GC51B-1057 0800h
Evaluation of Seismic Properties and Their Dependence on CO$_2$ Phases in a Porous Sandstone; Insights From Laboratory Injection Experiments
Carbon dioxide (CO$_2$) sequestration in geological formations would be the most straightforward strategy to manage CO$_2$ derived from fossil fuels. For practical sequestration, we need to develop monitoring methods of the underground CO$_2$. Thus, it is essential to understand physical properties of the strata and their dependence on CO$_2$ phases of liquid, gas and supercritical. Since seismic monitoring has been a useful technique, seismic wave responses have priority to be examined. In this study, seismic velocity of a porous sandstone is measured during injection of three different CO$_2$ phases using a tomography method and the results are compared with numerical simulations. The cylindrical rock samples were water-saturated then injected CO$_2$ normal to the bedding plane. Both P- and S- seismic waveforms were acquired using an array of piezoelectric transducers, which were glued to the side-wall of the samples. As the injection proceeds, two basic features can be observed in the waveform; a delay in the arrival time and a strong amplitude attenuation. The magnitude of the delay depends on the phase of the injected CO$_2$. These features suggest that CO$_2$ injection reduces the seismic velocities and increases the elastic viscosity of the sandstone. The observed data can be compared well with synthetic waveforms and this approach makes it possible to analyze the detail effects of injection on the seismic waveforms. In order to visualize the velocity change due to the injection, travel time tomography has also been carried out to the specimen with transducer arrays. The area of the injected CO$_2$ showed a lower value of P- or S- wave velocity and the progressive development of the injecting pathways was examined. These results demonstrate that the 4D seismic method should be a key technology and the phase of subsurface CO$_2$ can also be identified by this technique.
GC51B-1058 0800h
Mineral Sequestration of CO2 mixed with H2S and SO2 in Sandstone-Shale Formation
Carbon dioxide (CO2) injection into deep geologic formations can potentially reduce atmospheric emissions of greenhouse gases. Sequestering less-pure CO2 waste streams (containing of H2S and/or SO2) is less expensive or requires less energy than separating CO2 from flue gas or a coal gasification process. The long-term interaction of these injected acid gases with shale-confining layers of sandstone formations has not been well investigated. We therefore have developed a conceptual model of injection of CO2 with H2S and/or SO2 into a sandstone-shale sequence, using hydrogeologic properties and mineral compositions commonly encountered in Gulf Coast sediments. We have performed numerical simulations using a 1-D radial well region considering sandstone alone and a 2-D model using a sandstone-shale sequence under acid-gas injection conditions. Results indicate that shale plays a limited role in mineral alteration and sequestration of gases within a sandstone horizon for a short time period (10,000 years in present simulations). Unlike H2S, the co-injection of SO2 results in different pH distribution, mineral alteration patterns, and CO2 mineral sequestration. Simulations generate a zonal distribution of mineral alteration and formation of CO2 and SO2 trapping minerals that depends the pH distribution. Co-injection of SO2 results in a larger and stronger acidic zone close to the well. Precipitation of CO2 trapping minerals occurs in the higher pH ranges beyond the acidic zones. In contrast, SO2 trapping minerals are stable at low pH ranges (below 5) in the front of the acidic zone. Corrosion and well abandonment caused by co-injection of SO2 is a very significant issue. Significant CO2 is sequestered in ankerite and dawsonite, and some in siderite. CO2 mineral-trapping capability can reach 76 kg per cubic meter of medium. Most of SO2 is trapped by alunite precipitation, while some of the SO2 is trapped by anhydrite and pyrite precipitation. Addition of the acid gases and induced mineral alteration result in changes in porosity. The limited information currently available on the mineralogy of natural high-pressure acid-gas reservoirs is generally consistent with our simulations.