Lionel Esteban

Infrared Attenuated Total Reflectance Spectroscopy: An Innovative Strategy for Analyzing Mineral Components in Energy Relevant Systems

The direct qualitative and quantitative determination of mineral components in shale rocks is a problem that has not been satisfactorily resolved to date. Infrared spectroscopy (IR) is a non-destructive method frequently used in mineral identification, yet challenging due to the similarity of spectral features resulting from quartz, clay, and feldspar minerals. This study reports on a significant improvement of this methodology by combining infrared attenuated total reflection spectroscopy (IR-ATR) with partial least squares (PLS) regression techniques for classifying and quantifying various mineral components present in a number of different shale rocks. The developed multivariate classification model was calibrated using pure component mixtures of the most common shale minerals (i.e., kaolinite, illite, montmorillonite, calcite, and quartz). Using this model, the IR spectra of 11 real-world shale samples were analyzed and evaluated. Finally, the performance of the developed IR-ATR method was compared with results obtained via X-ray diffraction (XRD) analysis.

Physical properties of Mesozoic sedimentary rocks from the Perth Basin, Western Australia

The Perth Basin (PB) hosts important aquifers within the Yarragadee Formation and adjacent geological formations with potential for economic exploitation by both geothermal energy and carbon capture and sequestration. Published studies on the reservoir quality of the sedimentary units of the PB are very few. This study reports some petrophysical and lithological characteristics of the sedimentary units of interest for geothermal and geosequestration scenarios and help interpolation toward non-sampled intervals. A new fluvial-dominated lithofacies scheme was developed for the Mesozoic stratigraphy from four wells drilled in the central PB (Pinjarra-1, Cockburn-1, Gingin-1 and Gingin-2) based on grainsize, sorting, sedimentary structures and colour that relate to the environment of deposition. Systematic laboratory measurements of permeability, porosity, and thermal conductivity were conducted on core samples to investigate a variety of lithofacies and depths from these wells. Empirical correlations are established among the different physical properties, indicating encouraging relationships for full PB basin interpolation such as between porosity and permeability, when the samples are grouped into ‘hydraulic units’ defined by a ‘flow zone indicator’ parameter. The common principal controls on the PB thermal conductivity are the pore space arrangement and mineralogical content, which are strongly lithofacies-specific. Therefore, the lithofacies type could be a good first-order discriminator for describing spatial variations of thermal conductivity and then estimate their flow zone indicator.

Fault reactivation in travertine and its impact on hydraulic transmissibility: laboratory experiments and mesoscale structures

Direct shear experiments were undertaken to investigate the effect of faulting and reactivation on the hydromechanical characteristics of faults in continental carbonate samples. The tested rock is a travertine of continental, microbial origin with a calcite content of 99 wt%, with a strongly laminated texture. Analyses of the intact and sheared samples performed using medical X-ray computed tomography (CT) revealed that the porosity is mainly composed of subplanar pores and vugs. Permeability is high along the laminations, controlled by interconnected pores and fractures. The travertine is a lithological analogue for Aptian pre-salt oil reservoir rocks found in South Atlantic offshore basins. Three samples, with dimensions of 240 × 110 × 150 mm, were sheared to a maximum displacement of 120 mm under different levels of effective vertical stress (6–45 MPa), resulting in the formation of cataclastic fault gouge surrounded by a dense fracture network. A new experimental method was used to reactivate the artificially formed fault by performing cyclic vertical loading at different shear displacements on a previously sheared sample, while keeping a constant pore-pressure differential throughout the test. Pore-fluid responses across the fault zone were monitored continuously during both deformation (dynamic transmissibility) and hold periods (static transmissibility). Results show that the transmissibility reduces in all the samples for all values of the applied effective vertical stress and during shear reversal. The static transmissibility also decreases over time, which may be a result of creep deformation or the blocking of pore channels with gouge material. Our results indicate that once the gouge material is developed in the core of a carbonate fault zone, the dynamic transmissibility across that fault is permanently decreased, with little dependence on subsequent kinematics of reactivation, or changes in stress, so long as the gouge zone is not breached by a new structure.

Characterization of Darai Limestone Composition and Porosity Using Data-Constrained Modeling and Comparison with Xenon K-Edge Subtraction Imaging.

Data-constrained modeling is a method that enables three-dimensional distribution of mineral phases and porosity in a sample to be modeled based on micro-computed tomography scans acquired at different X-ray energies. Here we describe an alternative method for measuring porosity, synchrotron K-edge subtraction using xenon gas as a contrast agent. Results from both methods applied to the same Darai limestone sample are compared. Reasonable agreement between the two methods and with other porosity measurements is obtained. The possibility of a combination of data-constrained modeling and K-edge subtraction methods for more accurate sample characterization is discussed.

Experimental Evidence of Calcite Dissolution and Induced Precipitation during supercritical CO2 Residence

Prior to injecting CO2 in water-saturated carbonate reservoirs, one needs to investigate the effect of the residence of supercritical CO2 (SCCO2) on the rock integrity and overall physical properties. In this study, a Savonnieres limestone is characterised in terms of its physical properties, pore chemistry and textural features prior and after 2 or 4 hours SCCO2 residence under in situ stress/temperature conditions. More precisely, elastic waves (Vp and Vs) at ultrasonic frequencies, electrical resistivity (Rt), helium porosity-permeability and pore chemistry are measured before and after SCCO2 aging. In addition, X-ray CT monitoring is carried out during the different steps. While water chemistry highlights an enhanced calcite dissolution related to the duration of SCCO2 residence, a change in the physical properties is observed between the two residence steps. It is shown from the physical properties that (i) the rock building minerals were dissolved after 2 hours; and (ii) the rock overall integrity increases after 4 hours, highlighting a possible re-precipitation phenomenon.

