Fifth EAGE Shale Workshop

An extensive laboratory testing program and round robin test suggests that:-

• Good agreement can be obtained between laboratories in terms of grain density, bulk density and porosity as long as samples are cleaned in a similar way and helium pycnometry is used for grain density.
• It is not recommended making porosity measurements using nitrogen or methane due to gas adsoption.
• Stress corrections need to be applied for bulk density measurements due to the presence of induced-microfractures; Hg injection analysis may provide a cheap way of providing data to make suc6h stress corrections.
• The crushed shale method is not recommended for estimating permeability.
• Core plug permeability measurements are often overly influenced by damage (fractures) and it is recommended inverting results using a dual permeability model.
• Models incorporating transitional flow and Knudsen diffusion seem too complicated to be used to effectively invert the results from transient flow experiments. Instead, simple gas slippage models are recommended.

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.

The inherent complexity of shale rocks together with their relatively low adsorption capacity as compared to commercial adsorbents represent a new scientific and technical challenge in the study of adsorption at supercritical conditions. Some of these issues are discussed in this paper. The adsorption of CO2 on a sample of Eagle Ford shale has been measured at 50°C and up to 20 MPa, and a maximum adsorption capacity of 300 SCF/ton of dry (granulated) shale sample was obtained. The analysis has focused on the estimation of the density of adsorbed gas in the pores of the material, a parameter that is key to quantify the storage capacity of shale rocks. A wide range of values was obtained (0.3–0.8 g/cm3) depending on the assumed skeletal volume of the shale. Whether these variations in the adsorbed density are related to the distinct pore structure of the materials considered and/or to uncertainties associated to the experimental techniques requires more research work under different conditions. In this context, the design an experimental protocol to accurately quantify the inaccessible volume of shale would allow improving the reliability of storage capacity estimates in these rocks.

Marcellus Shale samples were used to quantify mineralogy and texture, evaluate the abundance and thermal maturity of organic matter, describe porosity and interpret the diagenetic history of this postmature shale-gas reservoir.

A multidisciplinary approach was adopted comprising X-ray diffraction to quantify the mineralogy, neutron diffraction to quantify the texture of the rock-forming minerals, electron microscopy to visualise porosity in the shale and distinguish between detrital and diagenetic phases and Raman spectroscopy to quantify thermal transformation in the organic matter.

Results indicate that the samples are composed of quartz, illite, calcite, chlorite, albite, and pyrite with a total organic content ranging between 3 and 7 wt %. There is a significant crystallographic preferred orientation in the diagenetic illite and calcite that can be well modelled assuming transverse isotropy; quartz shows random texture. Nano sized pores are observed within the organic matter as well as at mineral junctions. Raman geothermometry indicate that the sediment witnessed maximum temperatures of approximately 250°C commensurate with the high optical reflectance (R0 > 4.5%) reported on the same material. This and the analysis of illite cristallinity indicate that the Marcellus Shale has been exposed to prehnite-pumpellyite metamorphic facies and a maximum burial depth of 6–8 km

The properties of organic matter change during diagenesis and catagenesis, potentially altering the way shales deform and fracture. Although kerogen in mudrocks is thought to become stiffer during thermal maturation, few studies have been able to confirm this by direct measurement, as standard mechanical testing techniques cannot easily be used to measure the micrometer sized organic components in shales. Here, we use a new non-destructive atomic force microscopy technique to map the elastic modulus of organic and inorganic components at the nanometer scale in shales containing Type II kerogen from three different levels of thermal maturation. We found that when vitrinite reflectance increases from 0.40 to 0.82, the average Young modulus of kerogen increases from 6.1 GPa to 16.0 GPa. However, as %Ro increases further from 0.82 to 1.25, the modulus values for kerogen do not change significantly. In the samples that experienced catagenesis, the modulus maps reveal that individual kerogen macerals possess soft regions - interpreted as exuded bitumen - which act to soften the overall structure of the kerogen. As well as providing high resolution mechanical data, this technique could be used to track the way bitumen and other compounds are generated from kerogen during catagenesis.

