[SCG56-03] An idea of borehole measurements at Mantle drilling: insights from the OmanDP
Keywords:crust-mantle boundary, Moho, geophysical logging, OmanDP
One of the most important scientific targets would be the detailed structure around the Moho, and there may be a complicated transition zone as we can seen in the Oman Ophiolite. We need two different types of information for this purpose: physical property and mineral composition. Physical property is vital to understand the mechanical architecture of the boundary and to compare with the seismic characteristics identified on the seismic profile. Among the properties, acoustic impedance is of importance defined by sonic and density, which can be measured in-situ by using geophysical logging tools. Mineral composition could be examined continuously by cuttings when riser pipes are connected, otherwise we need to run geophysical logging operation for this purpose as well. Geochemical logging, e.g. lithoscanner, was tested at the CM sites of the OmanDP and showed potential in-situ elemental measurements at every 50 cm.
Internal stress environment of oceanic plates, a basic question on how the plate moves, is another important scientific interest, which borehole images may bring key information. A possible method to extract the information would be identification of borehole failure, i.e. drilling induce tensile fractures (DITFs) for this purpose. DITFs, an indicator of the SHmax direction, may be produced by cooling of the borehole wall-rock by circulating drilling fluid.
As for geophysical logging operations, wireline tools have advantages in term of variety and accuracy of the data, comparing to the logging-while-drilling (LWD) tools. The wireline tools, however, have lower temperature limit, e.g. 260°F (126°C), whereas LWD tools may have less problem in temperature as far as the drill mud can be circulated. For the greater depth, we also need to consider possible pressure due to the heavy drill mud affecting on the tools.
Drilling parameter data, commonly used for operation purposes, such as WOB, ROP, RPM, TOR may be useful to draw continuous strength profile along the borehole. The data quality would be much better by using MWD, compared with the data acquired by the drilling system. MWD can be run under high temperature environments but may not be used with combination of downhole motor.