Ida Lykke Fabricius

Diffusion-driven changes in pore water salinity are prevalent in natural environments. We show that the electrical conductivity of saturated porous rocks at a given salinity depends on the history of salinity change, that is, whether salinity was reached by increasing or decreasing concentration. Understanding this salinity-induced conductivity hysteresis is crucial for accurate geophysical interpretation of subsurface processes. We measure complex conductivity spectra across sixteen diatomite and chalk samples subjected to both increasing and decreasing pore water salinity. For a given salinity, in-phase conductivity is up to ~120% higher on the decreasing salinity path. The hysteresis is minor above 3 S/m bulk water conductivity but becomes increasingly pronounced below that threshold, where bound water increasingly contributes to conduction. Our data indicate that the hysteresis arises from distinct equilibrium states of the electrical double layer for each salinity path, not from kinetics. The magnitude of the consequent surface conductivity hysteresis scales with Archie's m-exponent and the fraction of pore space occupied by bound water. In contrast, the induced polarization (IP) response, characterized by the Cole-Cole parameters of relaxation time, distribution exponent, and normalized chargeability, shows minimal dependence on salinity history. This stability indicates a decoupling between conduction and polarization, where IP timescale and distribution are governed by pore geometry, while its magnitude reflects the equilibrium state of the interface. Using the equilibrium surface conductivity at high- and low-frequency limits, we introduce a new method to estimate the bound water fraction involved in polarization. The analysis shows that ~9–26% of the bound water contributes, with greater spatial extent along the increasing salinity path. This trend reflects that adsorbed ions become more hydrated during the increasing salinity path, and more dehydrated during the decreasing salinity path. These findings provide a new framework for interpreting geophysical data in variable-salinity environments.

We investigate the underlying mechanisms governing electrolytic current transmission within porous media, focusing on understanding the relationship between interface processes and the resulting complex conductivity signals. Leveraging insights from interface science literature, we aim to clarify how adsorbed hydrated ions shape surface conduction. To achieve this, we conduct experiments on ionic adsorption with diatomite powders alongside the complex conductivity of 26 saltwater-saturated diatomite and chalk samples. Our findings highlight a significant shift in current transmission, progressing from bound-water-dominated or slightly bulk-water-dominated conduction to solely bulk-water-dominated conduction with increasing NaCl concentration. At high salinity, ions in the adsorbed ion-water layer solidify, rendering bound water conduction insignificant compared to bulk water. As a result, a universal relationship emerges between the sample surface area and Archie’s porosity exponent that allows for the estimation of the bulk and bound water volumes from the porosity exponent. Based on these findings, a parallel conduction model using Archie’s equation is developed as an upgrade to the dual-water model to predict sample conductivity across varying bulk water conductivity. Furthermore, our results show that chargeability peaks when the bound-water-driven current transmission is significant but decreases with increasing pore water salinity due to surface ion dehydration and a shift in current transmission toward bulk water. Although the phase-shift magnitude varies with pore water salinity, the frequency distribution pattern remains unchanged, reflecting the mineralogical composition of each sample. In addition, normalized chargeability correlates with the nonconductive porosity fraction and specific surface area per pore volume. These findings hold significant implications for the petrophysical analysis of fine-grained rocks, such as permeability modeling.

Natural fractures are abundant and important components in many carbonate sedimentary rocks globally. In hydrocarbon and groundwater reservoirs of carbonate rocks they can form connected networks and thereby influence the permeability and f luid flow significantly. Outcrop studies of fractured carbonate rocks can provide an essential understanding of 3-dimensional fracture networks, thereby aiding in understanding fracture patterns and connectivity in subsurface carbonate reservoirs. The Arnager Limestone Formation is a naturally fractured silica-rich chalk of Coniacian age exposed in a coastal cliff on the island of Bornholm in the Baltic Sea (Denmark). This study examines the natural fractures in the Arnager Limestone Formation from a structural and geomechanical perspective. The Arnager Limestone Formation forms one, 12–20 m thick, main rock mechanical unit; bedding planes acts as weak interfaces and divides it into near-identical, cm- to dm-thick rock mechanical subuits. Flat-lying (horizontal) or low-angle dipping bedding-parallel fractures are intersected by two near-vertical or steeply dipping fracture systems, a major N–S-trending system and a less prominent W–E-trending fracture system. Rock mechanical analysis of the tensile strength and elastic moduli provides the foundation for discussing maximum burial depth of the Arnager Limestone Formation. The tensile strength gives information on the bedding-parallel fractures, which can have formed due to stress relief during uplift and erosion, possible accentuated by glacial processes. The near-vertical fracture sets are interpreted to have formed in response to tectonic movements.

