S51A-1733
Verification and Monitoring of CO2 Storage by Using Self-potential Method
Accelerating of global warming is caused by increasing emissions of greenhouse gases such as CO2. Reduction approaches of the greenhouse gases is one of the urgent problems on a global scale and attempted in many countries. CO2 capture and storage is an economical and efficient technology to the reduce greenhouse gas emissions. However, geophysical exploration methods for monitoring the CO2 migration are now developing. Self-potential is caused by the electrokinetic phenomenon of streaming potential, which has been applied to investigation of subsurface fluid flow. In order to confirm whether self- potential can be used for the monitoring of CO2 migration or not, we calculated self-potential variation associated with CO2 storage by using a numerical simulation code (STAR + EKP post-processor, e.g. Ishido and Pritchett, JGR, 1999). The target field of this study was set at Ogachi test field located in the northern part of the Honshu island of Japan, where CO2 injection experiment has been conducted. We calculated self- potential responses on the ground in three cases of CO2 injection (Injecting rate:10 ton/day, 30 ton/day, 50 ton/day; Injecting time: 6h). The results show significant self-potential changes around the injection well, which are 1mV by 10 ton/day, 4.7mV by 30 ton/day, and more than 12mV by 50 ton/day, respectively. Considering electrical noise on the field, more than 30 ton/day of CO2 is needed by using self-potential monitoring. This injecting rate is within practical CO2 storage rate in the actual field. Therefore, self-potential is one of the useful methods for the monitoring of CO2 storage. This research was supported by the grants of the Programmed Research eDevelopment of evaluation technology for the CO2 in the exhaust gas sequestration into geothermal fieldsf of RITE under the fund from METI (Ministry of Economy, Trade and Industry).
S51A-1734
Monitoring Temporal Changes of Seismic Velocity due to CO2 Injection Using Time- Lapse VSP Data and Coda-Wave Interferometry
Coda-wave interferometry is a technique to estimate small temporal changes of seismic velocity within a target region using seismic scattering signals before and after alternation in reservoir conditions/properties. For each pair of dataset, the temporal velocity changes are estimated at the centers of a moving time window by cross-correlation of the time-lapse signals within the time window. The average velocity change is the mean value of the temporal velocity changes as a function of the central time. It has been investigated for probing the relative location of seismic sources, monitoring time-varying rock properties in a laboratory environment, etc. To explore the capability of the coda-wave interferometry method for monitoring geological carbon sequestration, we apply the method to time-lapse field VSP (vertical seismic profiling) data acquired at the Aneth oil field in Utah for monitoring CO2 injection. The aim of the work is to demonstrate combined enhanced oil recovery and geologic carbon sequestration, as part of the project of the Southwest Regional Partnership on Carbon Sequestration. The time-lapse VSP data are pre-processed to reduce noises, remove unwanted events, and separate into upgoing and downgoing waves. We estimate the average velocity changes using the pre-processed time-lapse upgoing data and the coda-wave interferometry method. The results show that the average velocity changes increase as the receiver positions approaching the CO2 injection layer, the oil reservoir situated at depth approximately 1750 m. This indicates that seismic velocities in the oil reservoir changes during CO2 injection. The preliminary results show that the coda-wave interferometry method can effectively monitor seismic velocity changes caused by CO2 injection/migration using time-lapse VSP data.
S51A-1735
High-resolution Seismic Imaging for CO2 Storage Site Assessment
3D seismic imaging is considered as one of the key investigation and monitoring techniques in the frame of CO2 underground storage. The knowledge about the geometry and the structure of the reservoir including its fault inventory is a prerequisite for a reliable assessment of storage capacity and safety. The recent development of advanced seismic imaging and inversion techniques directly suggests their exploitation in order to achieve these aims. We present one of these developments, 3D Fresnel-Volume-Migration (FVM). This technique is an extension of Kirchhoff-Prestack-Depth-Migration (KPSDM) and restricts the smearing of the wavefield along two-way-traveltime isochrons to the actual reflection/diffraction point using the concept of Fresnel volumes. The necessary information to perform this restriction is the emergence angle at the receiver, which is obtained from local slowness estimates using slant-stack or cross-correlation techniques. The method has been applied to the 3D SEG/EAGE salt model data set as well as a real 3D seismic data set from a potential CO2 storage site. The results show that FVM yields images of superior quality compared to KPSDM, both in terms of resolution as well as the suppression of artefacts. Therefore the quality of the resulting FVM images provide an excellent basis for further studies with respect to reservoir assessment and monitoring.
