Overview of Investigation of Potential CO2 Storage Sites Offshore Japan

Abstract

Japan CCS Co., Ltd. (hereinafter “JCCS”) has been conducting the investigation of potential CO2 storage sites in Japan undercontract to the Ministry of the Environment (hereinafter “MOE”) and Ministry of Economy, Trade and Industry (hereinafter“METI”) of Japan since 2014. We have been conducting a number of 2D/3D seismic surveys and geological interpretation at pre-selected areas in offshore Japan, in order to identify prospective sites from among these areas. Several candidate sites will be selected from these prospective sites, and exploratory wells will be drilled to confirm the presence of geological conditions necessary for storage. The drilling will be followed by the construction of a geological model and flow simulation. The final goal of the current scope of work is to recommend three potential sites for CO2 geological storage to MOE and METI by around 2021. The main selection criteria of a potential storage site are as follows: > The potential sites should be located in offshore areas > Capacity of each site is to store over 100 million tonnes of CO2 > There are no active faults close to the site > The existence of a CO2 emission source near the site is not taken into consideration > The method of CO2 transportation to the site is not taken into consideration > Producing oil and gas fields are excluded (site is not for EOR purpose) > There is no environmental protected area around the site In addition, a preferable condition is whether the site can be reached by directional drilling from onshore, as is the case for the Tomakomai CCS Demonstration Project. In this project, two directional CO2 injection wells were drilled with horizontal reach of over 3,000 meters, saving considerable construction cost as it was not necessary to bring in an offshore drilling rig, or to lay expensive subsea pipelines and related offshore facilities. The 2D/3D seismic survey conducted in the investigation of potential sites is the most effective method to delineate the geological features of the subsurface. 2D seismic surveys are conducted to grasp the regional geological features over a wide range such as 50 x 100 km, and if prospective structures for CO2 geological storage are identified, 3D seismic surveys over a smaller range (e.g., 10 x 10 km) will be executed to delineate the geological features more precisely. When necessary, we employ state-of-art seismic acquisition and processing technology utilized in the oil and gas industry to accomplish our objectives. In one of the 3D seismic surveys, we have conducted a seamless 3D seismic survey across the transition zone between shallow marine and onshore. As the possible presence of active faults was suspected in the transition zone, there was a need to collect data from the shallow marine continuously to the onshore area. Although the survey has taken time and effort, we were able to fill the gap in the existing seismic data between offshore and onshore with data of reasonable resolution utilizing an Ocean Bottom Cable (OBC). The geological interpretation found that the subsurface of the transition zone was a narrow synclinal valley along the current coastline with no active faults present, and therefore that it was possible to store CO2 in the offshore area as it was safely isolated from the populated onshore area. At another prospective site, we have conducted a 3D seismic survey utilizing broadband technology; the first application of this state-of-art technology in Japan. Because the target sandstone reservoirs in the survey area are composed of deep marine sediments such as turbidites and submarine fans, high-resolution 3D seismic data is required to differentiate the sandstone types. The high resolution data may be further utilized for special processing such as acoustic impedance (AI) inversion and amplitude versus offset (AVO) analysis which is help to evaluate rock properties in order to reduce geological risk. The high resolution near surface data acquired in broadband technology is also useful in identifying shallow gas hazards in the area. In BroadSeisTM(*), the broadband seismic technology we adopted, the slanted streamer cables are deployed deeper than conventional seismic surveys, mitigating the effects of sea surface swells and wave noises under rough sea conditions. As a result, we were able to reduce drastically the downtime due to rough weather during the survey period. (*: trademark of CGG) In summary, the application of state-of-art seismic acquisition technology in the investigation of potential CO2 storage sites is expected to reduce geological risk as well as contribute towards saving acquisition costs.