Effects of temperature and CO2/Brine cycles on CO2 drainage endpoint phase mobility – implications for CO2 injectivity in deep saline aquifers

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The coupled process of (seasonal) CO2 downhole temperature cycles and natural CO2/brine drainage-imbibition cycles is an inherent feature of large-scale CO2 injection schemes during the injection phase of a geologic storage site. Analysis of long-term field injection data shows that the effect of temperature cycles on injectivity is significant and cannot be explained by changes in CO2 properties alone, raising the question of whether CO2-rock properties such as relative permeability are also changing. We present the results of lab-scale coreflood experiments designed to measure CO2 drainage endpoint phase mobilities and relative permeabilities for cyclic CO2 displacing brine at representative reservoir drainage pressure/temperature conditions, using Basal Cambrian Sandstone (BCS) core samples from the storage reservoir of a large-scale (>1Mtpa) CO2 injection scheme in operation. The isothermal experiments are conducted at three temperatures: average seasonal low, average seasonal high, and reservoir. The measurements also incorporate the effects of repeated saturation cycles (or saturation hysteresis), between ∼100% brine saturation and irreducible brine saturation for the same core sample for up to 12 repetitive drainage-imbibition cycles. The results show that the CO2/brine two-phase flow characteristics change after each flooding cycle. CO2 drainage endpoint phase mobility increases as temperature increases, but the mobility increases cannot be fully explained by the decrease in CO2 viscosity and rock permeability changes alone. CO2 endpoint relative permeability (measured at about 0.20 ± 0.1) contributes marginally to the CO2 phase mobility changes with temperature and exhibits a weak inconsistent dependency on temperature. Field scale injectivity of CO2 can be optimised in large-scale CCS injection schemes design by exploiting the CO2 temperature-CO2/brine cycling-phase mobility functional relationship where CO2 phase mobility is enhanced at higher CO2 downhole temperatures. Field data show that this effect, though significant, may be weakened or even overridden by other temperature-dependent phenomena.