Static fatigue of saline rocks under different CO2 phase conditions

Abstract

Static fatigue tests were conducted on core samples saturated with different phase CO2 to evaluate the fatigue rate and bulk modulus under constant but stepped stress increment. There was re-appearance of primary fatigue at higher stress in all the samples. In the brine saturated sample, for a 4 MPa deviatoric stress increase, there was about 89% increase in primary fatigue rate with up to 16% more strain, up to 88% reduction in fatigue rate when fatigue goes from primary to secondary fatigue and a reduction in primary fatigue duration by 39% with a bulk modulus of 1.050 GPa. Also, the secondary fatigue rate in stress conditions that had the initial primary fatigue portion was up to 16% higher than the secondary fatigue rate for stress conditions that do not have a primary fatigue portion. Saturating the rock with different phase CO2-brine caused a significant change in the fatigue behaviour and bulk modulus depending on the phase of the CO2. While the effect of CO2 phase on bulk modulus is most pronounced in the supercritical CO2 phase (decrease of 22 %), change in fatigue rate is most pronounced on gaseous CO2 phase (an increase of up to 45 %) especially when it is combined with brine (an increase of up to 93 %). The results of this study provide new insights into the deformation and response of rock's bulk moduli to different states of CO2-brine that could exist in a CO2 storage site and will be useful for the future site selection and maintenance of the geological storage projects.

Effect of CO2 Phase on Pore Geometry of Saline Reservoir Rock

“…Dissolution of CO 2 in brine produces an acidic solution. Results from Peter et al (2020), Pimienta et al (2017) and Zou et al (2018) show that the impact of CO 2 on the properties of rock starts vigorously almost immediately. Hence, for this experiment, any duration greater than 5 days is considered sufficient for the phaseCO 2 -brine to have an effect on the rock properties.…”

Microscopy and image analysis of the micro-fabric and composition of saline rocks under different phaseCO2-Brine states

“…Li et al (2019) reported that the effect of CO2 on the properties of rocks increase with longer exposure time. Results from Peter et al (2020); Pimienta et al (2017); Zou et al (2018) and Olabode and Radonjic (2013) showed that CO2-brine causes marked effects on rock properties even within a short contact period and the severity increases over time. Pimienta et al (2017) had used 2 and 4 hours exposure time; Zou et al (2018) had used 0.5 hour exposure time and Li et al (2019) had used a maximum of 168 hours.…”

“…They all had significant changes in properties. Peter et al (2020), reported that excess of 5 days is good enough for CO2-brine to affect the properties of the rocks. Therefore, it was decided that for this experiment, more than of 5 days would be good enough for the phaseCO2 saturated-brine to have an effect on the properties of the rocks.…”

“…Al-Zaidi, Fan, and Edlmann (2018) showed that the pore pressure due to CO2 injection into porous rock depends on the phase of the CO2. Given the variation in pressure-temperature (P-T) conditions of reservoirs as shown by thousands of borehole bottom hole data (KGS, 2003), the thermodynamics and phase behavior (Ricketts, 2016), as well as the solubility of CO2 in brine (Rumpf, Nicolaisen, Öcal, & Maurer, 1994), there will be undissolved CO2 and the undissolved CO2 will exist in different phases alongside brine in the reservoir for a long time (Bachu, 2000;Holloway et al, 2006;Peter et al, 2020;Plug & Bruining, 2007;Xie et al, 2016). Ennis-King and Paterson (2001;2003) and Blunt et al (2013) predicted this time to be in thousands of years.…”

“…CO2 sequestration in underground is the injection and storage of CO2 in saline reservoirs and it is the most promising way of reducing anthropogenic CO2 (Quale & Rohling, 2016). The stored CO2 is contained in the pore spaces and will change the pore pressure and the effective stress (Peter et al, 2020;Rutqvist, 2012;Zhang, 2017) and furthermore, cause unique changes in the transport, petrophysical and geomechanical properties of the rocks depending on the phase of the CO2 (Al-Zaidi et al, 2018). However, there has been little attention to evaluating the micro-fabric and mineral topology in saline CO2 reservoir rocks under the different possible phaseCO2-brine conditions, which controls those properties.…”