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doi:10.2204/iodp.sp.313.2009

Operational strategy

Drilling platform

The required depth of the boreholes below seabed is 750 m. The water depth at all the sites is shallow (35 to 40 m), and thus a jack-up type rig has been selected to carry out the coring for the project. The drilling platform, chosen by the contractor and inspected by European Consortium for Ocean Research Drilling (ECORD) Science Operator (ESO), is the LB Kayd, which is a 245 class liftboat. Essentially this is a three-legged, self propelled jack-up.

The coring rig will be cantilevered off the bow of the liftboat, between the two forward jacking legs.

The Kayd will have sufficient capacity by way of food and accommodation for 24 h operation but will require frequent resupply, which will be carried out by a contractor-arranged supply boat provisionally on a 7 day cycle. Exact scheduling will be controlled by coring requirements and, particularly, the freshwater requirements of the rig.

Coring rig

The coring rig is an Atlas Copco CS4002 mining rig utilizing flush-jointed mining drill strings sized to allow the larger ones to act as casings if the coring requires it. The rig has a mast capable of handling 6 m string lengths, and coring is done with a top drive system installed in the mast. Wireline operation of the core barrel is conducted through the top drive.

Coring methodology

A conductor pipe will be run to the seabed to protect the first drill string from excessive movement and vibration. Initially it may not be run into seabed to allow core collection from the seabed, but following this it can be run to a suitable depth to maintain top-hole stability for the rest of the coring.

The first coring string will be a Deep Observation and Sampling of the Earth's Continental Crust (DOSECC) lake drilling system with a wireline core barrel and a PHD (mining pipe) or HWT (mining casing) flush-jointed drill string (114.3 mm outer diameter [OD], 101.6 mm inner diameter [ID]). This system has interchangeable inner core barrels. The outer core bit size will be ~160 mm, and the core collected will be ~62 mm diameter. This is the "standard" IODP core size and is collected in identical liners.

The annulus between the hole and drill string is small, which is key to obtaining a stable borehole in delicate formations. The outer core barrel is able to accept a variety of inner core barrels, which will help to maximize core recovery in unconsolidated and consolidated formations. The inner core barrel bit or cutting shoe is interchangeable with the inner core barrel, allowing a good degree of flexibility to ensure that best core recovery and quality are obtained.

The maximum core run length will be 3 m. However, the length of a core run will be geared to maximize the core recovery and maintain hole stability, even if this reduces overall penetration speed. In unconsolidated, sandy, or silty formations, these core runs could well be less than the 3 m maximum length.

Drilling mud will be used to condition the borehole as dictated by the circumstances and the driller's requirements. Fluorescent microspheres will be added to the mud to assist the evaluation of contamination of samples for microbiology studies.

Should it be impossible to reach the required total depth (TD) with a single drill string, the first string will be used as a casing for the completed part of the borehole. With the inner core barrel removed, sufficient clearance through the outer core barrel bit will be made available to allow a second string and core barrel to pass through and thus progress the borehole. This second drill string will also be a flush-jointed mining system (HQ; 96.0 mm OD, 63.5 mm ID) and has a matched HQ-size mining core barrel. The core is also collected in a liner and, because the hole diameter and annulus spacing are smaller, the core collected is essentially the same size as the first core (61 mm diameter).

The liner is standard mining type and different from the IODP liner. Both types of liner were provided to the curation and petrophysics teams for calibration.

Drilling mud will also be used to condition this section of the borehole and to ensure the top of the section does not deteriorate further than the point when it caused the second string to be deployed.

Appropriate systems for gas monitoring will be used throughout the expedition. They will be contractor- and ESO-supplied. Note that although such systems will be able to detect and identify gas, they are empirical or semiquantitative at best and are not analytical instruments.

Core on deck

Once the drilling operation commences and core begins to arrive on deck and after initial labeling of cores, the operations team will be responsible for delivering the cores to the curation container. The operation will proceed using a changeover of inner core barrels to ensure continuity of the coring operation in as timely a fashion as possible. The deck operators will deploy an empty core barrel immediately after the previous one has been retrieved and then address the core removal and subsequent readying of that core barrel for reuse. As the cores will be collected in a plastic liner, the usual IODP curation procedures will be followed and will be documented in an ESO Handbook.

