CO_2-triggered gelation for mobility control and channeling blocking during CO_2 flooding processes

CO2 flooding is regarded as an important method for enhanced oil recovery （EOR） and greenhouse gas control. However, the heterogeneity prevalently dis- tributed in reservoirs inhibits the performance of this technology. The sweep efficiency can be significantly reduced especially in the presence of ＂thief zones＂. Hence, gas channeling blocking and mobility control are important technical issues for the success of CO2 injection. Normally, crosslinked gels have the potential to block gas channels, but the gelation time control poses challenges to this method. In this study, a new method for selectively blocking CO2 channeling is proposed, which is based on a type of CO2-sensitive gel system （modified polyacry- lamide-methenamine-resorcinol gel system） to form gel in situ. A CO2-sensitive gel system is when gelation or solidification will be triggered by CO2 in the reservoir to block gas channels. The CO2-sensitivity of the gel system was demonstrated in parallel bottle tests of gel in N2 and CO2 atmospheres. Sand pack flow experiments were con- ducted to investigate the shutoff capacity of the gel system under different conditions. The injectivity of the gel system was studied via viscosity measurements. The results indi- cate that this gel system was sensitive to CO2 and had good performance of channeling blocking in porous media. Advantageous viscosity-temperature characteristics were achieved in this work. The effectiveness for EOR in heterogeneous formations based on this gel system was demonstrated using displacement tests conducted in double sand packs. The experimental results can provide guideli- nes for the deployment of theCO2-sensitive gel system for field applications.

Electrical Conductivity and Sensitive Characteristics of Ag-Added BaTiO_3-CuO Mixed Oxide for CO_2 Gas Sensing

Carbon dioxide gas sensors based on BaTiO3-CuO composite with different concentrations of Ag addition（1,1.5 and 2 wt%） have been prepared by stander ceramic method and sintered at 500 and 700℃ for 5 h.Electrical conductivity and gas sensing properties of the prepared samples were investigated.Electrical conductivity measurement was used to characterize the obtained sintered sensor pellets.It was found that electrical conductivity and the sensitivity to CO2 were improved with Ag addition and sintering.The correlation between Ag content at different sintering temperature and sensing characteristics towards CO2 is discussed.

Sensitivity of on-line wavelength during retrieval of atmospheric CO2 vertical profile

Accurately measuring the differential molecular absorption cross section is the key to obtaining a high-precision concentration of atmospheric trace gases in a differential absorption lidar（DIAL） system. However, the CO2 absorption line is meticulous at 1.6 μm, easily translating and broadening because of the change of temperature and pressure. Hence, measuring the vertical profile of atmospheric temperature and pressure to calculate the vertical profile of the CO2 weight parameter is necessary. In general, measuring atmospheric temperature and pressure has a certain amount of uncertainty. Therefore, this study proposes the concept of a balanced on-line wavelength,where the differential molecular absorption cross section is larger and the CO2 weight parameter is insensitive to the uncertainty of atmospheric temperature and pressure. In this study, we analyzed the influence of uncertainty on the CO2 weight parameter at every preselected wavelength, as well as determined an appropriate wavelength near one of the absorption peaks. Our result shows that 1572.023 nm should be one of the appropriate balanced online wavelengths. The measurement errors of the mixing ratio of CO2 molecule in this wavelength are only 0.23%and 0.25% and are caused by 1 K temperature error and 1h Pa pressure error, respectively. This achievement of a balanced on-line wavelength will not only depress the requirement of the laser’s frequency stabilization but also the demand for measurement precision of the atmospheric temperature and pressure profile. Furthermore, this study can achieve the exact measurement of the vertical profile of atmospheric CO2 based on an independent differential absorption laser.