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doi:10.2204/iodp.sp.334.2010

Downhole tools and logging strategy

Downhole logs will be acquired at the CRISP sites to measure in situ physical properties, estimate compaction, and evaluate permeable horizons. Electrical resistivity images referred to magnetic north will be used to determine fracture orientations, to infer stress directions from borehole breakouts, and to orient core samples. Formation density and velocity measurements will allow for constructing synthetic seismograms to correlate depth in the hole with traveltime in seismic sections. In situ velocity data will also be used to build a velocity model for a 3-D seismic survey planned for the Expedition 334 study area.

Logging operations will include one or more of the following: wireline logging, LWD, and logging while coring (LWC). Wireline logging is expected to be challenging at the CRISP sites because the deep holes are likely to be unstable after coring. LWD is expected to acquire better data in this environment, and it was successful in previous expeditions at convergent margins (e.g., Leg 170 and IODP Expeditions 311 and 314). LWC is a less expensive alternative that does not acquire as many measurements as LWD; however, LWC takes core samples that are precisely located with respect to the logging measurements. The development of LWC is also an important engineering advance for IODP. At the time of the writing of this prospectus, funds to support LWD and/or LWC were not available. The default operations plan (Table T1) is for wireline logging, with an alternative plan for LWD (Table T2). The operations plan for LWC would be similar to that for LWD. A decision on whether funding is available to log the CRISP sites by LWD and/or LWC is expected by the third quarter of 2010.

Wireline logging

Three wireline tool strings are planned for the CRISP sites (Fig. F7). The first tool string to be deployed in each hole is the triple combination (triple combo) tool string, which measures hole diameter, natural gamma ray, bulk density, neutron porosity, and electrical resistivity. The second Formation MicroScanner (FMS)-sonic tool string will consist of a FMS resistivity imaging tool, a Dipole Sonic Imager (DSI) that measures P- and S-wave velocities, and a natural gamma ray sensor. The third tool string will be deployed in a check shot experiment where a Versatile Seismic Imager (VSI) tool records the arrival of acoustic pulses generated by air guns fired from the JOIDES Resolution. The FMS-sonic and VSI tool strings will be run depending on ship heave and borehole conditions. Detailed descriptions of wireline, LWD, and LWC tools and operational constraints can be found at iodp.ldeo.columbia.edu/TOOLS_LABS/index.html

The operational time estimates for the wireline log deployments are in Table T1. The CRISP operations plan calls for drilling two holes at each site: Hole A to ~500 mbsf (cored by APC/XCB) and Hole B to total depth (cored by RCB). The triple combo and FMS-sonic tool strings will be deployed to log the shallow interval in Hole A and the deeper interval in Hole B. To maximize the chances of successful wireline logging, it will be important to set the drill pipe in Hole B as deep as possible during logging, ideally just above the depth of the shallower Hole A. The drill pipe allows the logging strings to pass through borehole obstructions that may make it impossible to reach the deep interval to be logged. The final drill pipe depth will be determined based on hole conditions and after consultation between the IODP Operations Superintendent, the drilling subcontractor, and the logging staff scientist. The operations plan also includes a check shot survey with the VSI tool string in Hole B of proposed Site CRIS-4A. If borehole conditions are favorable and time is available, a check shot may also be performed at proposed Site CRIS-3B.

Logging while drilling

The LWD bottom-hole assembly presently planned for the CRISP sites consists of two Schlumberger tools. From bottom to top, these are the EcoScope (measuring resistivity, neutron porosity, images of bulk density, natural gamma ray, and borehole diameter, and annular pressure while drilling [APWD]) and the TeleScope (measurement-while-drilling [MWD] telemetry and power). This tool combination provides a basic set of in situ physical property measurements and real-time pressure in the borehole, needed for gas monitoring (see below). If funding is available, additional LWD tool strings to be added are, in order of priority, the geoVISION (resistivity images for fracture and breakout interpretation), sonicVISION (P- and S-wave velocity), and seismicVISION (check shot data acquired while drilling).

LWD measurements will be made in a dedicated Hole A drilled first at each site. The advantage of this strategy is that detailed physical property logs will be available to optimize coring in subsequent holes. The LWD operations plan in Table T2 takes ~3 days/hole. This time will allow 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, we will log as much as possible of the basement interval given the time constraint of ~3 days/LWD hole.

Because the first hole at each site will not be cored, we will need to monitor for gas with the LWD measurements. The key measurement to be carefully monitored is the APWD, transmitted in real time by the MWD tool. The possible occurrence of gas should be indicated by a sharp pressure decrease, which could be preceded by a pressure increase. If the final plan for CRISP includes LWD, a detailed protocol for gas monitoring will be developed following the protocol that was applied in Expedition 311. The protocol required preventive actions for any pressure anomaly exceeding 100 psi; no significant anomalies were detected in the Expedition 311 holes.

Logging while coring

An LWC system that combines a modified geoVISION resistivity LWD tool and a core barrel was tested during ODP Legs 204 and 209 (Goldberg et al., 2006). The geoVISION resistivity tool measures resistivity images and natural gamma ray, and the core barrel retrieves a 2.5 inch diameter core. The LWC system recovered up to 68% in sediment, whereas recovery was poor in hard rock because of a mismatched core catcher. The core catcher has since been modified, and better recovery is expected in hard rock.

LWC will be carried out in a dedicated hole drilled last at each site. The plan is to log the entire interval while taking cores in intervals selected to test the LWC system and to supplement core recovery in the previously drilled holes. The operations plan will be similar to that for LWD (Table T2) and take ~3 days/hole.

Temperature, pressure, and core orientation measurements

The scientific objective of the temperature measurement plan is to provide sufficient data to reconstruct the thermal gradient at each site. This information will be a key input to estimate heat flow, infer fluid flow, model pore water geochemistry, and constrain the sediment diagenesis history. We plan on deploying the APCT-3 in the interval where cores will be taken by APC and the SET tool further downhole where sediments are more consolidated. Temperature measurements will be carried out every 40–50 m down to the maximum depth where the SET tool can penetrate sediment. This maximum depth depends on the formation, and we estimate it to be ~500 mbsf. We also plan to deploy the SET-P tool to measure in situ formation pressure. Taking these measurements will require a stable hole because the probe needs to be seated in the formation for ~10 min (45 min for pressure) with no drill string rotation and no drilling fluid circulation. Finally, APC cores will be taken with a nonmagnetic core barrel and oriented with the FlexIt tool for paleomagnetic studies. The nonmagnetic core barrels will be used until overpull limits are exceeded.

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