RITE has promoted technological developments related to storage performance evaluation of a CO₂ injection site in aquifer storage, analysis of CO₂ behavior in a reservoir and analysis of CO₂ migration (leaks) outside the reservoir as a fundamental technology necessary for the establishment of enhanced safety and reliability of CCS as a project sponsored by the Ministry of Economy, Trade and Industry. In addition, RITE is compiling a best practices manual which will serve as guidelines for CCS operators.

For geological CO₂ storage in Japan, supercritical CO₂ is injected into deep saline aquifers (reservoir). In order to verify the safety and performance of such CO₂ storage, it is necessary to characterize a reservoir complex and to evaluate its storage capacity at an initial stage of the CCS project, for example, at the site characterization phase.

In order to make sure that CO₂ will be stably stored for a long time, CO₂ behavior in the reservoir should be predicted and examined. Injected CO₂ replaces formation water in the pore of rocks in the reservoir. The behavior of CO₂ depends on porosity, absolute permeability and relative permeability functions of the formation. With these parameters, the extent and state of CO₂ in the reservoir can be predicted numerically. A spatial distribution of these parameters in the ground forms a geological model. In order to predict the behavior of the injected CO₂ accurately, highly-accurate geological models should be developed.

Geological modeling techniques have been developed by oil and gas companies. They usually use many well log data to build a geological model of a site. In geological CO₂ storage projects, however, the number of wells is severely restricted to reduce leakage risks and costs in the projects and as a result their geological information is generally much limited compared to that of oil and gas fields. Thus, the geological models in CO₂ storage projects need to be constructed with limited geological information.

RITE have been developing and applying a technique of geological modeling suitable for Japan's heterogeneous formations which consist of sand, mud and gravel to the Nagaoka CO₂ Pilot-Scale Injection Site (Iwanohara Test Site in Nagaoka City, Niigata: INPEX CORPORATION). At the Nagaoka site, CO₂ of 10,400 tonnes in total was injected into saline aquifer (reservoir) at a depth of 1,100min a period from July 2003 to January 2005.

Nagaoka CO₂ Pilot-Scale Injection Test Site

Figures 1 and 2 show geological models at the Nagaoka Site. Figure 1 is a geological model of 1.5 km × 1.5 km made by combining various data from rock core samples, well logging data (one injection well and three observation wells) and three-dimensional seismic explorations. The red areas in the figure represent sandy formations with high porosity, which are suitable for CO₂ storage. Figure 2 was a geological model of 4 km × 4 km constructed by stochastic processing based on the spatial distribution of properties of rocks. These geological models are kept updated with data acquired in loggings that RITE periodically conducts.

CO₂ injected into deep saline aquifers (reservoir) spreads with replacing formation water in the reservoirs. It is important to confirm how the injected CO₂ is distributed and to predict how the distribution changes for safety assessment of geological CO₂ storage. RITE investigates the storage mechanism by comprehensively analyzing various data obtained at the Nagaoka CO₂ Pilot-Scale Injection Site. Further, monitoring of formation deformation during injection and observation of micro-seismic to monitor, for example, induced seismic activity, are also necessary to ensure safety. These monitoring techniques are referred as CO₂ behavior analysis.

Figure 3 shows long-term CO₂ trapping mechanisms. There are four mechanisms; structural and stratigraphic trapping, residual CO₂ trapping, solubility trapping and mineral trapping (mineralization), and degree of contribution of each mechanism changes as the function of time of post-injection phase. Safety against leakage increases in the order of structural and stratigraphic trapping, residual CO₂ trapping, solubility trapping and mineral trapping. Structural and stratigraphic trapping is due to geological structures such as cap rock. Residual CO₂ trapping is a mechanism in which CO₂ is once captured in small pores, and then cannot move due to capillary pressure. Solubility trapping is a mechanism in which CO₂ is dissolved into formation water and then trapped. Mineral trapping is a mechanism in which dissolved CO₂ chemically reacts with minerals in the reservoir and precipitates as carbonates.

The last figure shows mineral trapping. Formation water is sampled and analyzed three times: pre-injection, 2 years and 5 months and 8 years and 2 months after the start of injection. The figure at the lower left shows that whereas the total carbonic acid in the formation water increased Ca ion decreased. This is possibly because Ca ion reacted with the dissolved CO₂ and precipitated as CaCO3 (calcite). The figure at the lower right shows saturation indexes of calcite, which indicates that calcite precipitates or dissolves depending on the degree of saturation. The saturation index at a depth of 1118 m is positive. This may indicate that there are precipitation of calcite - in other words, mineral trapping has taken place there.

CO₂ behavior and the status of replacement of formation water with CO₂ when CO₂ is injected into samples of rock that forms reservoir can be observed in a non-destructive manner by using an X-ray CT. A CO₂ injection experiment of this kind is conducted under temperature pressure conditions where CO₂ is brought into a supercritical state because injected CO₂ is in a supercritical state in most cases.
RITE is working on technological development in order to quantify the relationship between porosity, degree of fluid saturation and physical properties of rock by analyzing X-ray CT image data. These physical property values can be used for interpretation of monitoring data such as seismic survey data and resistivity data.

The figures at the left show visualized images of a CO₂ distribution state in rock core samples. We can observe how injected CO₂ replaces water and enters pores. The injected CO₂ is not uniformly distributed in rocks. CO₂ is trapped with mixture of high CO₂ concentration areas and low CO₂ concentration areas in rocks. Analysis of such trapping status contributes to evaluation of actual storable quantities and behavior predictions.