The Role of Specific Surface Area and Cation Exchange Capacity in Determining Shale Rock Properties

Surface area, surface charge and the exchangeability of cations have long been known as factors in determining the physical and mechanical properties of shales. Such properties are related to the fine grain size of the various clay minerals and also the particle shapes, edge-face microstructural arrangements and are impacted by the likes of salinity and depositional environment. In general, the siltier a given clay or shale, the lower their cation exchange capacity (CEC) and specific surface area (SSA). CEC and SSA have direct impacts on mechanical and flow properties. Wellbore instability can be caused by the development of osmotic pressures resulting from differences between pore fluid composition and drilling fluid composition and the resultant time dependent effects based on permeability. Such instability can be mitigated by the use of oil-based muds or for example high KCl water-based muds. Electrical and dielectric properties are also dependent on mineral surface charge and related directly to the CEC of a given shale, especially properties such as resistivity (or conductivity) and permittivity (dielectric constant). This paper will detail SSA and CEC values of shales from multiple basins worldwide and also from individual boreholes to demonstrate their role for shale rock properties.

Pressure-dependent elastic and transport properties of porous and permeable rocks: Microstructural control

Although several studies aimed at linking electrical and hydraulic transport properties in rocks, the existing models remain at most incomplete. Based on this observation, in addition to the transport properties, this contribution investigates the pressure dependence of P wave velocities and porosity for three porous rocks. Apart from hydraulic conductivity, all physical properties show an important dependence to the confining pressure. In particular, electrical resistivity reaches an asymptote at low confining pressures. Using the measured P wave velocities and effective medium theories, the microcrack density (ρ) and its evolution with confining pressure are estimated. For the three rocks, the microcrack density at which electrical resistivity reaches a plateau is of ρ ~0.13. This value corresponds to the threshold for crack percolation in media containing microcracks exclusively. This suggests that in porous and microcracked rocks, electrical resistivity is controlled by two independent hydraulic pathways of tubes and cracks acting in parallel. In contrast, this percolation threshold is not observed in the permeability data. A simple conceptual model is finally introduced that explains the fundamental differences between transport properties.

Compaction of Quartz-kaolinite Powders with Aggregated Initial Microstructure - Elastic Properties and Anisotropy

The effects of compaction on elastic properties of shales and their anisotropy are important for seismic imaging, seismic to well tie and borehole stability issues. Compaction affects microstructure and as a consequence all the physical properties of shales including their elastic moduli, electrical conductivity and permeability. The results of this experimental study of compacted artificial shales allow to conclude that elastic anisotropy grows with increase of quartz amount and during porosity reduction. At a given porosity, the increase of the quartz fraction results in higher ultrasonic velocities. The anisotropy of P-waves does not depend on quartz grains orientation. There is a necessity to further investigate the relationships between the microstructural properties and the physical properties of these artificial shales.

Linking Thermal and Elastic Properties in Sandstones Reservoir Rocks

Summary Although not measurable at the field scale, thermal properties of reservoir rocks at depth are important for many applications. Motivated by the fact that field-scale elastic properties may be obtained from measurements at the surface, a new physics-based theoretical model has recently been developed to link thermal and elastic properties through their common dependences in rocks. The model aims to ultimately predict thermal properties from elastic ones. But verifying this model with existing datasets obtained from different rock samples measured under different experimental conditions proves to be challenging. New joint measurements of thermal and elastic properties acquired on reference sandstone samples at identical experimental conditions allow an improved assessment.

Experimental Determination of the Stress Sensitivity of Elastic Wave Dispersion in a Fluid-saturated Cracked Rock

Summary We investigate the effects of micro structure, stress, pore fluids and frequency of perturbation on the elastic response of a thermally-cracked Carrara marble. The following data are reported at various effective pressure steps up to 50 MPa: (i) helium porosity and permeability under dry conditions; (ii) low-field NMR scanning under water-saturated conditions; (iii) low amplitude-high frequency (0.5 MHz) and low amplitude-low frequency (0.01 Hz) elastic moduli in dry and water-saturated conditions. This data set suggests that: (1) the natural rock contains a small amount (<1%) of micro-porosity and is isotropic and homogeneous at the laboratory scale; (2) thermal cracking induces the appearance of micro-cracks located at the interface between calcite grains (∼300 micron); (3) the spatial and orientation distributions of these micro-cracks are uniform; (4) crack closure occurs around a threshold effective pressure of 30–40 MPa. Below this pressure threshold dispersion in the thermally cracked and water-saturated rock between 0.01 Hz and 0.5 MHz reaches ∼25% for S-waves and ∼10% for P-waves; above this threshold, dispersion virtually cancels. Gas permeability at low pressure is larger in the thermally cracked rock than in the natural rock and decreases toward the value before thermal cracking with increasing effective pressure.