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Shale rocks have proven to be a challenge for their characterization. Even more so when trying to resolve porosity in the nanoscale (1 y 2). The extremely small size of the porosity, the role of the complicated composition, the lability of certain compounds and the fluids present in the pores are a few of the characteristics that make a very complex system to study. In terms of composition, the low scale porosity is associated with organic matter (OM) and clay compounds. (3)
The goal is to fully understand the porosity of a shale sample. To do so we work with a petrophysical model based on previous experience and a workflow methodology that includes NMR measurements (4). Laboratory measurements are complemented with imaging, both SEM and optical microscopy (petrography) (5). Although this allows us to achieve a deeper understanding of the rock, the smallest porosity and its characteristics remain unrevealed. In this work we show how gas? adsorption techniques provide information not shown by other techniques, thus complementing other techniques and improving the overall characterization of the rock’s poral system.
Gas adsorption technique is an indirect measurement that allows the characterization of meso and microporosity. Analysis was performed on source rock samples from regions in Argentina with good production or high potential. We show results from the Vaca Muerta Formation of the Neuquén Basin and Palermo Aike Formation from the Austral Basin.
In the first case we found trends between OM and clays with the resulting BET area (see Figure 1). The procedure consists in measuring adsorption/desorption of nitrogen for samples as received and after OM oxidation. BET area measurements on samples after OM oxidation follow the change in clay content (see Figure 1 A). Meanwhile, the difference in BET area between the as received sample and after OM oxidation, follows the change in TOC value (see Figure 1 B). These findings allow us to conclude that clays control porosity when OM is absent. Fractal dimension values can be obtained by applying the FHH model. Values between 2.7 to 2.9 were obtained for as received samples, while fractal dimensions between 2.5 to 2.7 for samples without OM.
The results for samples from the Austral Basin differ in that OM seems irrelevant in the characterization of nanoscale porosity. Clays and inorganic matrix porosity seem to be the dominant pore type at the nanoscale. There is a strong trend between clay content and BET area, with low fractal dimension values ranging from 2.4 to 2.7 which is an indication of low complexity.
In future works we are considering measuring rocks at different maturity states and trying different gases. We are also interested in looking into other characteristics of adsorption, such as adsorption enthalpy which might give insight on the affinity of the surfaces.
| References | 1. Hierarchical integration of porosity in sales. Lin Ma, Thomas Slater, Patrick J. Dowey, Sheng Yue, Ernest H. Rutter, Kevin G. Taylor & Peter D. Lee. Scientific Reports, 2018. 2. Integrated source rock evaluation along the maturation gradient. Application to the Vaca Muerta Formation, Neuquén Basin of Argentina. J.B. Spacapan, M. Comerio, I. Brisson, E. Rocha, M. Cipollone, J. C. Hidalgo. Basin Research, 2021. 3. Nano-scale texture and porosity of organic matter and clay minerals in organic-rich mudrocks. Utpalendu Kuila, Douglas K. McCarty, Arkadiusz Derkowski, Timothy B. Fischer, Tomasz Topór, Manika Prasad. Fuel, 2014. 4. An integrated petrophysical analysis based on NMR, organic geochemistry and mineralogy. The Vaca Muerta source rock-unconventional play at different thermal maturities. Diana Masiero, Marcos Comerio, Esteban Domené, Gabriela Vila, Bernarda Epele, Mariano Cipollone, Mariela Silka, Carlos Camacho, Lourdes Vera López and Silvina Chiappero. SCA 2022. 5. Semi-Quantitative SEM Analysis of |
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| Country | Argentina |
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