Measurement of CO2 Solubility in Saline Groundwater in Northern China’s main Basin
Xueli Zhang, Qingchun Yu
China University of Geosciences (Beijing)
CO2 attracts widespread concern as an important factor leading to the greenhouse effect. Geological carbon storage (GCS) is now recognized as a relative effective method for mitigating the greenhouse effect. Depleted oil-gas reservoirs, unminable coal seams and deep saline aquifers are potential sites for CO2 geological storage. There are vast sedimentary basins with advantageous geological condition in China, and deep saline aquifers are potential CO2 sequestration sites. The solubility of CO2 in saline is an important parameter to estimate the dissolve storage capacity of corresponding aquifer. Study of CO2 solubility in specific saline groundwater is very few. This paper selected a few potential aquifer for CO2 storage and measured CO2solubility in them. We selected Carboniferous Xiaohaizi strata of Tarim Basin, Triassic Middle strata of Junggar Basin, Paleogene Dongying group, third section of River Street group and Kongdian group of Bohai Bay Basin.
In this experiment, the CO2 we used was of 99.99% purity. Analytical pure reagents, including KCl, NaCl, MgCl2, CaCl2, NaHCO3, NaOH and HCl were provided by the Beijing Reagent Factory with mass fraction purities of 99.95%. Test water samples were made up artificially according to the chemical composition of the specific formation saltwater. Experimental temperature and pressure were in the range of 40℃~80℃ and 8~12MPa. The experimental used the method of static reaction. Reactor pressure was read out by precision pressure gauge(accuracy of 0.25), and temperature control was achieved through a water bath whose temperature controller error was less than 0.2℃. Before experiment, we injected 5.010×10-1L water sample into the reactor, and adjusted the temperature controller to the desired temperature. The reactor was placed in water bath heated to the desired temperature. Before being injected into the reactor, the CO2 was cooled to liquid in cold box(set temperature of 4.5℃). After the CO2 was cooled, we turned on the valves between cold box and piston pump and between reactor and piston pump. Piston pump then was turned on to pump the liquid CO2 into the reactor. When the pressure in reactor reached to about 20MPa, piston pump and all valves would be turned down. When the pressure stayed stable, rotate the valve to adjust pressure to the proper value. Set it aside for at least 12h, so that the reaction could reach dissolution equilibrium. After that, a part of the CO2 was dissolved in water and the other part of CO2 was in a supercritical state. Then the reactor was connected to the gas collection system which collected gas by the drainage method, and the water was saturated with CO2. The exhaust valve was turned on slowly to let out the gas from the reactor and that the saline would not be bought out by the gas. When the pressure reduced to atmospheric, room temperature and volume of water was recorded. The solubility of CO2in water sample at atmospheric pressure was calculated by the double indicator titration. The titration needed to use phenolphthalein reagent, 0.1mol/L NaOH solution, methyl orange reagent and 0.1mol/L HCl solution.
The aquifers have the potential of CO2 storage in Tarim Basin, Junggar Basin and Bohai Bay Basin was selected for study, and the solubility of CO2at reaction temperature (313.15K~353.15K) and pressure(0~12MPa) were measured. The reliability of the experimental dates were verified by contrasting with literature data of the solubility in pure water.
Through vertical and horizontal comparison of experiment dates, we can draw the following conclusions:
(1) In the range of experimental temperature, CO2 solubility in the water samples increased with the pressure increasing. CO2 solubility changed greatly with different pressure at low pressure, while the effect of pressure on CO2solubility declined at the higher pressure.
(2) In the range of experimental pressure, CO2 solubility decreased with temperature increasing. By the graph it can be learned that the effect of temperature on the solubility of CO2was greater than the effect of pressure.
(3) CO2 solubility differed with the saline that CO2 solubility in high-salinity water samples was significantly lower than its solubility in low-salinity water samples. Because the higher the concentration of ions in the brine was, the stronger the ions interacted, and the molecule was more difficult to dissolve in water, leading to a decrease in the solubility of CO2.
(4) In the experimental conditions, solubility of CO2 in Carboniferous Xiaohaizi strata of Tarim Basin, Triassic Middle strata of Junggar Basin, Paleogene Dongying group, third section of River Street group and Kongdian group of Bohai Bay Basin were 0.683~1.137mol/kg, 0.564~1.168mol/kg, 0.765~1.178mol/kg, 0.837~1.213mol/kg, 0.697~1.018mol/kg, respectively.