Comprehensive Studies of Formation Water for Achimov and Bazhenov Formations — Revitalizing Archived and Old Cores

The sediments of Bazhenov (BF) and Achimov (ACh) formations have been studied for more than 50 years, and to date, a large amount of core material obtained during drilling at the stage of exploration is stored in the core stores of Russian industrial companies. The rise in the cost of the complex of geological exploration and the emergence of new research methods enabled studies of the old/dry core from previously drilled exploration wells. BF and ACh reservoir rocks have low permeability and initially low water saturation, and therefore, during storage, rock samples could partially retain pore water. To study its composition and quantitative content, we used the previously proposed integrated approach with proven effectiveness in fresh low-permeability BF rocks. The studied materials include BF rock samples from 3 different fields stored for 2 years after sampling in the laboratory, and ACh rock samples from 12 fields, cored more than 10 years ago at the geologic exploration stage. The complex technique includes determining free and bound water contents by the evaporation method with isotopic analysis (δ18O and δD) of the released water. The modified water extraction method provides a range of pore water salinity, while the modified alcohol ammonium chloride method measures the cation exchange capacity (CEC). Auxilary methods include Rock-Eval pyrolysis to determine organic matter content, X-ray diffraction analysis (XRD) to assess the mineral composition of sediments. The verification of the results for the BF rock samples was carried out according to the data obtained for the new/fresh core, while for the ACh rock samples — according to the results of the direct chemical composition analysis of the formation water. Despite the low permeability, we found that the BF and ACh rock samples during storage lost almost all (up to 90%) free water due to evaporation. At the same time, salts from formation water remained in the pore space, which made it possible to estimate the range of its NaCl salinity: 1.84–14.7 g/L for ACh rock samples and 4.49–20.19 g/L for BF rock samples. The obtained values set the lower limit of the possible pore water salinity of the studied ACh and BF rock samples. Moreover, the results match those from direct salinity measurements in the ACh depth intervals and the results of fresh BF core laboratory studies. We showed that the old/dry BF and ACh core could be effectively used to assess the bound water content with subsequent determination of its genesis from isotopic composition data and CEC measurements. This opportunity relies on the high clay content in the studied BF and ACh rock samples. The obtained results show the fundamental capability of informative laboratory studies of BF and ACh rock samples from public and private core storage facilities and give new life to archived and old/dry core.

Differential pressure dependence of the complex conductivity of sandstones

SUMMARY An experimental work is undertaken to understand the effect of the differential pressure (in the range 3–20.7 MPa) upon the complex conductivity of sedimentary rocks. We use five sandstone core samples from outcrops and a sandstone analog built from sintered glass beads. The spectra were fitted with a Cole–Cole complex conductivity model and the four Cole–Cole parameters were plotted as a function of the differential stress (in the range 3–20.7 MPa). The Cole–Cole relaxation times are analysed in terms of the evolution of the pore size with the differential pressure. Neither the relaxation time nor the Cole–Cole exponent show a strong dependence with the differential pressure indicating that the distribution of the relaxation times remains here roughly the same when the differential stress increases. More specifically, the Cole–Cole exponent does not describe the entire distribution of relaxation times, but the broadness of this distribution. Since the relaxation times are related to the pore sizes, this means that the pore sizes do not depend on the differential pressure in this case. The chargeability is essentially independent of the differential pressure and close to the upper value that can be reached in rocks without metallic particles. This also means that the conductivity of these rocks is dominated by their surface conductivity contribution considering the low pore water salinity used in this work. These results are interpreted thanks to the Stern layer polarization model. The Stern layer denotes the inner part of the electrical double layer coating the surface of the grains. The predictions of this model are mostly consistent with the data.

