Pieter Bertier

Observational evidence confirms modelling of the long-term integrity of CO2-reservoir caprocks

Storage of anthropogenic CO2 in geological formations relies on a caprock as the primary seal preventing buoyant super-critical CO2 escaping. Although natural CO2 reservoirs demonstrate that CO2 may be stored safely for millions of years, uncertainty remains in predicting how caprocks will react with CO2-bearing brines. This uncertainty poses a significant challenge to the risk assessment of geological carbon storage. Here we describe mineral reaction fronts in a CO2 reservoir-caprock system exposed to CO2 over a timescale comparable with that needed for geological carbon storage. The propagation of the reaction front is retarded by redox-sensitive mineral dissolution reactions and carbonate precipitation, which reduces its penetration into the caprock to ∼7 cm in ∼105 years. This distance is an order-of-magnitude smaller than previous predictions. The results attest to the significance of transport-limited reactions to the long-term integrity of sealing behaviour in caprocks exposed to CO2.

On the use and abuse of N2 physisorption for the characterization of the pore structure of shales

PIETER BERTIER , KEVIN SCHWEINAR, HELGE STANJEK, AMIN GHANIZADEH, CHRISTOPHER R. CLARKSON, ANDREAS BUSCH, NIKO KAMPMAN, DIRK PRINZ, ALEXANDRA AMANN-HILDENBRAND, BERNHARD M. KROOSS, and VITALIY PIPICH Clay & Interface Mineralogy, RWTH-Aachen University, Bunsenstr. 8, D-52072 Aachen, Germany Department of Geoscience, University of Calgary, Calgary, Canada Shell Global Solutions International, Kessler Park 1, 2288 GS Rijswijk, The Netherlands Dynchem, Saarstrasse 98, D-52062 Aachen, Germany Institute for Petroleum & Coal, RWTH-Aachen University, Lochnerstr. 2, D-52062 Aachen, Germany Jülich Centre for Neutron Science JCNS, Forschungszentrum Jülich GmbH, Outstation at MLZ, Lichtenbergstrasse 1 85747 Garching, Germany e-mail: Pieter.Bertier@emr.rwth-aachen.de

Determining the porosity of mudrocks using methodological pluralism

Abstract Porosity of shales is an important parameter that impacts rock strength for seal or wellbore integrity, gas-in-place calculations for unconventional resources or the diffusional solute and gas transport in these microporous materials. From a well section obtained from the Mont Terri Underground Rock Laboratory in St Ursanne, Switzerland, we determined porosity, pore size distribution and specific surface areas on a set of 13 Opalinus Clay samples. The porosity methods employed are helium-pycnometry, water and mercury injection porosimetry, liquid saturation and immersion, and low pressure N2 sorption, as well as small-angle to ultra-small-angle neutron scattering (SANS–USANS). These were used in addition to mineralogical and geochemical methods for sample analysis that comprise X-ray diffraction, X-ray fluorescence, total organic carbon content and cation exchange capacity. We find large variations in total porosity, ranging from approximately 23% for the neutron-scattering method to approximately 10% for mercury injection porosimetry. These differences can partly be related to differences in pore accessibility, while no or negligible inaccessible porosity was found. Pore volume distributions between neutron scattering and low-pressure sorption compare very well but differ significantly from those obtained from mercury porosimetry: this is realistic since the latter provides information on pore throats only, and the two former methods on pore throats and pore bodies. Finally, we find that specific surface areas determined using low-pressure sorption and neutron scattering match well.

Water vapour sorption on mudrocks

Abstract High-resolution water sorption isotherms were measured on 13 representative mudrock samples in order to assess the mechanisms of water vapour sorption and their relationship to the pore structure of mudrocks. The isotherm measurements were performed at 303 K on a gravimetric, dynamic vapour sorption device. Experimental data were interpreted by traditional physisorption models for which the validity was evaluated by relating model parameters to those obtained from nitrogen physisorption measurements. No direct relationships with the pore structure were observed, except for the Gurvich total pore volumes and the corresponding porosity data. Specific surface areas from Brunauer–Emmett–Teller theory are ambiguous and do not relate to nitrogen data, suggesting that water molecules do not adsorb as (multi-) layers covering pore walls. The volume filling theory (Dubinin–Astakhov equation) fits the water sorption data well but no relationship to the nitrogen data was observed in the studied sample set. A lower affinity of water for micropores was evident from the higher filling pressures of N2-based micropore volumes. The Barret–Joyner–Hallenda theory combined with N2 physisorption measurements on moist mudrocks revealed that capillary condensation prevails close to saturation but not below about 0.94 relative pressure (p/p0). A distinct low-pressure hysteresis was observed from hysteresis scanning that was attributed to surface chemistry since capillary condensation occurs only at very high relative pressures. Analysis of mineralogical composition, total organic content (TOC) and organic maturity in relation to water sorption revealed only a weak correlation with the total clay content. In contrast, cation-exchange capacity (CEC) strongly correlates with water uptake, which evidences a surface-chemistry-controlled sorption mechanism. Tests of the influence of the exchangeable cation were inconclusive because pore system alteration due to cation-exchange probably superimposed the effect. To further assess the sorption mechanisms of water, nitrogen physisorption isotherms were measured on moisture-equilibrated mudrocks (11, 52, 75, 94% relative humidity at 298 K). Micropore analysis and cumulative pore-size distributions denote that water blocks pore throats rather than fills pore volumes at lower relative humidities. Over the entire humidity range, no direct relationship between water sorption and pore size was observed. These findings imply that water adsorption does not sequentially fill pores with increasing radii in mudrocks as relative humidity increases, as would be expected from water sorption by capillary condensation. This conclusion has important implications for the interpretation and measurement of geomechanical and petrophysical properties of mudrocks. Capillary pressures, particularly at low water saturations, are often calculated from water saturation using a concept based on the Kelvin equation for capillary condensation. Since water sorption in mudrocks seems to be controlled by surface chemistry rather than pore size, this approach is questionable. The observations reported here suggest that the water distribution in mudrock pore systems resulting from vapour equilibration differs from that obtained by fluid displacement (i.e. capillary drainage or imbibitions). A further consequence is that water vapour equilibration is a convenient, but not necessarily representative, method to obtain partially water-saturated mudrock samples for laboratory measurement of saturation-dependent geomechanical or petrophysical properties.

