Permeable weak layer in the gas hydrate reservoir presumed by logging-while-drilling log data

Abstract:

One of the specific intervals attracted attention to analyze the 2012 gas-production test from methane-hydrate reservoir, because its pressure and temperature behavior was different from other intervals of the production zone. The pressure and temperature behavior implied the interval should be high permeability.

We analyzed the interval to characterize the properties before gas-production test; i.e. the original properties of the interval. We checked the data of the logging-while-drilling data of AT1-MC, which was one of the monitoring wells at the gas-production test. The specific interval was described as 1290-1298m, where was boundary between upper sand and mud alteration layer and middle clayey zone. The first, we noticed that there were several layers that showed broad T2 distributions of nuclear magnetic resonance (NMR). On the basis of the T2 distributions and the resistivity data of the interval, there were large pores that showed the T2 distribution around 100ms, even though some amount of methane hydrate were contained. This result could be explained the interval showed high permeability below the 1294m. After checking their ultra-sonic caliper data in detail, we found interesting difference in the interval. The specific interval of 1294-1295m had different borehole-enlargement direction from other intervals of the methane-hydrate bearing zone, even though diameter of borehole was slightly enlarged. Other layers in the methane hydrate reservoir showed NW-SE directions of enlargement, however, the specific interval had NE-SW direction of enlargement. Hence, H-max stress and H-min stress of this specific interval could be very close values. Thus, near the 1294m, the lithology of the layer was permeable and weak. It might be useful to understand many phenomena occured during the gas-production test. This research was conducted as a part of the MH21 research, and the authors would like to express their sincere appreciation to MH21 and the Ministry of Economy, Trade and Industry for providing the permission to disclose this research.