Unsteady state supercritical CO2 flooding experiments on samples from Stenlille-6, Gassum formation: CO2FLOW Project WP1

This report presents the findings from the unsteady-state core flooding experimental study conducted as part of CO2FLOW project, titled "Experimental Study and Modelling of CO2 Propagation in Geological Storage." Funded by INNO-CCUS.
To investigate the effect of dry and wet CO2 injection into Stenlille’s saturated brine sandstone reservoir cores, three core flooding experiments were conducted: EXP-1 (dry CO2 injection), EXP-2 (wet CO2 injection), and EXP-3 (wet CO2 injection using the same cores as EXP-1). This experimental study aimed to evaluate the injectivity as well as investigation on effects of both dry and saturated CO2 injection on the Stenlille samples through pre- and post-analysis of the rock properties. The study also involves calculating the relative permeability and capillary pressure curves by performing history matching of these experimental results which will be reported further.
Sandstone core seal piece 3 from well Stenlille-6 was received, and 14 horizontal plugs were successfully extracted and cleaned. The cores are called C1 to C14, and their petrophysical properties, porosity, and permeability were measured. Then, a CT scan was conducted, and based on all the information obtained, two sets of cores for the experiments were selected. The criteria for choosing pairs of core plugs were identical parameters such as porosity, permeability, same lithology and grain size, which were 25.8% and 157.8 mD for the first set (C10-C13) and 25.5% and 538.7 mD for the second set (C7-C8), respectively. The core plugs were saturated with synthetic formation brine and prepared to match the produced brine chemistry from stenlille-1 well. The saturation was then tested using the Archimedes method to ensure full saturation of the core. Results indicate that the cores are fully saturated with an average water saturation of 99% PV.
The three core flooding experiments were performed at a fluid pressure of 160 bara, a hydrostatic confining pressure of 185 bara, and a temperature of 50 °C, mimicking the conditions of the actual reservoir. The initialization of all experiments was the same: the brine-saturated core was assembled into the core holder and pressurized to the desired pressure by increasing the temperature. The absolute brine permeability of samples at reservoir condition as measured to be 121.23 mD and 123.58 for the core plugs (C10-C13) used for EXP-1 and EXP-3 and 365.50 mD for EXP-2.
The dry CO2 flooding in experiment EXP-1, which employs both drainage and vaporisation mechanisms, reduced the brine saturation to 4% PV after 200 PVI (pore volume injected). In contrast, the CO2 flooding in EXP-3, which used CO2 equilibrated with brine before injection through the same set of core plugs, achieved 49.73% PV residual water saturation after 200 PVI. Saturation results for EXP-2, which employed equilibrated CO2 using high-permeability cores, suggested 40.3% PV after 60 PVI. This experiment was terminated when no more water was produced, and no dry-out was expected due to equilibrated CO2 injection.
The comparative study of final water saturation from the two experiments with the same set of core plugs (EXP-1 and EXP-3) underscores the significance of dry-out in EXP-1. The CO2 can vaporize a significant amount of brine, demonstrated by a substantial decrease in water saturation after liquid phase water production stops completely, attributed to salt precipitation and permeability alteration through the activation of vaporisation mechanisms. The water production in EXP-2 and EXP-3 occurs due to displacement mechanisms, in contrast to EXP-1, where water saturation decreases substantially after liquid phase production ends. The final water saturation in EXP-2 was 9.43% PV lower than in EXP-3. The analysis concludes that this difference is due to the high permeability of the cores used in EXP-2. The substantial difference between residual water saturation of EXP-1 and EXP-3 highlights the potential of the dry-out mechanism to reduce water saturation around the injection point and cause permeability alteration of the formation due to the potential risk of salt precipitation.
Post-experiment observations revealed changes in porosity and permeability of the cores in EXP-1: a 6.9% PV change in porosity and a 31.67% change in permeability for core number C10 (inlet core), and a 4.12% PV change in porosity and a 10.85% change in permeability for core number C13 (outlet core). The inlet end face of the first plug (C10) showed significant salt precipitation that could virtually be observed. Even though the cores in EXP-1 were mostly dried, the endpoint relative permeability of the system was 0.6, indicating that the salt precipitated in pathways impacted the effective permeability of the core. Despite having a very low remaining water saturation of 4%, the system's permeability compared to the air absolute permeability changed by nearly 21%.
The overall effect of dry CO2 injection started at the inlet of the core and substantially changed the core's physical properties. The permeability measurements after the experiment support this conclusion, showing that the reduction in effective permeability in an almost dried-out core was significant, indicating that the decrease was due to salt precipitation. Furthermore, no drying out or salt precipitation was observed even after 200 PV of equilibrated CO2 injection in EXP-3. However, compared to wet-scCO2 injection experiment, EXP-3, the final effective permeability to scCO2 is higher for the dried core plugs in EXP-1. This indicates that the overall effect of water vaporisation during Dry-scCO2 core flooding in this study leads to an increase in the effective permeability of the core sample, even though salt precipitation occurred. It is important to highlight that these results were obtained using Gassum formation brine at elevated reservoir pressure and temperature (50 °C and 160 bar).