Knowledge of the elastic properties of clays and clay-rich sediments is essential when performing quantitative seismic interpretation and geomechanical modeling of reservoirs where they are present. Such is the case for the marly chalk reservoirs of Lower Cretaceous age in the Danish North Sea. We develop a new method to derive the elastic properties of the clay-rich insoluble residue (IR) fraction in marly chalks, using petrophysical logs and rock physical modeling. We calculate compressional (M), shear (G), and bulk (K) moduli of the formation along the borehole using the density and sonic (shear and compressional) logs and model the elastic properties of minerals using the isoframe (IF) rock-physics model. In our model, the marly chalk is composed of two materials: calcite, with well-known elastic properties, and the IR fraction, whose elastic properties are the sought variable. We systematically vary the assumed elastic properties of the IR fraction, each time achieving equality between the measured and modeled elastic moduli solving for the IF values of calcite (IFc) and IR fraction (IFIR) for each of the three elastic parameters one at a time, deriving the modeled values for the other two. We add up the error between the modeled and logged values for each set of assumed elastic properties of the IR fraction. We adopt the values of KIR and GIR that minimize the compound error as the actual elastic properties of the IR fraction. We run the algorithm in the Boje-2C and Valdemar-2H wells in different stratigraphic units of the Lower Cretaceous: the Fanø Formation and the marly chalk of the Tuxen and Sola formations. Elastic properties derived for the IR fraction compare well to data for wet clays from ultrasonic velocity laboratory tests. In a clean chalk section (almost 100% calcite), the model outputs values similar to calcite.

In seismic data analysis, acoustic impedance may be used for predicting the porosity of a sedimentary layer with known lithology, where the influence on impedance from pore fluid is known. This method typically assumes that the stiffness of the rock frame is solely a function of porosity and that there is no variation in stiffness between rocks with the same composition and porosity. However, contact cementation can cause an increase in rock frame stiffness with no significant reduction in porosity. This means that rocks with similar porosities and varying degrees of cementation would have significantly different elastic moduli; thus, attempting to derive porosity from seismic impedance alone would incur errors. The primary goal of this study is to assess how significant this error is for porosity prediction. The secondary goal is to calculate the dry rock frame stiffness across the field and evaluate to what extent the time of hydrocarbon placement affects the lithification of the rock. To fulfill these objectives, we studied petrophysical logs and core data from the Dan field and derived seismic impedance, porosity and stiffness expressed in terms of Biot's coefficient for 14 wells across the field. Trends defined between acoustic impedance, porosity, and Biot's coefficient in the Maastrichtian unit of the Dan field showed that, for a given porosity and water saturation in the oil zone, the chalk located at the crest of the reservoir is softer than the chalk located at the flanks of the field. We also found that chalk is stiffer in the down‐faulted north‐western part of the field for a given porosity and water saturation in the oil zone. In the water zone, no difference was found. We speculate that contact cementation downflank in the northwest block caused an increase in the chalk stiffness without reducing the porosity, while earlier hydrocarbon filling of the south‐eastern block and the crest of the structure delayed contact cementation between the chalk particles. This difference in contact cementation between rocks of the same porosity causes porosity prediction from acoustic impedance to vary by up to six porosity units.

Dansk Geologisk Forening (DGF) blev stiftet i 1893 for at fremme interessen for geologi og for at være samlingspunkt for geologer og geologisk interesserede.

Improved oil recovery from chalk reservoirs by water‐flooding may cause mechanical weakening and change in elasticity. Confined compressive strength testing of chalk from a North Sea reservoir was done in water‐saturated and oil‐saturated conditions. During testing, elastic wave velocities were sampled by ultrasonic transducers, so that subsequently Biot's coefficient could be modelled. The porosity declined via an ‘elastic phase’, a ‘transitional phase’, an ‘elastoplastic phase’ and a ‘strain hardening phase’, but Biot's coefficient indicates that these terms may be partly misleading. In the ‘elastic phase’, porosity and Biot's coefficient decrease, indicating elastoplastic deformation. In the ‘transitional phase’, Biot's coefficient increases as a reflection of breaking contact cement (pore collapse), whereas Biot's coefficient remains stable in the ‘elastoplastic phase’, indicating elastic deformation on the virgin curve. Plastic deformation takes place during phases of creep, where both porosity and Biot's coefficient decrease. Similarly, in the ‘strain hardening phase’, both porosity and Biot's coefficient decrease as a reflection of elastoplastic deformation. For chalk with 45%–47% porosity, the ‘transitional phase’ begins at 8 MPa axial stress when water‐saturated and at 12 MPa when oil‐saturated. For chalk with 41%–43% porosity, the corresponding stresses are 16 and 20 MPa. For chalk with 32%–36% porosity, the corresponding stresses are 23 and 31 MPa. Chalk samples with irreducible water saturation and movable oil were water‐flooded. They yield at stresses close to corresponding oil‐saturated samples, but after flooding show compaction trends not significantly different from the water‐saturated samples. Water‐flooding promotes pore collapse as reflected in an increasing Biot's coefficient. The consequent softening effect on acoustic impedance is small as compared with the effect of increasing fluid density. With respect to 4D seismic, water‐flooding causes distinctly higher acoustic impedance and Poisson's ratio irrespective of compaction.