Modeling of CO2-LPG WAG with asphaltene deposition to predict coupled enhanced oil recovery and storage performance.

Combined carbon capture and storage and CO2-enhanced oil recovery (CCS-EOR) can reconcile the demands of business with the need to mitigate the effects of climate change. To improve the performance of CCS-EOR, liquefied petroleum gas (LPG) can be co-injected with CO2, leading to a reduction in the minimum miscibility pressure. However, gas injection can cause asphaltene problems, which undermines EOR and CCS performances simultaneously. Here, we systematically examine the mechanisms of asphaltene deposition using compositional simulations during CO2-LPG-comprehensive water-alternating-gas (WAG) injection. The LPG accelerates asphaltene deposition, reducing gas mobility, and increases the performance of residual trapping by 9.2% compared with CO2 WAG. In contrast, solubility trapping performance declines by only 3.7% because of the greater reservoir pressure caused by the increased formation damage. Adding LPG enhances oil recovery by 11% and improves total CCS performance by 9.1% compared with CO2 WAG. Based on reservoir simulations performed with different LPG concentrations and WAG ratios, we confirmed that the performance improvement of CCS-EOR associated with increasing LPG and water injection reaches a plateau. An economic evaluation based on the price of LPG should be carried out to ensure practical success.

Effects of Aqueous Solubility and Geochemistry on CO2 Injection for Shale Gas Reservoirs.

In shale gas reservoirs, CH4 and CO2 have finite aqueous solubilities at high-pressure conditions and their dissolutions in water affect the determination of the original gas in place and the CO2 sequestration. In addition, the dissolution of CO2 decreases the pH of connate water, and the geochemical reactions may thus occur in carbonate-rich shale reservoirs. The comprehensive simulations of this work quantify the effects of aqueous solubility and geochemistry on the performance CO2 huff-n-puff process in shale gas reservoir. Accounting for the aqueous solubility of CH4 increases the initial natural gas storage and natural gas production. The effect of the aqueous solubility of CO2 enables to sequester additional CO2 via solubility trapping. Considering the geochemical reactions, the application of the CO2 huff-n-puff process causes the dissolution of carbonate minerals and increases the porosity enhancing the gas flow and the gas recovery. Incorporation of geochemistry also predicts the less CO2 sequestration capacity. Therefore, this study recommends the consideration of aqueous solubility and geochemical reactions for the accurate prediction of gas recovery and CO2 sequestration in shale gas reservoirs during the CO2 huff-n-puff process.

Thermochemical Nitriding and Oxynitriding of Ti Alloys.

Nitrided and oxynitrided coatings that formed on α alloy (c.p.-Ti), near-α alloy (Ti-2.1Al-2.5Zr), (α + ß) alloy (Ti-6Al-4V), and ß alloy (Ti-6Al-2Zr-1Mo-1V) were microstructurally characterized. The nitriding at 950 °C and PN2 ═ 105 Pa for 5 h formed TiN, Ti2N, and α-Ti(N) layers from the surface. The nitriding tendency increased in the order of ß alloy, (α + ß) alloy, near-α alloy, and α alloy. The Ti-N coatings transformed to Ti-N-O coatings when the nitrided alloys were exposed to PO2 ═ 10-2 Pa during cooling at the final stage of the nitriding. This oxynitriding process led to the formation of TiNxO1-x, Ti2N, and α-Ti(N,O) layers from the surface where a small amount of rutile-TiO2 coexisted. Oxynitriding was more effective than nitriding in increasing the surface microhardness, with the former accumulating more compressive residual stress than the latter.