The overall strategy for Expedition 308 was to complete continuous coring, wireline logging, in situ measurements, and logging-while-drilling (LWD) operations at each primary site. As time allowed, we also visited alternate locations. To minimize the time that expensive LWD tools and personnel were kept on board the ship, the strategy was to first core and perform downhole measurements in Brazos-Trinity Basin #4 and then to perform continual LWD measurements first in Brazos-Trinity Basin #4 and then in Ursa Basin. Thereafter, the LWD tools would be offloaded and final coring and downhole measurements would be performed in Ursa Basin.
We planned to use LWD because we expected to encounter thick unconsolidated sands in Brazos-Trinity Basin #4 and because we might encounter unconsolidated sands in Ursa Basin. Unconsolidated sands can result in unstable boreholes where it is not possible to deploy wireline tools. LWD would ensure that a good logging suite was obtained.
In addition to LWD, we also planned to use measurement while drilling (MWD). In MWD, logging data are communicated in real time from near the drill bit to the ship so that the borehole conditions can be continuously monitored. In Ursa Basin, we expected to encounter overpressured mudstones and it was possible we would encounter overpressured and unconsolidated sands. If we encountered unconsolidated and overpressured sands, there was a risk that "shallow-water flow" would occur. Shallow-water flow results when overpressured unconsolidated sands flow into a borehole that has a lower pressure than the formation pressure. With MWD, we would be able to monitor both the downhole annular pressure and the formation lithology. When shallow-water flow occurs, an elevated pressure is recorded as sand flows into the borehole. Through continuous monitoring we would be able to decide whether it was necessary to increase the mud weight to offset the flow into the borehole or terminate the drilling and kill the well.
Finally, to minimize shallow-water flow, we planned to use heavy mud during drilling and coring of portions of proposed Sites URS-1B and URS-2C. We also planned to deploy heavy mud during long-term in situ measurements with penetration probes (Davis-Villinger Temperature-Pressure Probe [DVTPP] or temperature/dual pressure [T2P] probe).
It was planned to use two downhole tools to measure pressure and temperature: the T2P, which was designed by the Massachusetts Institute of Technology (MIT), Pennsylvania State University, and Integrated Ocean Drilling Program (IODP) U.S. Implementing Organization Science Services, Texas A&M University (TAMU) and the DVTPP. The T2P is designed to measure pore pressure more rapidly than the DVTPP. It has a narrow tip with a temperature and pressure sensor; a second pressure measurement is collected slightly up-probe from the sensors at the tip (Fig. F12). The design allows for rapid measurement of pressure in low-permeability sediments. In addition to temperature measurements made with the T2P and DVTPP, temperature measurements were made during coring with the advanced piston corer (APC) using the advanced piston corer temperature (APCT) tool. Multiple 1020 cm long whole-round cores were planned to assist interpretation of the T2P and DVTPP data, directly measure formation pressures, and infer the in situ stress state through laboratory analysis.
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