Site GUATB-03C, the proposed site for all drilling operations, was selected from the region surveyed to take advantage of faster upper crustal velocities, which may indicate the presence of massive basalt flows, and to avoid thrust faults that occur elsewhere in the region. The estimated sediment thickness of 240 m is more than sufficient for installing a reentry cone with 20-in casing and establishing a reentry hole for deep penetration into basaltic basement.
Our plan is to first core the sedimentary section and upper couple meters or so of basement with the advanced piston corer (APC) and extended core barrel (XCB) system (Table T2). Second, we will conduct a jet-in test to establish what length of 20-in casing can be jetted into the sediment. The test is conducted by setting the XCB bit on the seafloor and then circulating fluids through jets in the bit. The bit is gradually jetted into the sediments until further progress is inhibited by the increased induration of the sediments. Next, we will trip the pipe to the surface to switch to the rotary core barrel (RCB) system, trip the pipe back to the seafloor, drill through the sedimentary section to within about 20 m of basement, and then proceed with RCB coring to bit destruction. The RCB bit life is likely to be ~60 hr of rotation in basalt, which, at the estimated rate of 2 m/hr, will allow us to core ~120 m into basement. After preparing the hole for logging, the bit will be dropped in the bottom of the hole and the pipe raised to ~100–150 mbsf for logging. The basement and lower portion of the sedimentary section will be logged with the triple combination (Triple Combo) and the FMS/sonic tool strings and the BGR borehole magnetometer as described in the "Logging Plan" section.
This pilot hole will allow us to assess basement hole stability and to determine the depth to which the 16-in casing string should be run. Ideally, the casing will only need to extend into the upper few meters or tens of meters of basement, with the goal of casing off the lower sedimentary section, the sediment/basement contact, and any rubbly or fractured basalt that occurs at the top of basement. It is possible, however, that a greater amount of the basement will need to be cased. Thus, we provide two scenarios for the reentry hole (Table T2). In the first scenario (Fig. F12), only two casing strings are required, the 20-in and 16-in strings, with the 16-in casing string extending 20 m into basement. In the second scenario (Fig. F13), three casing strings are required. The depth of each casing string is purely hypothetical at this point, as the actual depths will depend on where or if unstable hole conditions are encountered.
Both scenarios begin with jetting-in the reentry cone with ~80 m of 20-in casing. A pipe trip is required to change to a new 18-in-diameter bi-center bit that cuts a 211/2-in-diameter hole. Bi-center bits have not been used previously by ODP. Their advantage is that they can fit into holes or casing with a diameter as narrow or narrower than the diameter of the hole they cut. Prior to acquiring the bi-center bit, we had planned to use an 18.5-in-diameter tricone bit in basement. The larger diameter hole created by the bi-center bit provides a greater safety margin for installation of the 16-in casing. The bi-center bit will thus be used to drill a 211/2-in hole through the sediment and into basement.
Assuming favorable hole conditions are encountered in the pilot hole, then we will follow the first scenario. In this scenario, the 211/2-in hole is drilled 30 m into basement. The hole is then cased with 16-in casing 20 m into basement, leaving a 10-m rat hole below casing. The casing is cemented into basement, and the pipe is tripped to the rig floor. Operations then consist of RCB coring, with multiple bit changes as necessary, until coring time expires. Given the estimated bit life and rate of penetration, we anticipate coring to ~1050 mbsf, or ~810 m into basement. Following completion of coring, the hole would be logged with the triple combo, the FMS/sonic, the BGR borehole magnetometer, the Ultrasonic Borehole Imager (UBI), and the three-component WST-3 logging tools as described in the "Logging Plan" section.
If unstable hole conditions are encountered in the basement, then operations may follow something like this second scenario. Again, after drilling through the sediments, the 211/2-in hole is drilled, though this time it penetrates 120 m into basement. This scenario assumes the upper 100 m or so of basement is found to be unstable in the pilot hole. The 16-in casing is thus installed and cemented 110 m into basement. RCB coring then proceeds. In this scenario, an unstable portion of the hole is encountered above 480 mbsf, though below the total depth of the pilot hole. In order to continue operations in the hole, casing would need to be set through the interval. First, however, we would attempt to log the interval because it would not have been logged in the pilot hole. The same tool strings as used in the pilot hole will be used for this interval. Depending on the depth of the unstable interval, either 133/8-in or 103/4-in casing could then be used. A decision to use 133/8-in casing would allow an additional 103/4-in casing string to be used later if necessary, though it also requires that we use a new bi-center bit. A 141/2-in-diameter bi-center bit that cuts a 181/2-in-diameter hole will be used if 133/8-in casing is installed. Again, the larger diameter hole cut by the bi-center bit provides a greater safety margin for installation of the 133/8-in casing. In this hypothetical second scenario, we open the hole with the bi-center bit to a depth of 490 mbsf and then install and cement the 133/8-in casing to 480 mbsf. Operations then consist of RCB coring, with multiple bit changes as necessary, until coring time expires. We anticipate coring to ~770 mbsf, or ~530 m into basement. Following completion of coring, the hole would be logged with the triple combo, the FMS/sonic, the BGR borehole magnetometer, the UBI, and the WST-3 logging tools as described in the "Logging Plan" section.
Should the primary site prove to be unstable or should operations in the reentry hole become impossible for other reasons, we would plan to move to one of the alternate sites and repeat as much of the above plan as possible.