Numerical Study on Flow Behavior of CO2 around Injected Well for Risk Assessment of Carbon Capture and Storage

摘要

For reliable risk assessment of carbon dioxide capture and storage (CCS), it is very important to predict the behavior of migration of injected CO2 in relation with time and space under the environment of underground. Especially, it is important to estimate the amount of storage and leakage of injected CO2 by way of both experimental and simulation study. In this study, we conducted experimental study about flow behavior of supercritical CO2 and water in porous media. The experiment was conducted to optimize relative permeability curves in the process of CO2 injection, and it resulted that 1) water mobility was relatively high compared with that of COj when grain size was large and 2) relative permeability to CO2 became higher under the condition below critical point of CO2. In addition, we interpreted transport phenomena of CO2 after shutoff of CO2 injection that 1) liquid CO2 easily migrated into geological formation in the cases of small grain size and low temperature and 2) dissolved CO2 migration due to groundwater flow contributed to the change of CO2 distribution under the condition of high water saturation. Then we considered some scenarios of leakage around injection well and migration of CO2 through numerical study. We constructed a simple strata model, which is consisted from a reservoir layer, a cap rock layer and upper layer. To discuss the effect of reservoir depth, we set a couple of model strata: (a) upper limit of reservoir-800 m level, thickness of cap rock is 200 m, and (b) upper limit of reservoir is-200 m, thickness of cap rock is 100 m. In addition, we set four combinations of horizontal and vertical permeability for reservoir: (a) 500 mD, 100 mD, (b) 50 mD, 10 mD, (c) 500 mD, 10 mD and (d) 50 mD, 100 mD. Porosity is uniformly set to 0.4. On the other hand, permeability value of upper layer is set to 1000 mD. The surface temperature is 20 deg.C and thermal gradient is 3 deg.C/100m. To analyze effect of fail in well casing and/or cementing, we defined a gap between the well casing and surrounding layer. The gap has thickness and different geological conditions with surrounding layers. For gap, we set various permeability and 2 scenarios of leakage: (a) crack covers cap rock level, (b) crack covers entire well depth from bottom to surface. The CO2 injection rate is 1 million ton/year and duration is 50 years. Using the constructed injection model, we carried out some simulation run using TOUGH2-CO2 by LANL. As the result we obtained the relation between injection volume and the amount of CO2 leakage through the gap model, and distribution of CO2 in the model layer. We considered the effect of each parameter on flow behavior of CO2 on gap model and quantified the amount of CO2 leakage. These results will be implemented to the risk assessment system GERAS-CO2 of AIST.