Multiscale approach to (micro)porosity quantification in continental spring carbonate facies: Case study from the Cakmak quarry (Denizli, Turkey)

Carbonate spring deposits gained renewed interest as potential contributors to subsurface reservoirs and as continental archives of environmental changes. In contrast to their fabrics, petrophysical characteristics - and especially the importance of microporosity (< 1 mu m) - are less understood. This study presents the combination of advanced petrophysical and imaging techniques to investigate the pore network characteristics of three, common and widespread spring carbonate facies, as exposed in the Pleistocene Cakmak quarry (Denizli, Turkey): the extended Pond, the dipping crystalline Proximal Slope Facies and the draping Apron and Channel Facies deposits formed by encrustation of biological substrate. Integrating mercury injection capillary pressure, bulk and diffusion Nuclear Magnetic Resonance (NMR), NMR profiling and Brunauer-Emmett-Teller (BET) measurements with microscopy and micro-computer tomography (mu-CT), shows that NMR T-2 distributions systematically display a single group of micro-sized pore bodies, making up between 6 and 33% of the pore space (average NMR T-2 cut-off value: 62 ms). Micropore bodies are systematically located within cloudy crystal cores of granular and dendritic crystal textures in all facies. The investigated properties therefore do not reveal differences in micropore size or shape with respect to more or less biology-associated facies. The pore network of the travertine facies is distinctive in terms of (i) the percentage of microporosity, (ii) the connectivity of micropores with meso- to macropores, and (ii) the degree of heterogeneity at micro- and macroscale. Results show that an approach involving different NMR experiments provided the most complete view on the 3-D pore network especially when microporosity and connectivity are of interest.

Water Vapour Sorption by Shales

Porosity and pore structure are very important parameters in many geological and engineering applications. With respect to shales or mudrocks, these properties are essential in the assessment of multiphase transport, geomechanical behaviour and storage/sealing capacity. A multitude of methods are currently in use for determination of porosity, each of which has advantages and pitfalls. Though porosity is theoretically an intrinsic property of a rock, different techniques often yield substantially different results. This is above all the case for shales, of which the pore structure comprises substantial micro-( 50 nm) porosity. Particularly the lower range of pore sizes makes it hard to adequately characterize porosity by routine methods, such as those used for conventional reservoir rocks (e.g. petrography, computer tomography or mercury intrusion porosimetry).

Shale Porosity - What Can We Learn from Different Methods?

While the determination of porosity on sandstones is well established, porosities determined on shales are much less straightforward due to limited coring or inadequate pore preservation. Porosity in shale has an important control on many petrophysical, geomechanical and geochemical parameters of shales. Most of the porosity in shales is associated with small pore throat sizes, ranging in diameter from few up to about 100 nm. Pore throat sizes in carbonate or sandstone reservoir rocks are typically determined using mercury injection porosimetry (MIP). It is however well understood that MIP on shales underestimates porosity due to its limited accessibility. It is well known that using different methods for determining shale porosity results in different porosity values which is due to the different accessibility. Nonetheless, porosity is generally used as an absolute, intrinsic parameter without considering the method for determination. To address this issue we compare porosity, specific surface areas and pore volume distributions from fluid invasion and radiation methods on a total of 14 different Opalinus Clay samples recovered from the shaly facies at the Mont Terri underground laboratory in St. Ursanne, Switzerland.

Bridging pore and macroscopic scale - Scanning SAXS-WAXS microscopy applied to shales

The determination of fabric and pore structure of shales remains a challenging task which is mainly due to the wide range of pore sizes (and shapes) ranging from molecular dimensions to microns. High resolution imaging techniques fail to provide information over representative regions of interest, while more conventional characterization techniques may only assess volume averaged properties of the pore systems. Thus, open questions remain regarding the effects of the multi-scale pore network of shales in the retention and transport of hydrocarbons during unconventional production processes. We apply scanning small- and wide-angle X-ray scattering (SAXS and WAXS) microscopy to obtain averaged but detailed information from the micro- and meso-pore structures of shales. By combining SAXS/WAXS with raster-scanning microscopy, we obtain local scattering information from 1-100 nm-size pores in micrometer-size volumes over a large (2 x 2) mm2 scanning area. We derive porosity, pore size distribution and orientation, as well as mineralogy of specially prepared thin section samples, covering length scale ranges of nm to sub-microns and from microns to millimeters, with a gap that can potentially be closed The method further enables the linking of porosity to shale matrix components, which is integrated in a multi-scale imaging workflow involving μCT, and SEM/EDX analysis, aimed at allowing for the full pore network characterization of shales.

Clay/CO2 Interactions in the Context of Geological Storage of Carbon Dioxide

A major concern when storing CO2 in geological formations is the sealing efficiency of lowpermeable sequences overlying potential storage reservoirs. The long-term integrity of these sealing layers (caprocks) is a prerequisite to maintain CO2 in place and avoid dissipative loss to the atmosphere. Such leakage will occur either by capillary leakage, via diffusion or through existing or induced faults and fractures The assessment of leakage risks and leakage rates, considering different potential mechanisms, is therefore an important issue for site approval and public acceptance.