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High-pressure methane sorption isotherms were measured on one Paleozoic and five Mesozoic shales, considered as targets for shale gas exploration in The Netherlands. The samples varied in mineralogy, organic richness, and thermal maturity. Four of the samples were clay-rich (total clay content 60−71 wt %), one contained equal amounts of clays and quartz (36 wt % and 33 wt %, respectively) and one was a marl sample (clays 34 wt %, carbonates 49 wt %). The total organic carbon contents (TOC) ranged from <1 wt % to 10.5 wt %, and the thermal maturity, as inferred from Rock-Eval analysis, from immature to overmature. Excess (Gibbs) sorption isotherms for methane were measured at 65 °C on dry samples up to 25 MPa. The maximum excess sorption capacities within this pressure range varied from 0.05 to 0.3 mmol/g (1.1−6.8 m 3 STP/t). No correlation of excess sorption capacity with TOC was found. Low-TOC, clay-rich shales had comparable or even higher methane sorption capacities per unit rock mass (mmol/g) than organic-rich shales, and a positive correlation was found between the maximum Langmuir capacity (n L ) and the clay content. This observation supports the notion that clay minerals can contribute significantly to the sorption capacity of shales. Furthermore, we demonstrate that significant errors in TOC-normalized sorption capacities may result from the uncertainties in TOC contents, especially at low TOC values. A comparison between the immature and the overmature sample (both organic-rich with equal clay contents) did not show any enhancement of the sorption capacity with thermal maturity. However, the excess sorption isotherm of the overmature sample had a distinct maximum, while no maximum was observed for the immature sample in the experimental pressure range. A Langmuir-type absolute sorption function, with a term taking the volume of the adsorbed phase explicitly into account, gave a good representation of the measured excess sorption isotherms. The three-parameter fit yielded the Langmuir parameters (n L and p L ) and a nominal density value for the adsorbed phase (ρ ads ). Two-parameter fits of n L and p L using different fixed values of ρ ads are discussed.

23This paper presents the initial results of a scientific drilling project to recover core 24 and pressurized fluid samples from a natural CO 2 reservoir, near the town of Green River, 25Utah. The drilling targeted a stacked sequence of CO 2 -charged Jurassic sandstone reservoirs 26 and caprocks, situated adjacent to a CO 2 -degassing normal fault. This site has actively 27 leaked CO 2 from deep supercritical CO 2 reservoirs at depth >2km within the basin for over 28 Geyser constrain mixing models which show that, within the Navajo Sandstone, the 49 reservoir fluids are undergoing complex mixing of: (i) CO 2 -saturated brine inflowing from 50 the fault, (ii) CO 2 -undersaturated meteoric groundwater flowing through the reservoir and 51 (iii) reacted CO 2 -charged brines flow through fracture zones in the overlying Carmel 52Formation caprock, into the formations above. Such multi-scale mixing processes may 53 significantly improve the efficiency with which groundwaters dissolve the migrating CO 2 .

This paper reports the results of an international interlaboratory study led by the National Institute of Standards and Technology (NIST) on the measurement of high-pressure surface excess carbon dioxide adsorption isotherms on NIST Reference Material RM 8852 (ammonium ZSM-5 zeolite), at 293.15 K (20 °C) from 1 kPa up to 4.5 MPa. Eleven laboratories participated in this exercise and, for the first time, high-pressure adsorption reference data are reported using a reference material. An empirical reference equation [nex-surface excess uptake (mmol/g), P-equilibrium pressure (MPa), a = −6.22, b = 1.97, c = 4.73, and d = 3.87] along with the 95% uncertainty interval (Uk = 2 = 0.075 mmol/g) were determined for the reference isotherm using a Bayesian, Markov Chain Monte Carlo method. Together, this zeolitic reference material and the associated adsorption data provide a means for laboratories to test and validate high-pressure adsorption equipment and measurements. Recommendations are provided for measuring reliable high-pressure adsorption isotherms using this material, including activation procedures, data processing methods to determine surface excess uptake, and the appropriate equation of state to be used.Electronic supplementary materialThe online version of this article (10.1007/s10450-018-9958-x) contains supplementary material, which is available to authorized users.

The geological storage of carbon dioxide (CO 2 ) is a well-studied technology, and a number of demonstration projects around the world have proven its feasibility and challenges. Storage conformance and seal integrity are among the most important aspects, as they determine risk of leakage as well as limits for storage capacity and injectivity. Furthermore, providing evidence for safe storage is critical for improving public acceptance. Most caprocks are composed of clays as dominant mineral type which can typically be illite, kaolinite, chlorite or smectite. A number of recent studies addressed the interaction between CO 2 and these different clays and it was shown that clay minerals adsorb considerable quantities of CO 2 . For smectite this uptake can lead to volumetric expansion followed by the generation of swelling pressures. On the one hand CO 2 adsorption traps CO 2 , on the other hand swelling pressures can potentially change local stress regimes and in unfavourable situations shear-type failure is assumed to occur. For storage in a reservoir having high clay contents the CO 2 uptake can add to storage capacity which is widely underestimated so far. Smectite-rich seals in direct contact with a dry CO 2 plume at the interface to the reservoir might dehydrate leading to dehydration cracks. Such dehydration cracks can provide pathways for CO 2 ingress and further accelerate dewatering and penetration of the seal by supercritical CO 2 . At the same time, swelling may also lead to the closure of fractures or the reduction of fracture apertures, thereby improving seal integrity. The goal of this communication is to theoretically evaluate and discuss these scenarios in greater detail in terms of phenomenological mechanisms, but also in terms of potential risks or benefits for carbon storage.