Drilling and coring strategy

Proposed drill sites

Proposed Site IND-01A (alternate proposed Site IND-02B) targets the recovery of sediment that will be used to investigate the role of orbital processes in monsoon and environmental development. This site is intended to provide high-resolution weathering and erosion records on millennial timescales. Based on experience at nearby shallow core sites (Gupta et al., 2011; Singh et al., 2011), this proposed site will also provide paleoceanographic data from foraminifers hosted within the sediment. In contrast, a short transect designed to sample a continuous long-term record (proposed Sites IND-03C and IND-04A) by sampling as deep as the acoustic basement will provide information for the entire Cenozoic history of weathering and erosion within the Indus basin. These deeper sites will also elucidate the syn- and postrift history of the region. Special attention will be paid to the 258 Ma period, during which crucial environmental and geodynamic changes are known to have occurred. We particularly focus on the time around the initial exhumation of the Greater Himalaya (~23 Ma) in order to test the model that this was climatically triggered, as well as also reconstructing the interval at ~15 Ma during the mid-Miocene Climatic Optimum to assess the nature of the monsoon under those forcing conditions.

Drilling and coring operations

The overall operations plan and time estimates for Expedition 355 are summarized in Tables T1 and T2. An alternate site has also been established and could be cored if time is available (Tables T1, T3). Time estimates are based on formation lithologies and depths inferred from seismic and regional interpretation. After departing Colombo, we will transit for ~3.6 days to the first site and prepare for drilling operations.

Our coring strategy will consist of advanced piston corer (APC) coring using nonmagnetic core barrels in two holes at each site to ~250 meters below seafloor (mbsf) or APC refusal. In addition, the FlexIT core orientation tool will be deployed above the core barrel during APC operations in at least the first hole (A) at each site so that the cores can be oriented. For planning purposes, APC refusal depth is estimated at 250 mbsf, although this could be exceeded at some sites. APC refusal is conventionally defined in two ways: (1) the piston fails to achieve a complete stroke (as determined from the pump pressure reading) because the formation is too hard or (2) excessive force (>60,000 lb; ~267 kN) is required to pull the core barrel out of the formation because the sediment is too cohesive or “sticky.” In cases where a significant stroke can be achieved, but excessive force cannot retrieve the barrel, the core barrel can be “drilled over” (i.e., after the inner core barrel is successfully shot into the formation, the bit is advanced to some depth to free the APC core barrel). Time permitting, the half-length APC (HLAPC) with a 4.8 m advance may be used to advance the APC total depth, although the FlexIT orientation tool cannot be used with the HLAPC. When APC refusal occurs in a hole before the target depth is reached, the extended core barrel (XCB) may be used to advance the hole.

The target depth at all primary sites is greater than the anticipated APC refusal depth. The second APC hole at each site will be advanced by XCB coring to either the total depth planned for the site (690 mbsf for proposed Site IND-01A) or to a predetermined depth of ~500–600 mbsf for the two sites with basement targets (proposed Sites IND-03C and IND-04A) (Table T2). XCB coring could be terminated before reaching the planned total depth because of XCB refusal or if the consensus is that rotary core barrel (RCB) coring will improve core recovery, core quality, and/or penetration rates. In this event, we would begin a new hole and drill down to a short distance above the total depth of the XCB hole (~10–20 m) and then begin RCB coring. This procedure is planned for proposed Site IND-04A, where a third hole will be drilled without coring to ~500 mbsf (with the upper part of the section already recovered by APC/XCB coring in the first two holes) and then RCB cored using nonmagnetic core barrels to the target depth of ~950 mbsf, which includes 50 m of basement (Table T2). The total amount of basement cored at this site may be reduced, depending on time available during the expedition and if enough basement material has been recovered to meet the scientific objectives of the expedition.

The total depth proposed for proposed Site IND-03C is ~1570 mbsf, which includes 100 m of basement. We have included a reentry system consisting of three strings of casing in our operations plan for this site to improve our chances of achieving the basement objective. Estimated depths for the three casing strings are 20 inch casing to ~60 mbsf, 16 inch casing to ~250 mbsf, and 10¾ inch casing to ~600 mbsf; however, these depths may be modified to accommodate hole conditions and lithologies encountered in previous holes at the site. In addition to the two holes that will be cored using the APC and/or XCB, an additional hole will consist of a jet-in test for the 20 inch casing string. The reentry system will be installed in the final hole at the site, which will then be RCB cored below the reentry system through the sedimentary section and into basement, with a minimum target depth of 50 m into basement (Table T2). If time is available, we plan to core ~100 m into basement.

After coring is completed at each site, holes will be conditioned, displaced with logging mud, and logged as per the logging program (see “Downhole measurements strategy”). At proposed Site IND-03C, two holes will be logged. The shallow section will be logged in the XCB hole (planned penetration to ~600 mbsf), with logging of the deep section (~600–1500 mbsf) below the reentry system completed in the final hole (Table T2).

Risks and contingency

There are a number of challenges associated with drilling operations in deep water that could impact the drilling and coring operations strategy of this expedition. Weather is always a potential issue, as sea state and the resulting heave can have adverse effects on drilling and coring. Although we are sailing during a reasonable weather window, the end of the expedition will overlap with the initiation of the summer monsoon season. Although this should not seriously hamper operations, the expedition could experience some weather delays depending on conditions during critical operations, such as casing and reentry system deployment and RCB basement coring.

In order to fit into the time available for operations, one of the original primary sites has been designated an alternate site (proposed Site IND-02B). Based on our operations plan, this should allow the depth objectives to be reached at the three remaining primary sites; however, installation of a reentry system with three strings of casing, followed by deep coring, could take longer than anticipated. Should this be the case, the remaining operations plan will be modified to maximize achievement of expedition objectives at the remaining sites. This could be accomplished through shallower penetration, coring at an alternate site with a shallower target, reducing the number of holes cored at the remaining sites, or a combination of these possibilities.

The significant depth of coring at proposed Site IND-03C and the basement objective at the bottom of the hole present several challenges for successful drilling. Hole stability is always a risk during coring operations and the longer the open-hole sections, the higher the risk. Casing has been planned to ~600 mbsf to mitigate the risk of hole collapse and to provide a smaller annulus for improved annular velocity for hole cleaning. Hole cleaning also becomes a problem in the deeper sections of the hole, particularly when dense basement material is cored. Additional mud sweeps with larger volumes of mud will be planned for this section. The same problems apply to proposed Site IND-04A, but no casing has been planned to achieve the depth objective at that site, as it requires penetration to only ~950 mbsf. Lower annular velocities will make hole cleaning more difficult in the deeper sections of these holes. Increasing flow rates to ensure hole cleaning could result in washed-out sections of sediment in the upper part of the hole. This can also cause hole stability problems toward the end of the drilling and logging process.

Although we do not anticipate using a free-fall funnel (FFF) for any of the expedition sites, we have the ability to deploy one to reduce the amount of time required to reach the planned objective if we are short on operational time. There are several risks associated with FFF deployment. The FFF can be dislodged while pulling out of the hole. The FFF can become buried or impossible to use for reentry. The use of the FFF leaves the open-hole section open longer, which can contribute to hole instability.

A stuck drill string is always a risk during coring operations and can consume expedition time while attempting to free the stuck drill string or, in the worst case, severing the stuck drill string. This can result in the complete loss of the hole, lost equipment, and lost time while starting a new hole. The JOIDES Resolution carries sufficient spare drilling equipment to enable the continuation of coring, but the time lost to the expedition can be significant.