Operations plan and drilling strategy

The overall operations plan for Expedition 336, including target depths and anticipated cored intervals, is presented in Table T2. The highest-priority objective for this expedition is the installation of the three long-term CORKs. The exact sequence of operations presented here and in Table T2 may change based on the results of ongoing CORK design and development as well as continuing evaluation of risks and contingency plans.

APC coring at Sites 1074 and 395

We will begin with a single APC-cored hole at Site 1074 to recover the entire ~64 m thick sedimentary section. Without retrieving the drill string, we will move to nearby Hole 395A and APC core a single hole to recover the entire ~93 m sedimentary section. These cores will be the focus of intensive onboard incubation studies and analytical programs. In parallel with sediment coring, we will begin preparing all of the experiments that will be deployed with the CORKs.

Hole 395A

Our next operation will be to remove the old-style CORK and thermistor string currently in Hole 395A. We will then deploy the water-sampling temperature probe (WSTP) and a self-contained temperature logging device to collect a deep sample of the borehole fluid at 550 mbsf. This process will be completed without circulating fluids to minimize borehole disturbance. After logging operations (see "Logging/​Downhole measurements strategy"), the status of the borehole will be checked and the hole will be cleaned to 610 mbsf with the least amount of fluid circulation possible to ensure that the hole is open for the CORK and that the hole is not overly disturbed with the introduction of surface seawater. Given the underpressure relative to hydrostatic at Hole 395A, we expect bottom seawater that flows into the borehole to flow primarily into the upper permeable formation and less so into the lower, warmer, less permeable formation below 350 mbsf. Provided the hole remains open to 600 mbsf, we will install a new multilevel CORK with downhole and surface experiments, as illustrated in Figure F5. Should the hole contain more than the anticipated amounts of fill, circulation will be used to clean the hole so that it is suitable for CORK emplacement.

Site NP-1

Following the transit to Site NP-1, we will install the "deep" CORK observatory. After installation of a reentry cone and surficial 20 inch casing, we will drill a hole into uppermost basement for the 16 inch casing with a pilot bit and a 21½ inch underreamer through the ~85 m of sediment and into uppermost basement. We will then install and cement the 16 inch casing into uppermost basaltic basement. To minimize the risk of the casing strings and reentry cone sinking below the mudline (as happened during Expedition 301), we will use a special cement program (with lost circulation material and perhaps accelerants). We will then reenter the hole with a 14¾ inch tricone bit to drill out the cement and make a hole to ~215 mbsf for the 10¾ inch casing. This section will not be cored in order to allow rapid penetration in the most unstable uppermost basement. Whenever possible, while drilling in basement we will use a bottom-hole assembly (BHA) that consists of extra 8¼ inch drill collars to maximize the amount of slick pipe that is exposed to the unstable basement formations and keep the top of the BHA inside the 16 inch casing. After thoroughly cleaning the hole and verifying the amount of fill on the bottom, we will install the 10¾ inch casing. To ensure rapid emplacement of the casing string and unimpeded landing of the 10¾ inch casing hanger, we will install this casing without stopping to install the cementing manifold and subsea release system. We intend to use a special cement program (with lost-circulation materials and/or accelerants) for this casing string as well.

If we encounter substantial difficulties while drilling the 14¾ inch hole for the 10¾ inch casing, we may elect to use Site NP-1 as the shallow basement CORK observatory.

After tripping out of the hole to change to a 9⅞ inch RCB bit, we will drill out the cement plug and cut RCB cores from ~215 to ~415 mbsf in basaltic basement. After we change the RCB bit, RCB coring will continue to ~565 mbsf. The actual depth of penetration may differ, depending on rates of penetration, drilling conditions, and the nature of the rocks recovered. However, all of our operations (including anticipated hole conditions and penetration rates) are based on previous drilling in North Pond. Once coring is finished, wireline logs will be obtained to identify optimal placement of straddle packers as well as to provide formation properties of the oceanic crust (see "Logging/​Downhole measurements strategy"). Our next step will be to conduct drill string hydrologic (packer) tests at three different depths. After verifying the depth of the open hole, the multilevel CORK will be deployed. This CORK will be configured to isolate three intervals of upper basement and will include borehole and surface instrument packages (Fig. F6).

