Electric and elastic properties of low-permeability sediments

As part of the 2020 North Sea Agreement, Denmark will decommission many hydrocarbon-producing wells, necessitating the evaluation of petrophysical and elastic properties of reservoir chalks and overburden diatomaceous shales to ensure safe operations. This PhD research refined geophysical logging interpretations by studying electrolytic conduction and elasticity of fine-grained sediments like chalks and diatomites.

Given the high surface area of these fine-grained sediments, initial research focused on understanding electrolytic conduction in porous media by exploring the role of the electrical double layer. This study analyzed ionic adsorption dynamics of diatomite powders which complemented complex conductivity measurements on twenty-six saltwater-saturated diatomite and chalk samples. Results indicated a significant shift in current transmission from being predominately surface-water-dominated or slightly bulkwater-dominated, to being exclusively bulk-water-dominated with increasing NaCl concentration. At high salinity, bulk water governs conduction, raising Archie’s mexponent above 1.5 and enabling the calculation of the proportion of pore space occupied by bulk versus surface water. A model predicting sample conductivity as a function of bulk water conductivity was proposed and enabled the calculation of surface conductivity. Chargeability was found to gradually decrease with increasing salinity, reflecting the dominant role of bulk water in current transmission. Although the phase shift magnitude varied with pore water salinity, the distribution patterns remained consistent, reflecting the distinct mineralogical composition of each sample.

Geomechanical uniaxial compaction tests on twelve chalk samples showed that transitions in mechanical behavior could be consistently described by strain under different saturation conditions: dry, isopar-L oil-saturated, and tap-water-saturated. During these tests, significant noise and misleading wave arrivals in the recorded S-wave trains complicated the detection of the actual S-wave arrival. To add confidence in the Swave arrival detection, a method was proposed combining graphical representations of stacked wave trains. The method produced results congruent with the iso-frame modeled P-wave, shear, and bulk moduli.

The principles derived from studying the electric and elastic properties of diatomites and chalks were applied to improve well log interpretation in two Danish North Sea wells: the water-saturated diatomaceous shale in the Sten-1 well and the water-wet hydrocarbon-bearing clay-rich chalk reservoir in the Boje-2C well. The composition of formation solids was quantified using cuttings or core sample information, and porosity was assessed from neutron and density logs. Applying the new electrolytic conduction model, petrophysical properties such as surface area, permeability, irreducible water saturation, water saturation, and water salinity were acquired without using Archie's m- or n-exponents. Additionally, elastic moduli calculated by sonic and density logs were substituted to dry state allowing the calculation of Biot's coefficient and vertical elastic strain; the latter facilitated the identification of significant compaction in the Sten-1 well.