Downhole logging

During all expeditions the downhole logging program will be integrated with the scientific objectives to ensure maximum scientific output. This may include the use of specialist third-party tools.

To facilitate downhole measurements and core petrophysics for mission-specific platforms, the European Petrophysics Consortium (EPC) has been developing protocols for use both offshore and as part of the Onshore Science Party.

Unlike the D/V Chikyu and riserless vessels where the pipe size will be constant and allows a standard set of logging tools to be deployed, mission-specific platforms have variable pipe sizes and drill in a variety of water depths, each of which provides constraints on the anatomy of logging operations. Pipe diameter is the controlling factor, and it is envisaged that a wide range, from slim-line memory-mode tools to standard oilfield tool suites, may be utilized. Water depth is also an important constraint because some mission-specific platform expeditions will operate in very shallow territorial waters where the deployment of nuclear sources may be prohibited or be severely restricted.

This service will be contracted as part of the services for the New Jersey shallow shelf expedition and will be managed by the EPC. The logging equipment and team will be interfaced for a seamless operation on the platform, ready to undertake any requirements as the project progresses.

Priorities and potential program of work

All aspects of the following work program are subject to change as our knowledge of hole conditions and the timing of operational activities improves.

The New Jersey sites will be attempted in the following order: MAT-1, MAT-2, and MAT-3. Site MAT-2 is considered to be the most scientifically critical site. Site MAT-3 is considered to be the least scientifically critical site and may not be cored in its entirety if time runs out toward the end of the operation. If coring at Site MAT-1 is problematic and slow, the Eocene section below the "o1" reflector at Site MAT-1 will not be cored, as it is considered to be a low-priority interval.

In all holes, we will core to TD (750 m) or the maximum depth possible, using the PHD string. Before continuing with the smaller diameter HQ string if required, a total gamma ray log will be taken through the PHD casing. After HQ coring, a total gamma ray log will be taken through the HQ-cased section of the hole. Drilling will be paused to take in situ borehole temperature measurements if borehole conditions are favorable.

Open-hole logging will commence after coring, most probably in increments to reduce the risk of hole instability halting the logging runs. The exact pull-back increments will be defined on site when we will have better knowledge of the formations. If HQ coring is utilized, the minimum pull-back of the first increment is at the depth where HQ coring commenced.

Normally, and where hole stability allows, five wireline logging tools will be run at each site in this order: induction resistivity, magnetic susceptibility, sonic measurements, spectral and total gamma ray, and mechanical caliper. The induction tool is the most robust tool in this suite and, with minimal risk of damage, will confirm whether the hole is open for additional logging tools while providing information about porosity and formation fluids.

Vertical seismic profile (VSP) logging will commence over the entire open hole after all wireline logging is finished. If an unstable hole is indicated, a wiper trip to recondition the hole will be conducted before VSP logging is attempted. If the hole is found to be very unstable during the wireline logging runs, VSP logging will be attempted in increments, essentially as the fifth tool in the logging suite.

VSP logging of Site MAT-2 is critical to meet the scientific objectives of the expedition. If VSP logging of the first hole at Site MAT-2 is unsuccessful, a dedicated hole for VSP logging may be drilled using a full-face bit (no coring) despite the potential reduction in available coring time this might cause at Site MAT-3. This will only be undertaken if VSP logging was successful at Site MAT-1, which will have demonstrated that the expedition can run VSP logging in difficult, sand-prone formations. If Sites MAT-1 and MAT-2 both prove too challenging for VSP logging, then VSP logging will be attempted at Site MAT-3, which is suspected to be the least sand-prone site.

If the borehole remains open after VSP logging, and if time allows, a second low-priority suite of wireline tools may be run: hydrogeological, and acoustic imaging (including acoustic caliper).

Marine mammal observation will be conducted by trained observers during VSP surveying.

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