Site NP-2

Initial activities at Site NP-2 will nearly duplicate those at Site NP-1, except that the reentry cone will be installed with 16 inch casing to ~62 mbsf. We will then use a 14¾ inch tricone bit to drill out the remaining sediment and basement to ~105 mbsf for the 10¾ inch casing. The 10¾ inch casing will be cemented in place using the same procedure employed at Site NP-1.

After performing a round-trip of the drill string to change to a 9⅞ inch RCB bit, we will drill out the cement with the RCB bit and then RCB core from ~105 to 175 mbsf. Collection of downhole logs is included in this plan; however, there will be a very short interval of open hole, we expect poor hole conditions in this uppermost basement, and the priority is to conduct hydrologic (packer) experiments and install the CORK. Hydrologic (packer) tests will be conducted, and then the open hole depth will be verified before deploying a single-level CORK observatory. The CORK will be configured to isolate the uppermost basement below the sediment. This CORK will be instrumented with borehole and surface instrument packages (Fig. F7). The final configuration of the deepest portion of each CORK will be similar (Fig. F8). The retrievable experiments within the CORKs are intended to have identical instrument packages with four different basic configurations for different experiments (Fig. F9).

The last operation at Site NP-2 will be the recovery of a single copy of the entire ~85 m thick sedimentary section with the APC coring system, tagging basement. In the current operations plan and time estimate, there will be time to APC core only one hole at Site NP-2. If additional time is available, we may core a second or third hole at this site.

Site NP-1

Our final operation will be the recovery of a single copy of the entire ~64 m thick sedimentary section at Site NP-1 with the APC system. Once again, additional time at this site may be used to core extra APC holes or perhaps a single RCB hole to collect upper basaltic basement.

Hydrologic (drill string packer) testing at Sites NP-1 and NP-2

In situ hydrogeologic testing is essential for quantifying crustal properties (transmissive and storage) that control ridge-flank hydrothermal circulation. For this testing, we will use a drill string packer, which is reliable and relatively easy to integrate, as part of a comprehensive program of basement drilling, sampling, and experiments. Tests at multiple depth intervals will be conducted to discern difference between test results at two depths and to quantify differences in hydrologic properties within discrete depth intervals.

The drill string packer will be made up as part of a BHA that is compatible with logging so that a separate pipe trip is not required. Wireline logs are run before packer testing to assist with identification of suitable zones (massive, in gauge) at depths where the packer element can be inflated. Tests at multiple depth intervals within the open borehole at Site NP-1 will follow the standard approach: (1) inflate and set the packer at the deepest setting point first, (2) complete testing at that depth, and (3) deflate the packer and raise it up to a shallower depth and repeat the tests. The difference between test results at two depths can be used to quantify differences in hydrologic properties within discrete depth intervals. Tests are generally repeated at each testing depth, using two or more different pumping rates, to verify test response and formation properties.

Neglecting the time required to trip pipe into and out of the borehole, an experimental program involving packer testing at 2–3 depths in open hole and casing in a 500 m deep basement borehole will generally require 24–30 h. Testing of the shallower penetration hole at Site NP-2, based on setting the packer at a single depth (likely in the base of the 10¾ inch casing), might require 12–18 h. Additional time may be used to collect reliable baseline pressure data, set the packer at additional depths, or test at additional flow rates.

Pressure data are generally collected using autonomous downhole pressure gauges that are suspended below a "go-devil" that is dropped into the packer when it is positioned at depth in the borehole. These gauges are recovered when the complete testing sequence has been run. The shipboard Rig Instrumentation System (RIS) is used to record key pumping parameters: time, stroke rate, total number of strokes, BHA depth, and standpipe pressure, which is a backup for downhole pressure records.