CO2 wettability of seal and reservoir rocks and the implications for carbon geo-sequestration

We review the literature data published on the topic of CO2 wettability of storage and seal rocks. We first introduce the concept of wettability and explain why it is important in the context of carbon geo-sequestration (CGS) projects, and review how it is measured. This is done to raise awareness of this parameter in the CGS community, which, as we show later on in this text, may have a dramatic impact on structural and residual trapping of CO2. These two trapping mechanisms would be severely and negatively affected in case of CO2-wet storage and/or seal rock. Overall, at the current state of the art, a substantial amount of work has been completed, and we find that: 1. Sandstone and limestone, plus pure minerals such as quartz, calcite, feldspar, and mica are strongly water wet in a CO2-water system. 2. Oil-wet limestone, oil-wet quartz, or coal is intermediate wet or CO2 wet in a CO2-water system. 3. The contact angle alone is insufficient for predicting capillary pressures in reservoir or seal rocks. 4. The current contact angle data have a large uncertainty. 5. Solid theoretical understanding on a molecular level of rock-CO2-brine interactions is currently limited. 6. In an ideal scenario, all seal and storage rocks in CGS formations are tested for their CO2 wettability. 7. Achieving representative subsurface conditions (especially in terms of the rock surface) in the laboratory is of key importance but also very challenging

Brine/CO

It has been long recognized that interfacial interactions (interfacial tension, wettability, capillarity and interfacial mass transfer) govern fluid distribution and behaviour in porous media. Therefore the interfacial interactions between CO2, brine and reservoir oil and/or gas have an important influence on the effectiveness of any CO2 storage operation. There is a lack of experimental data related to interfacial properties for all the geological storage options (oil & gas reservoirs, coalbeds, deep saline aquifers). In the case of deep saline aquifers, there is a gap in data and knowledge of brine-CO2 interfacial properties at storage conditions. More specifically, experimental interfacial tension values and experimental tests in porous media are necessary to better understand the wettability evolution as a function of thermodynamic conditions and it’s effects on fluid flow in the porous media. In this paper, a complete set of experimental values of brine-CO2 Interfaciale Tension (IFT) at pressure, temperature and salt concentration conditions representative of those of a CO2 storage operation. A correlation is derived from experimental data published in a companion paper [Chalbaud C., Robin M., Lombard J.-M., Egermann P., Bertin H. (2009) Interfacial Tension Measurements and Wettability Evaluation for Geological CO2 Storage, Adv. Water Resour. 32, 1, 1-109] to model IFT values. This paper pays particular attention to coreflooding experiments showing that the CO2 partially wets the surface in a Intermediate-Wet (IW) or Oil-Wet (OW) limestone rock. This wetting behavior of CO2 is coherent with observations at the pore scale in glass micromodels and presents a negative impact on the storage capacity of a given site