Figure 1. Diagram showing wireline and LWD technologies for in situ logging devices can make accurate physical measurements including porosity, resistivity, density, and natural gamma radiation, among others, only minutes after the drill bit cuts through a gas- or hydrate-bearing formation. The resolution of these LWD sensors is similar to that of wireline logging tools; neutron porosity measurements have a vertical resolution of about 12 inches, while density and gamma-ray measurements have a vertical resolution of 6 inches, depending in part on maintaining a consistent drilling rate. Unfortunately, the velocity of typical hydrate-bearing formations is less than the minimum value that can be resolved using LWD devices and wireline sonic tools continue to be required for logging in low-velocity sediments [Goldberg, 1997].

A primary advantage of LWD over wireline logging in marine environments is that data can be acquired with high resolution without gaps below the seafloor or at the bottom of the drill hole. In gas- and hydrate-bearing sediments, an important additional benefit of LWD is that data is recorded almost immediately after drilling and provides an accurate measurement of in situ properties without allowing time for them to change significantly. The reduced time before the measurement is taken minimizes drilling-induced hydrate dissociation or changes in gas concentration in the vicinity of the drill hole that may seriously affect a wireline log.

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