Development of a permanent OBC system for CCS monitoring in shallow marine environments

In the CCS monitoring, time-lapse 3D seismic survey is necessary and microseismic monitoring is also required to assure the safe CCS operations. In order to fulfill these requirements efficiently, we are developing a permanent Ocean Bottom Cable system (OBC) in shallow marine environments. The system can figure out the extent of CO2 injected area and assure that CO2 is not escaped above seal rocks. The use of the system is advantageous from the long term view of the cost. In this paper, we, firstly, present a simulation study based on the data from the Nagaoka CCS test site. 4D response was evaluated using crossplot of NRMS and PRED indices so that we could estimate the discrimination degree between 4D responses caused by CO2 injection and other 4D noises. Basic test experiments of a permanent OBC system have been conducted in Sites A and B in Japan and repeated seismic surveys using airgun source were shot for evaluating the 4D noise response. Tidal correction is found not to be simple as the wavelet shape itself is modified in the shallow water condition. P-SV reflections are recorded with three-component geophones installed in the OBC which will contribute to the CCS reservoir characterization and monitoring. A microseismic event with the magnitude of 0.6 occurred at the depth of 103 kilometers just below Site A was detected. As the CO2 geological sequestration will be conducted within several thousand meters, detectable limit is estimated down to magnitude minus two in an ideal condition. (C) 2013 The Authors. Published by Elsevier Ltd.