ReviewWettability of rock/CO2/brine systems: A critical review of influencing parameters and recent advances
Graphical abstract
Introduction
The injection of carbondioxide into geological formations have been acknowledged as an auspicious method of improving hydrocarbon recovery, as well as minimization of the greenhouse effect that results from the anthropogenic emissions of CO2 gas through carbon geo-sequestration (CGS) [1], [2], [3]. An agreement was reached by almost 200 countries during the Paris climate conference to ensure that the global warming is maintain lower than 2 °C beyond the pre-industrial level [4], [5]. Carbon geo-sequestration was identified as a crucial route for achieving this goal [5].
However, during geo-sequestration, the CO2 is stored in the formations at depths beyond 800 m in which CO2 exists as buoyant and supercritical fluid [6], [7]. At this condition, the density of the supercritical CO2 is lower than that of the formation brine, resulting in upward movement and leakage of CO2 through the caprock to the surface [2], [7], [8], [9]. Hence, it is essential to establish that the injected CO2 will be stored for a long time in formation during the geo-storage schemes [9]. According to International Energy Agency Greenhouse Gas (IEA-GHG) emissions database, the CO2 storage period could span to several centuries and even to millennia [10].
The success of CO2-enhanced oil recovery (EOR) and CO2 geo-storage projects are determine by the rock/fluids interfacial interactions, mainly the interfacial tensions (IFT) between the fluids and the wettability of the reservoir rock in contact with the formation brine, and the supercritical CO2 [1], [7], [8], [11], [12], [13], [14]. The wettability alteration, due to the interaction between the reservoir rock, brine and CO2, and the interfacial tension between CO2 and brine affects the capillary sealing capacity of the caprock, as well as the residual and structural trapping processes during CO2 sequestration schemes [15]. Decreasing sealing efficiency of caprock have been predicted to occur with the increasing wetting of the pore surfaces of the rock matrix by CO2 [9], [16]. Water-wet surfaces were identified to be favorable for CO2 storage due to slower vertical migration of CO2, as well as higher capillary and stratigraphic trapping capacities [2], [11], [16], [17], [18]. Whenever the alteration of oil-wet sytems into water wetting conditions occur, the ease of spontaneous imbibtion of water and oil displacement from the oil-wet rock matrix is increased [19].
Generally, the rock/CO2/brine wettability, macroscopically expressed as the contact angles, governs the fluid flow distribution/characteristics, displacement mechanisms, as well as the structural/capillary spreading [7], [12]. Likewise, the wettability controls the movement of CO2 within the reservoir through its influence on capillary force, surface chemistry phenomenon, and relative permeability [1], [20], [21]. The likely occurrence of capillary imbibition of the injected CO2 through the brine filled caprock can also be inferred from the wettability data (contact angle datasets) [22]. During hydraulic fracking stimulation, the rock wettability determines the uptake/loss of fracturing fluid into the rock matrix [23], [24]. Hence, the detailed and systematic characterization of the interfacial interactions between the reservoir rock, brine and CO2 at downhole conditions is vital for successful CO2-EOR and CO2 geological storage projects [2], [15], [25], [26], [27], [28].
A good number of experiments and molecular dynamic simulation predictions have been conducted to investigate the wettability of rock/CO2/brine systems. Most of the previous studies focused on wettability of the rock/CO2/brines at varying pressure, temperature and brine salinity. The contact angle datasets of natural rock samples, such as carbonate and sandstone rocks or representative minerals for these rocks such as clean glass surfaces and quartz (for sandstone), as well as pure calcites and limestone (for carbonates) are published in several technical papers. Recently, the wettability of shale/CO2/brine systems is being investigated to demonstrate the feasibility of CO2-EOR and carbon geo-sequestration projects in unconventional shale reservoirs. The injection of surface-active nanofluids/surfactant solutions into the reservoir for modification of reservoir rock wettability in presence of CO2 is also currently being explored.
Despite these developments, the impact of wettability alterations due to rock/CO2/brine interactions at reservoir conditions on CO2-EOR and CO2 geo-storage projects is far from being understood [11]. Several conflicting reports are presented in literature on the influence of temperature, pressure, salinity and surface roughness on the wettability of rock/CO2/brine systems. Moreover, the literature that discussed the CO2 wettability of shale, as well as the wettability of rock/CO2/nanofluids systems are yet to be reviewed. Thus, a comprehensive review of existing literature on wettability of rock/CO2/brine systems is essential for identifying the knowledge gaps, providing guideline for further studies, optimum field design and successful applications of CO2-EOR and carbon geo-sequestration projects in the conventional and unconventional reservoirs.