Effect of Pore-Water Salinity on the Electrical Resistivity of Partially Saturated Compacted Clay Liners

The aim of this paper is to investigate the effect of pore-water salinity on the electrical resistivity (ER) of different compacted clay liners (CCLs) in terms of its mineralogical composition. For this purpose, an experimental programme was conducted where ERs of different kaolin-dominant CCL specimens, reconstituted using water having different concentrations of NaCl (0 M, 0.5 M, and 1.0 M), were measured. The kaolin-dominant CCL specimens tested in this study include pure kaolin, three different kaolin-bentonite mixtures, and three different kaolin-sand mixtures. The experimental results show that the ERs of CCL specimens decrease as the salt concentrations in pore water, moisture content, and dry density increase. At constant density and moisture content, the test results also indicate that increasing the sand content in kaolin-dominant CCL specimens increases its ER regardless of the water salinity level. This behaviour could be attributed to the lower surface conduction of sand compared to kaolin. However, at constant density and moisture content, increasing the bentonite content in kaolin-dominant CCL specimens decreases its ER in the distilled water environment as surface conduction of bentonite is higher compared to that of kaolin. On the contrary, in saltwater environments, ER increases as the bentonite content increases. This behaviour could be explained in terms of the expected aggregated microstructure of bentonite in the saltwater environment that could reduce the number and area of interparticle contacts, and consequently, increase the ER of CCL specimens.

Influence of structure on the compression behaviour of two very stiff fissured high plasticity Eocene clays

A series of one-dimensional compression tests have been performed on reconstituted specimens of Søvind Marl. A clear correlation is obtained between the location and the gradient of the intrinsic compression lines and the void ratio at the liquid limit. The found correlation has subsequently been used to normalise the compression curves of 12 one-dimensional compression tests on intact specimens of Søvind Marl and Little Belt Clay obtained from various depths at two sites at Aarhus Harbour. Søvind Marl and Little Belt Clay are both marine sedimentary clays of Eocene age, which can be characterised as very stiff fissured clays with a very high plasticity. From a comparison of the reconstituted and intact compression paths the influence of structure is analysed and discussed in the paper. The majority of intact specimens of Søvind Marl and Little Belt Clay from Aarhus Harbour show similar behaviour; a very stiff recompression path and yielding at high stress level after crossing the intrinsic compression line. The displayed behaviour indicates an insignificant influence of fissuring on the pre-yield behaviour. Post-yield the compression paths indicate a stable structure dominated by fabric rather than metastable interparticle bonds. The paper furthermore highlights the importance of considering the effect of pore water salinity when assessing the influence of structure on the compression behaviour of smectite rich natural marine clays.

Effect of evaporative drying on complex conductivity spectra of sandstones

Dehydration is a common process occurring in natural environments, rocks, and building materials. Suitable methods for monitoring the changing moisture content should be identified. We have investigated the impact of dehydration of sandstones on complex conductivity spectra. Spectral induced polarization measurements were performed on five sandstone samples during evaporative drying. The complex conductivity spectra indicate considerable changes with decreasing saturation. The relationship between conductivity and saturation can be described by a power law known as the second empirical Archie equation. Separate saturation exponents have been determined for the real and imaginary parts of conductivity. The imaginary part of conductivity indicates higher saturation exponents for the investigated sandstones compared to the real part. Obviously, the saturation exponents depend on the method used for changing water saturation. Evaporative drying, which is used in our experiments, causes an increase of pore-water salinity and results in lower saturation exponents for the real part of conductivity but higher exponents for the imaginary part in comparison with the classical imbibition/drainage technique. We evaluate a theoretical approach that considers the influence of pore water salinity on the saturation exponents of the real and imaginary parts of conductivity. The complex conductivity spectra are processed by a Debye decomposition procedure. The resulting integrating parameters such as direct current resistivity, total chargeability, normalized chargeability, and mean relaxation time indicate a power law dependence on saturation. Our experiments indicate that the imaginary part of conductivity and normalized chargeability have a high sensitivity related to the drying process of rocks and might be suitable indicators to monitor the changes of moisture content in sandstones.