Coring and drilling strategy

The main aim of Expedition 334 is the thorough characterization of sediment coverage and basement in at least two different sites along the Costa Rica erosive margin offshore the Osa Peninsula: Site U1379 (proposed Site CRIS-4A, alternate Site CRIS-11A) above the locked portion of the subduction zone and Site U1378 (proposed Site CRIS-3B, alternate Site CRISP-10A) above the unlocked portion of the subduction zone. Generally, these objectives involved logging and coring as much of the sedimentary sequence and the basement at both sites as possible in the specified time window, and drilling operations were adjusted accordingly.

The originally proposed drilling strategy determined at the precruise meeting in College Station, Texas (USA), in May 2010 was to begin drilling at Site U1378 followed by Site U1379 and to core two holes at the each site. At both sites, we planned to core the first hole (Hole A) with the advanced piston coring (APC) system to refusal, followed by extended core barrel (XCB) coring to refusal. The estimated refusal depth was ~500 meters below seafloor (mbsf). We planned to drill the second hole (Hole B) at both sites to a depth slightly above the refusal depth of Hole A (e.g., ~490 mbsf), followed by rotary core barrel (RCB) coring to the target depth (~950–1000 mbsf). While drilling/coring, we planned to take a number of advanced piston coring temperature tool (APCT-3) and Sediment Temperature Tool (SET)/sediment temperature pressure (SET-P) probe measurements to calculate temperature and pressure gradients at both sites. Core orientation measurements with the Flexit tool were also planned during the APC-cored sections at each site.

The downhole logging program of Expedition 334 was designed to complement the core sample record at both sites by measuring continuous, in situ profiles of physical properties such as bulk density, porosity, resistivity, and natural gamma radiation. In addition to these formation properties, downhole logging provides oriented images of the borehole wall that are useful for determining the directions of bedding planes, fractures, and borehole breakouts. In the conventional technique of wireline logging, downhole measurements are taken by tools lowered in a previously drilled borehole. Wireline logging has had limited success in deep holes in unconsolidated clastic sequences because these holes tend to be unstable after drilling. It may be difficult to lower wireline tools in an unstable borehole, and hole irregularity can compromise the quality of the measurements. In LWD, downhole measurements are taken by instrumented drill collars in the bottom-hole assembly (BHA) near the drill bit. Hence, LWD measurements are made shortly after the hole is drilled and before the adverse effects of continued drilling or coring operations. LWD has been successful in previous scientific drilling expeditions to convergent margins such as Nankai trough during ODP Leg 196 and IODP Expeditions 314 and 319 (Mikada et al., 2002; Kinoshita et al., 2009; Saffer et al., 2010), Barbados during ODP Legs 156 and 171A (Shipley et al., 1995; Moore et al., 1998), and Costa Rica during ODP Leg 170 (Kimura et al., 1997). LWD was selected as the logging technique for Expedition 334. The LWD equipment used during this expedition was provided by Schlumberger Drilling and Measurements under contract with the Lamont-Doherty Earth Observatory Borehole Research Group.

The Schlumberger LWD tools used during Expedition 334 were the geoVISION 675 (near-bit electrical resistivity, resistivity images, and natural gamma radiation), the arcVISION 675 (annular borehole pressure, resistivity, natural gamma radiation), the adnVISION 675 (bulk density, neutron porosity, an density and ultrasonic caliper), and the measurement-while-drilling (MWD) TeleScope 675. Detailed descriptions of all down hole logging tools can be found at index.html. In addition to collecting drilling mechanics data, the MWD tool also transmits a limited LWD data set by acoustic telemetry to the surface for real-time monitoring. The real-time measurements included the pressure of the borehole fluid in the annulus (the space between the drill string and the borehole wall). During Expedition 334, annular pressure was monitored while drilling to ensure that free gas did not enter the borehole.

In Expedition 334, LWD measurements were planned to be made in a dedicated hole drilled first at each site. The advantage of this strategy is that detailed physical property logs are available to optimize coring in subsequent holes. The operations plan estimated ~3 days/hole for LWD. This time allows for logging the whole sediment section at expected rates of penetration (ROPs) of ~20 m/h. These are "gross" ROPs that include time for pipe connections. The allotted time is enough to reach the total depth objective in each hole if ROPs in the basement interval can be maintained at ~10 m/h. If ROPs in the basement are significantly slower, the plan was to log as much as possible of the basement interval given the time constraint of ~3 days per LWD hole.