S51A-1736
Numerical modeling of time-lapse seismic monitoring of CO2 sequestration in layered basalt
As part of a feasibility study to sequester CO2 in layered basalt for the Center for Advanced Energy Studies (CAES), we numerically investigate the abilities and limitations of seismic methods as a non-invasive monitoring technique. While basalt has been shown to have geochemical advantages as a reservoir for CO2 storage (the notion is that CO2 mineralizes quite rapidly while exposed to basalt), it poses a considerable challenge in term of seismic monitoring: strong scattering from the layering of the basalt complicates surface seismic imaging. We perform numerical tests to identify possibilities and limitations of seismic monitoring of CO2 sequestration in a basalt reservoir. While surface seismic is unlikely to detect small physical changes in the reservoir due to the injection of CO2, the results from Vertical Seismic Profiling (VSP) simulations are encouraging. As we perturb reservoir conditions by small amounts, it produces -- what we think -- are significant changes in VSP coda waves of pre-injection and post-injection conditions. Hence, we perform an analysis using Coda Wave Interferometry (CWI), to quantify these changes in the reservoir properties due to CO2 injection.
S51A-1737
Quantitative Estimation of Seismic Velocity Changes Using Time-Lapse Seismic Data and Elastic-Wave Sensitivity Approach
Quantitative monitoring of reservoir property changes is essential for safe geologic carbon sequestration. Time-lapse seismic surveys have the potential to effectively monitor fluid migration in the reservoir that causes geophysical property changes such as density, and P- and S-wave velocities. We introduce a novel method for quantitative estimation of seismic velocity changes using time-lapse seismic data. The method employs elastic sensitivity wavefields, which are the derivatives of elastic wavefield with respect to density, P- and S-wave velocities of a target region. We derive the elastic sensitivity equations from analytical differentiations of the elastic-wave equations with respect to seismic-wave velocities. The sensitivity equations are coupled with the wave equations in a way that elastic waves arriving in a target reservoir behave as a secondary source to sensitivity fields. We use a staggered-grid finite-difference scheme with perfectly-matched layers absorbing boundary conditions to simultaneously solve the elastic-wave equations and the elastic sensitivity equations. By elastic-wave sensitivities, a linear relationship between relative seismic velocity changes in the reservoir and time-lapse seismic data at receiver locations can be derived, which leads to an over-determined system of equations. We solve this system of equations using a least- square method for each receiver to obtain P- and S-wave velocity changes. We validate the method using both surface and VSP synthetic time-lapse seismic data for a multi-layered model and the elastic Marmousi model. Then we apply it to the time-lapse field VSP data acquired at the Aneth oil field in Utah. A total of 10.5K tons of CO2 was injected into the oil reservoir between the two VSP surveys for enhanced oil recovery. The synthetic and field data studies show that our new method can quantitatively estimate changes in seismic velocities within a reservoir due to CO2 injection/migration.
S51A-1738
Evaluation of Temperature Changes to Measure Hydrate Formation Time
Formation of gas hydrates usually accompanies a considerable amount of latent heat. An evaluation of temperature, an accurate indicator measuring the heat, was conducted to validate it as a proper method to measure the hydrate formation time. We monitored the hydrate formation in solid suspensions (Na- montmorillonite, kaolinite mixture, and pyrite) at the temperature range of 0.3-3.0°C at 30 bar. CO2 was used as the gas hydrate former. The latent heat released during the hydrate formation was measured by resistance temperature detector sensors (RTD) placed laterally in a pressurized vessel and converted to temperature change. Multiple RTD sensors in the vessel monitored the heat flow and heat dissipation along the hydrate layer. An abrupt increase in temperature (0.2 to 2.0°C) was monitored when the initiation of hydrate nuclei formation was observed through tempered sight glasses. The decrease in temperature after the sharp peak indicates that the latent heat was being carried away by thermal conduction along the hydrate forming environments.