A systematic and technical review of CO2 wettability of the conventional and unconventional reservoir rocks was conducted in this study. The consequences of such wettability alteration on CO2 geological storage and CO2-enhanced hydrocarbon recovery potential were highlighted. The existing literature on wettability of sandstone-CO2-brine and carbonate-CO2-brine systems, as well as proxy minerals for these rocks such as clean glass surfaces and quartz (for sandstone), pure calcites and dolomite (for carbonates) were reviewed. We then extensively reviewed CO2-wettability of shale and the analogous mineral (mica), as well as rock CO2-wettability alteration using nanofluids/surfactants. Prior to this, the mechanisms of permanent CO2 trapping in the reservoir and influence of critical parameters on rock/CO2/brine wettability modifications were reviewed. From the reviewed literature, previous findings and state-of-art were reported, conflicting results and knowledge gaps were identified and direction for future studies were suggested. The review literature provides guidelines for future studies, selection of ideal formations and conditions for further assessment and implementations of CO2-enhanced hydrocarbon recovery and CO2 geo-sequestration projects.
Section snippets
Concept of wettability and capillary forces
In a 3-phase system comprising of the rock matrix, the non-aqueous or oleic phase, as well as the aqueous or brine phase, the rock surface wettability is generally defined in terms of the hydrophobicity or the aqueous phase hydrophilicity, and the inclination of the phase to adhere to or spread on the rock surface [29], [30]. The phenomena of wetting was originally identified by Galileo in 1612 [31] while the equations to describe the contact angles (?) were suggested by Young in 1805 [32]
CO2 storage mechanisms
There are four principal mechanisms by which the injected CO2 can be rendered immobile within the formation or the upward migration of the CO2 to the surface can be prevented [2], [25], [75]. These mechanisms have been identified as (1) structural or stratigraphic trapping [2], [15], [76], [77], (2) residual or capillary trapping [17], [75], [78], [79], [80], [81], (3) solubility or dissolution trapping [18], [25], [75], [82], [83], [84] and (4) mineral trapping [85], [86], [87], [88], [89],
Literature review
A review of previous literature on wettability of carbonate/CO2/brine systems, and its analogous minerals such as calcite and limestone, as well as wettability of sandstone/CO2/brine systems and its representative minerals such as quartz and pure glass is presented in this section.
Influence of critical parameters on the wettability of rock/CO2/brine system
A critical analysis of the existing literature on wettability of rock/CO2/brine systems presented in Table 1, Table 2, Table 3, Table 4 showed that the influence of temperature, pressure, salinity, surface roughness and total organic content (TOC) of shale were the most critical parameters that defines the wetting properties of rock/CO2/brine systems. An extensive discussion of influence of these parameters on wettability of rock/CO2/brine systems is presented in this section.
Recent advancement in rock/CO2/brine wettability studies
Recently, attentions are being focus on wettability of shale/CO2/brine systems as well as alterations of rock/CO2/brine wettability in presence of surface-active agents such as nanofluids and nano-surfactants for hydrocarbon recovery and CO2-geostorage. The review of existing literature on CO2 wettability of shale, as well as wettability of shale/CO2/nanofluids systems is presented in this section.
Conclusions and future outlook
The detailed and systematic characterization of the wettability of rock/CO2/brine system is vital for successful CO2-EOR and CO2 geological storage projects. A systematic and technical review of CO2 wettability of the conventional and unconventional reservoir rocks was conducted in this study. Precisely, the existing literature on wettability of sandstone/CO2/brine and carbonate/CO2/brine systems, as well as proxy minerals for these rocks such as quartz (for sandstone), pure calcites and
Conflicts of interest
The authors declare no conflicts of interest.
Acknowledgement
The authors would like to thank the Institute of Hydrocarbon Recovery, Universiti Teknologi PETRONAS for supporting this research through PETRONAS Research Fund (PRF) grant awarded to Eswaran Padmanabhan.
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