Capillary Alteration of Caprocks by Acid Gases

Abstract

The safety of acid gas geological storage is to a large extent controlled by the capillary properties of the caprock. This low-permeable (e.g., clayey) porous media usually saturated with water acts as a capillary barrier to the underlying stored acid gas, provided its water-wettability is preserved and water/acid gas interfacial tension (IFT) is high enough. The displacement or capillary breakthrough pressure, above which the stored acid gas intrudes into the caprock, is directly related to those two interfacial properties. Water/acid gas IFTs have recently been thoroughly characterized. However, little is known on the effect of acid gases (CO2, H2S and their mixtures) on the water-wettability of caprocks. We present an experimental setup and procedure for measuring contact angles on mineral substrates in the conditions of geological storage. Measurements have been carried out in a range of pressures extending up to 150 bar, both with CO2 and H2S, and with mineral substrates representative of caprock minerals such as quartz and mica, as well as with a substrate sampled from the caprock of a depleted gas reservoir. We observed that the wettability alteration of mica is moderate in the presence of dense CO2, but pronounced in the presence of dense H2S. In contrast, the wettability of quartz and of the 'real' caprock substrate is not altered by dense CO2 or H2S. In addition to those substrate- and acid gas-dependent wettability effects, the much lower water/acid gas IFTs as compared to water/hydrocarbon gas IFTs are responsible for a loss in capillary-sealing potential of a given caprock when a hydrocarbon gas is replaced with acid gas, especially when the acid gas is rich in H2S. This potential, as evaluated by the displacement or capillary breakthrough pressure, should be determined very carefully when planning an acid gas geological storage operation. 1. Introduction As an increasing number of H2S-containing (sour) gas reservoirs are being exploited around the world, there is a growing interest for injecting and storing in geological formations the H2S rich-acid gas that is separated from the (sour) natural gas in gas processing plants. For instance, acid gas disposal in geological formations has been practised over the past 15 years in Western Canada, where more than 3 Mt of H2S and 3 Mt of CO2 have been injected, with a maximum up to 83% of H2S in one of the 40 storage sites (deep aquifers or depleted hydrocarbon reservoirs; Bachu, 2007). The reinjection of H2S-rich acid gases in massive quantities is currently being considered in some reservoirs such as the Kashagan oil field in the North Caspian Sea. These reservoirs usually contain CO2 along with H2S as associated gases, which are both separated in the gas plant. The injection of the resulting acid gas stream in a geological formation is interesting for the two following reasons:to avoid atmospheric emissions of CO2, andto avoid H2S desulphurization through the Claus process, which has many drawbacks, both environmental and economical (Abou-Sayed et al., 2005). The implementation of this option on a large scale requires a proper assessment of the effects induced by the presence of acid gas on the integrity of the formation. This assessment is the subject of many research studies, mostly conducted in the context of CO2 geological storage. A large part of this effort addresses the different possible leakage mechanisms by which CO2 may escape from the geological formation where it is stored. This effort needs to be extended to acid gases containing significant amounts of H2S.

Molecular Dynamics Simulation of Water/CO2-quartz Interfacial Properties: Application to Subsurface Gas Injection

“…Figure 2) indicating that the H 2 S has a higher affinity for the quartz surface than CO 2 . Our simulation corresponds to surfaces that are more hydrophobic than the ones investigated experimentally by Shah et al [48]. The reason for these differences has been advanced in our previous work, Iglauer et al [26].…”

“…H2S/H2O/quartz contact angle from simulations (this work) compared to experimental data from Shah et al[48] (limited results) and simulations of the CO2/H2O/quartz water contact angle from Iglauer et al Density profile of CO2 and H2S above the quartz surface; from the (50mol% H2S:50mol% CO2)/water/quartz contact angle simulation.…”

Integrative Modeling of Caprock Integrity in the Context of CO2Storage: Evolution of Transport and Geochemical Properties and Impact on Performance and Safety Assessment

“…The pH changes due to the presence of CO 2 can also potentially modify the mineral wettability, although this is still a controversial matter (see e.g. Chiquet et al, 2007;Shah et al, 2008;Fleury et al, this issue).…”

Evaluating Sealing Efficiency of Caprocks for CO2Storage: an Overview of the Geocarbone-Integrity Program and Results

“…-a petrophysical characterization of caprock samples including diffusion measurements [7,9], and a study of the contact angle to evidence wettability changes [10,11];…”

“…The experimental setup and procedure used in a preliminary set of measurements [10] were improved and the final results show that the contact angles corresponding to the drainage process of interest (i.e., CO 2 displacing water) were barely affected by an increase in CO 2 pressure up to 14 MPa [11,22]. Hence, even though all types of rock substrates have not been tested, it can be reasonably assumed that dense CO 2 does not significantly alter the water-wettability of typical rock minerals, at least in the drainage process of CO 2 displacing water.…”