New subsurface storage technologies such as underground hydrogen storage (UHS) or carbon capture and storage (CCS) play an integral role in the energy transition. The use of depleted oil or gas reservoirs is discussed for both UHS and CCS. However, to guarantee the long-term safety of such storage operations, cap rock integrity studies need to be conducted for the respective storage gas. To obtain petrophysical parameters such as effective porosity or permeability, drill cores are needed. Unfortunately, core material is lacking particularly for the seal rock intervals, which in the Vienna Basin are often mudstones.
In absence of core material, the use of drill cuttings may be a viable alternative, provided that the cuttings material still inherits the original microstructural properties of the drilled formation. In this study, three different mudstone cores from the Vienna Basin were investigated. All cores are from shallow drilling depths in the range of 500-600 m burial depth and from Pannonian (Upper Miocene) intervals. The core material was compared to corresponding drill cuttings with respect to porosity and microstructure. In addition, different artificial cuttings were generated using a jaw crusher, LA test, and LCPC test. With these artificial cuttings, the mechanical impact of a drill bit should be simulated. In total, three core samples, nine artificial cuttings generated from these cores, and eight actual cuttings corresponding to the drilled intervals in the Pannonian were investigated. The main tasks were i) to determine the microstructural damage from drilling, and ii) to compare different porosimetry techniques including buoyancy-, pycnometry-, and imaging-based approaches regarding their suitability for the use with mudstone cores and cuttings.
The broad ion beam – scanning electron microscopy (BIB-SEM) technique provides pore space properties such as total SEM-visible porosity and pore size and geometry (e.g., aspect ratio and circularity) distributions. However, this technique is limited to a practical pore size resolution of > 30 nm. To determine total connected porosity including the pore fraction < 30 nm, helium pycnometry was performed. Bulk volumes were determined by applying the weighing method after Archimedes using isopropanol as a medium to avoid any water-based alteration of the mudstones. The different porosity data sets were then compared to evaluate their representativeness. Furthermore, based on the obtained microstructural and porosity data, the cuttings samples were screened for microstructural alteration compared to the core samples with a supposedly preserved microstructure.
The different porosity approaches yielded strongly deviating porosity values. For the core and artificial cuttings, the buoyancy- and pycnometry-based approaches show a good correlation (R2~ 0.8). However, adding the real drill cuttings, the correlation coefficient decreases (R2~ 0.6) which documents that the actual cuttings material reacts more sensitive to the saturation process, which may be indicative for microstructural disintegration. The artificial crushing with the jaw crusher also led to a porosity decrease, as well as a changed pore size distribution determined in BIB-SEM imaging. In general, the crushing led to a wider spread of pore geometry and size distributions compared to the original core material, documenting mechanical damage effects which were particularly visible with the jaw crusher method. BIB-SEM yielded significantly lower total porosity values compared to the petrophysical approaches, which is expected due to the practical resolution limit of BIB-SEM. However, the correlation between image-based and petrophysical porosity values was found to be weak, and great variability between different image captures within each sample were observed particularly for the cuttings samples. This documents the heterogeneity of the investigated cuttings material, which likely points to the formation of agglomerates of different lithologies rather than a representative bulk sample. In conclusion, the investigated cuttings samples are likely not suitable for the determination of representative seal rock properties in absence of drill core material. An improved pre-screening of the cuttings material, as well as the potential use of larger pieces, may help to overcome the limitations encountered in this study.