[H-RE20] 地球温暖化防止と地学（CO2地中貯留・有効利用,地球工学）

[HRE20-P06] 不均質堆積岩におけるCO₂流動の可視化と定量評価

*朴 赫¹、蔣 蘭蘭¹、木山 保¹、西澤 修¹、張 毅¹、上田 良²、中野 正則²、薛 自求¹ (1.公益財団法人 地球環境産業技術研究機構 (RITE)、2.石油資源開発株式会社 (JAPEX）)

キーワード：CO2流動、不均質性、X線CT、可視化、CO2-EOR

To investigate CO₂ flow mechanisms and fluid recovery processes in heterogeneous rock, we designed a laboratory experimental system which visualizes CO₂ movements during flooding experiments by using X-ray CT. We carried out laboratory experiments of CO₂ flooding in heterogeneous sandstone, together with porosity calculation, fluid saturation monitoring based on CT images, and mass flow measurements for ejected fluids. Based on the experimental results, we try to understand the flooding characteristics of CO₂ in heterogeneous rocks having complex sedimentary structures, which will contribute to CO₂ geological sequestration and oil recovery. Sarukawa sandstone (diameter: 34.80mm, length: 79.85mm, north central Japan) was used in this study. Porosity of the specimen determined by X-ray CT imaging was 31.2%. As shown in figure1a, the specimen has a heterogeneous structure. Especially, upper part of the specimen is more complex than the lower part. The experiment was conducted under the pressure and temperature conditions that simulate underground environments; pore pressure: 10MPa, temperature: 40 degrees Celsius. The confining pressure selected in this study was 12MPa. Fluid pressure and its injection rate were controlled by high-precision syringe pumps. A high-pressure vessel having high transparency for X-ray was utilized in this study. The specimen was first saturated with KI aqueous solution (12.5%), and then oil was injected to change the specimens into oil-water mixed state. Totally, ten steps of CO₂ flooding were performed for this experiment. For each step, KI aqueous solution and oil were carefully recovered from the syringe pump which plays a role of back pressure. The CO₂ flooding test was carried out until the CO₂ injection reaches 3.03PV (pore volume). Figure 1b shows the differential CT images when the CO₂ injection reaches 0.26PV. In the figure, almost all of the CO₂ preferentially moves through the upper part of specimen. This represents that the sedimentation heterogeneity is the main factor that affects the CO₂ flooding pattern. The oil recovery was identified as 48.9% when injected CO₂ reached 1.0PV in the specimen. We increased the differential pressure to examine the influence of differential pressure on oil recovery in heterogeneous media. The oil recovery was 69.7% when injected CO₂ reached 2.0PV. The increment of oil recovery from 1.0PV-step to 2.0PV-step, 20.8% corresponds to more CO₂ flooding into the non-recovering zone (low porosity and/or low permeability) due to increasing of capillary pressure.