In an overseas CO₂ geological storage project (In Salah Project), it was observed that an increase in pore water pressure due to CO₂ injection in the storage formation causes a rise of the ground surface around the injection well. If the deformation of geological formations is large, it could damage safety.
Deformation of formations can be observed by embedding underground displacement gauges. It is, however, necessary to determine the embedding depth of the displacement gauges in advance, and only a limited number of displacement gauges can be embedded from an engineering perspective.
RITE is developing a technology to monitor deformations of formations during injection using a distributed-type optical fiber which can continuously measure the deformation in the depth direction. Deformation (strain) of formations is measured by measuring and analyzing scattered light of a pulse laser beam passed into the core of the optical fiber. Further, because the optical fiber sensor can also continuously measure temperature and pressure, it may be possible to widely use the optical fiber sensor as a monitoring system such as post-injection monitoring of leaks from the reservoir in a period, and RITE is promoting the technological development of such sysmems.

Seismic surveys are most-widely used for monitoring of CO₂ geological storage.
Oscillating seismic waves and receiving their reflected waves can clarify underground structure and CO₂ distribution state in formations.
Surveys of storage under the seabed is usually carried out with a ship towing so-called streamer cables, which are cables equipped with a number of sensors, on the sea surface. On the other hand, permanent OBC (Ocean Bottom Cable) has sensors settled on the seabed. There is an advantage that the accuracy of repeated measurements is high because of the fixed sensors.
Furthermore, and the seismic survey with the streamer cables requires a large ship and also a large space on the sea as multiple oscillation-receiving sensors are towed. The survey using the permanent OBC, by contrast, only requires a small ship for oscillation.
In addition,the permanent OBC can observe measure natural earthquakes and microseismic events induced by CO₂ injection.in a real-time basis.

RITE conducted actual offshore area tests for the permanent OBC off the coast of Hiratsuka in Kanagawa from 2011 to 2012, and carried out seismic surveys and verified long-term stability and durability of the system.

The permanent OBC system is installed at the site of CCS demonstration project which is conducted by Japan CCS Co., Ltd., sponsored by the Ministry of Economy, Trade and Industry, in Tomakomai-City, Hokkaido.

The number of microseismic monitoring studies conducted for overseas CO₂ injection sites is very limited and most of them were conducted for areas with low seismicity. In earthquake-prone countries like Japan, this kind of study should be essential from the viewpoint of gaining public acceptance. RITE, therefore, develops an evaluation methodology for CO₂ injection induced microseismicity to secure safe CO₂ injection operation.

RITE develops mainly three kinds of methods: 1) a method to measure and locate microseismicity at a high accuracy, 2) a method to judge that an observed seismic event is caused by a natural earthquake or CO₂ injection and 3) a method to manage CO₂ injection based on the analysis of real-time data recorded at a CO₂ injection site.

RITE has been monitoring microseismicity at a CO₂ injection site (Cranfield oilfield, Mississippi, the U.S.A.) since December 2011, collaborating with the Lawrence Berkeley National Laboratory and the Bureau of Economic Geology, the University of Texas. The oilfield is one of the injection test sites run by the Southeast Regional Carbon Sequestration Partnership (SECARB), one of the seven regional carbon sequestration partnerships funded by the U.S. Government.
We study the relationship between CO₂ injection and occurrences of microseismicity by analyzing the recorded data.

RITE conducts research on the assumption that CO₂ has leaked in an offshore area from the viewpoint of safety assessment. CO₂ leaked from the seabed diffuses into seawater. How the leaked CO₂ diffuses depends on the flow and temperature of seawater. RITE is conducting research to numerically model the status of the ocean and to predict diffusion through simulations.

RITE sorted out data regarding what levels of CO₂ concentration could impact what kinds of marine organisms and complied the results in database. .The database is annually updated with accordance with latest research outcomes published each year.

In addition, RITE carries out technological development in order to detect CO₂ that has leaked into offshore areas. RITE conducts studies on the appropriate method to determine whether CO₂ that has dissolved in seawater is within the variations of natural phenomena or can be attributed to leaks of CO₂.
Furthermore, applicability of an acoustic probing method such as side-scan-sonar (SSS) for detection of CO₂ bubbles is also examined.

In order to further understand the impact of CO₂ on the marine environment, RITE is conducting a field experiment (UK QICS Project: Quantifying and Monitoring Potential Ecosystem Impacts of Geological Carbon Storage) where CO₂ is intentionally released from the seabed with research institutes in the U.K. In this experiment, RITE is researching the change in behavior of large benthonic organisms and impacts on microorganisms in saline mud, etc.

The first CO₂ aquifer storage project was started at Sleipner, Norway in 1996. After the that, many CO₂ geological storage projects were started in the world-. The obtained knowledge and experience was summarized as Best Practice Manual or Recommended Practice Manual.

For example, the European Commission provided Guidance Document published in four. These Guidance Documents "CCS directive" to present rules related to CCS implementation to the EU member countries. On the other hand, the United States EPA (Environmental Protection Agency) provided many "Class VI Well Guidance". DOE(The United States Department of Energy) was promoting preparation of a BPM based on the knowledge experience of the CCS projects of the RCSP (Regional Carbon Sequestration Partnership). As described above, accumulated experience and knowledge of pilot-scale tests involving CCS projects are analyzed and reviewed and adopted, and preparations are being made overseas for full-scale implementation of CO₂ storage underground and seabed formations.

A diagram for compilation of the best practices manual is shown below. As a first step, relevant information is collected from within Japan and overseas, sorted and analyzed, and in parallel key aspects and major data of the Nagaoka CO₂ Pilot-Scale Injection Test are sorted out and summarized. Then, based on results from research and development conducted by RITE, a RITE version of the CCS best practice manual is compiled, and as a final step, a Japanese version of the CCS best practices manual is to be developed, incorporating the large-scale demonstration project in Japan.