Low-net-to-gross floodplain stratigraphy contains thin-bedded crevasse splays that may have tough gas reservoir potential. Floodplain deposits are abundant in the distal part of dryland fluvial fans in endorheic basins, such as existed in the Permo-Triassic North and Central Atlantic margins. Outcrops of the Huesca fluvial fan (Ebro Basin, Spain) serve as an analogue to intervals of such deposits. Horizontally-laminated clay and fine silt are dominant, whereas low sinuous fluvial channels constitute only a fraction of the stratigraphy. Crevasse splays are common, and frequently occur in stacks of up to two metres thick. This results from aggradation of the active channel belt when sufficient accommodation space is available, forming an heterogeneous elevated fringe around the active channel. Lateral amalgamation, vertical stacking, and interaction with sand-rich channel fill deposits significantly increase the connected reservoir volume of the laterally extensive crevasse splays. The thickness of the stacked intervals can be used as a proxy to determine the channel lag thickness of their feeder channel. This allows to estimate the connectivity in a low-net-to-gross floodplain interval without data from well penetrations.

A comprehensive understanding of mechanical and fluid flow properties in fine-grained geo-materials like shales requires imaging the microstructure at a range of scales as the microstructures are small and the samples are heterogeneous. Broad Ion Beam (BIB) milling followed by Scanning Electron Microscopy (SEM) imaging provides access to these microstructures at nanometre resolution and large cross sections of up to several mm². Applying BIB-SEM on naturally and experimentally deformed or Wood’s Metal injected samples enables to resolve the related deformation processes and to image the pore connectivity, respectively. The pore fluid distribution, however, can be resolved by integrating BIB-SEM with cryogenic techniques. Examples of such integrated studies indicate calcite in shear fractures in Opalinus Clay and high connectivity in a fine-grained Boom Clay sample.

A new approximation for qP-wave velocities is developed for shales (TI medium). Proposed approximation is based on the linear relation between the velocity curvatures taken at zero and 90 degrees angles. We found that this relation is controlled by one parameter only. The resulting approximation is using the average value of this parameter from a large dataset of shale properties. Comparing with other known approximations for few shale samples, the proposed approximation shows the best accuracy.

We present new measurements of the bedding-parallel permeability of a single shale core to water and to supercritical CO2, under confined conditions. Furthermore, measurements were carried out on the same core plug after this had been split along its bedding.

Our results show that with increasing effective confining pressure, permeability decreases due to (permanent) compaction, with both an instantaneous and a time-dependent component. Furthermore, measurements performed on the core after splitting only show an effect of the crack at low effective confining pressure, whereas at higher confining pressures there was no significant effect. Finally, a first measurement performed using CO2 rather than water as the transport fluid gave a four times higher permeability. However, more measurements will be performed to confirm this preliminary observation.

Our results show that (time-dependent) compaction is an important factor controlling shale permeability. Furthermore, it is demonstrated that shale permeability may not be (permanently) affected by fracturing under confined conditions, as compaction may lead to crack closure. Accurate measurements of shale compaction and permeability are required to assess caprock integrity.

Black shales (BS) accumulate economic and biologically important trace metals such as Re, precious metals, U, V, Mo, Cu, Ni and Co above expected Clarke concentrations. We extracted colloid-salt fraction with particle size <1000 nm from BS with water under special conditions. In nano-fraction the detection limit for rare and trace elements are lower by 2–3 orders of magnitude, show maximum content for Re-22, 6 mg/l; Ag-9, 7 mg/l; Au-1, 4 mg/l; Pd-1, 7 mg/l; and Pt-0,5. Our data reveals a BS nanogeochemistry that could enhance geochemical ore prospecting methods and open new ways for extracting low concentration elements from BS.