned to safely entomb the sources. Following the incident, a report must be filed by the Operations Manager and delivered to the Logging Staff Scientist for possible use for insurance purposes. A copy of the ship's log must be included in this report.

Tool Replacement Strategies

If a wireline or LWD/MWD tool is lost downhole, a backup tool should be put into service only after an appropriate recovery effort. Duplicate LWD/MWD and wireline tools are often, but not always, available. Substitutes for all routine measurements are available. The backup strategy is shown in Table T6.

Hazardous Material Safety

Logging operations often involve the use of radioactive sources and, seldom, explosive sources. The Schlumberger or Anadrill engineer is trained and qualified in the safe handling and use of such sources. The radioactive and explosive materials must only be handled by authorized personnel. Several key safety steps must be followed by all other shipboard personnel when radioactive or explosive source handing is occurring: (1) all personnel besides the logging engineer must clear the vicinity of the source work and (2) when a source is loaded into a tool or collar, the tool must not be raised above the rotary table when personnel are on or near the rig floor. Additionally, electronic neutron generators (minitrons) must not be switched on when the tool is above rotary table.

The Schlumberger engineer will maintain an up-to-date hazardous material manifest, and a copy will be provided to the Master. Any changes to the hazardous material manifest are registered with the bridge.

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