CO₂–Water–Rock Wettability: Variability, Influencing Factors, and Implications for CO₂ Geostorage

Abstract

Carbon geosequestration (CGS) has been identified as a key technology to reduce anthropogenic greenhouse gas emissions and thus significantly mitigate climate change. In CGS, CO is captured from large point-source emitters (e.g., coal fired power stations), purified, and injected deep underground into geological formations for disposal. However, the CO has a lower density than the resident formation brine and thus migrates upward due to buoyancy forces. To prevent the CO from leaking back to the surface, four trapping mechanisms are used: (1) structural trapping (where a tight caprock acts as a seal barrier through which the CO cannot percolate), (2) residual trapping (where the CO plume is split into many micrometer-sized bubbles, which are immobilized by capillary forces in the pore network of the rock), (3) dissolution trapping (where CO dissolves in the formation brine and sinks deep into the reservoir due to a slight increase in brine density), and (4) mineral trapping (where the CO introduced into the subsurface chemically reacts with the formation brine or reservoir rock or both to form solid precipitates). The efficiency of these trapping mechanisms and the movement of CO through the rock are strongly influenced by the CO-brine-rock wettability (mainly due to the small capillary-like pores in the rock which form a complex network), and it is thus of key importance to rigorously understand CO-wettability. In this context, a substantial number of experiments have been conducted from which several conclusions can be drawn: of prime importance is the rock surface chemistry, and hydrophilic surfaces are water-wet while hydrophobic surfaces are CO-wet. Note that CO-wet surfaces dramatically reduce CO storage capacities. Furthermore, increasing pressure, salinity, or dissolved ion valency increases CO-wettability, while the effect of temperature is not well understood. Indeed theoretical understanding of CO-wettability and the ability to quantitatively predict it are currently limited although recent advances have been made. Moreover, data for real storage rock and real injection gas (which contains impurities) is scarce and it is an open question how realistic subsurface conditions can be reproduced in laboratory experiments. In conclusion, however, it is clear that in principal CO-wettability can vary drastically from completely water-wet to almost completely CO-wet, and this possible variation introduces a large uncertainty into trapping capacity and containment security predictions.

Impact of salinity on CO2containment security in highly heterogeneous reservoirs

“…However, although it is well established that brine salinity can vary widely between prospective storage reservoirs, there is a knowledge gap in terms of how this can influence storage efficiency. Importantly, experimental data showed that water salinity has a significant impact on the water‐CO 2 ‐rock contact angle θ (i.e., rock wettability) . For example, Arif et al .…”

“…This effect is caused by a better surface charge screening at higher salinities. Mechanistically, cations move to the negative surfaces charges and shield them, thus reducing the surface potential and surface polarity, which leads to a dewetting of the surface . Furthermore, salinity affects water viscosity (e.g.…”

A Realistic Look at Nanostructured Material as an Innovative Approach for Enhanced Oil Recovery Process Upgrading

“…Al-Anssari et al [129] investigated the influence of pressure on the wettability of calcites treated with and without nanoparticles in the presence of CO 2 to account for pressure variation with injection depth. An increase in pressure with increase in contact angle occurred for all surfaces tested which is attributed to an increase in intermolecular interactions between CO 2 and calcite, which increased significantly with increasing CO 2 density thus an implication of a reduction in structural and residual trapping capacities at reservoir conditions [126][127][128][130][131][132][133]. At ambient condition, oil-wet calcite was weakly CO 2 -wet (115 θa -0.1 MPa and 323 K) and strongly CO 2 -wet at storage conditions (148 θa -20 MPa and 323 K) -the high contact angle is an indication of possibilities of CO 2 leakage.…”

Enhancement of CO2 trapping efficiency in heterogeneous reservoirs by water‐alternating gas injection

“…These curves were built using experimental data . Intermediate‐wet conditions were chosen, as this is the most likely wettability scenario in CO 2 ‐brine deep saline aquifer systems . In addition, rock wettability has a significant effect on the residual water and gas saturations .…”

“…The effectiveness of the trapping mechanisms depends on various geological and hydraulic parameters, including geological reservoir heterogeneities, caprock properties, CO 2 ‐rock wettability, reservoir temperature, wettability heterogeneity, and brine salinity . Moreover, although many geological parameters cannot be changed for a particular site, it has been shown that intelligent selection of CO 2 injection technology (e.g.…”