Experimental evaluation of injection pressure and flow rate effects on geological CO2 sequestration using MRI

摘要

Deep Saline aquifers are promising long-term storage sites for CO2 emissions. The injection pressure is one of major factor that determines CO2 migration phase equilibrium, and the flow rate influences the injectivity behavior and plume migration. However, data and knowledge of the injection pressure and the variation of velocity in displacement process are scarce. In this paper, we describe a set of laboratory tests on two-phase flow drainage of CO2 and brine. Fundamental characteristics such as the CO2 front, the breakthrough time and sweep efficiency, residual water saturation, due to the injection pressure and rate of displacement fluid was explored. The measurement is conducted at 40 °C under two different pressure conditions and four CO2 injection rate, representative of a CO2 storage operation. Magnetic resonance imaging (MRI) technique was used to observe pore-scale events in drainage process. After the injection of 3-5PV CO2, saturation of CO2 keeps constant indicating that the rest part remains inaccessible. The CO2 saturation at the last time for 8MPa is higher than 6MPa. Residual saturation and breakthrough time of different injection rate displacement procedures are different. The packed porous media is not perfectly homogeneous, so the CO2 run through the high permeability regions vertically in a short period. With CO2 injection, the brine saturation decreased gradually from the inlet of the bead-pack core, and the MRI image turned into dark gradually. The CO2 front proceeding in the porous media could be measured clearly, we can find CO2 "channeling" phenomena obviously in the case of gaseous CO2 displacement due to the difference of fluid viscosities and densities, and it can be seen from the profiles of 76 and 80 min. This phenomenon causes premature breakthrough of the CO2 to occur, thereby reducing the displacement efficiency of the water. And yet, the piston-like displacement occurred in the supercritical CO2 drainage experiment, due to the viscosity of supercritical CO2 is higher than gaseous CO2. According to capillary number definition, the Darcy velocity U keep invariable with constant CO2 flow rate, and the non-wetting phase (CO2) viscosity increase with the injection pressure increase. In other words, the displacement process capillary number of 8MPa is higher than 6MPa. The sweep efficiency in 8MPa is slightly higher compared to the CO2 injection in 6MPa. Therefore, supercritical CO2 displacement process is more practical than gaseous CO2 displacement process. The sweep efficiency will decrease for higher capillary number. At a small CO2 injection rate, the breakthrough time is long. A longer break through time resulted higher sweep efficiency. The CO2 flooding increases the displacement efficiency by raising the capillary number due to the relatively low interfacial tension values between the brine and the injected CO2. At high injection rate, especially 0.8 ml/min, the MRI image near the outlet region remain at high signal intensity in a relatively long time, indicating that the CO2-brine distribution does not change, since the displacement is steady. The sweep efficiency decreases as the injection flow rate increase, but the decrease extent reduces gradually. This means there is a critical capillary number for CO2 sequestration, which results in minimum sweep efficiency. In future experimental work, we will go into more detail about the critical capillary number. Within the capillary number scope of 10~(-9) to 10~(-10), higher capillary number will decrease the CO2 sweep efficiency.