Open AccessEffect of electrostatic forces on the porosity of saturated mineral powder samples and implications for chalk strength
Author(s) -
Leonardo Teixeira Pinto Meireles,
Einar Madsen Storebø,
Ida Lykke Fabricius
Publication year - 2020
Publication title -
geophysics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.178
H-Index - 172
eISSN - 1942-2156
pISSN - 0016-8033
DOI - 10.1190/geo2018-0724.1
Subject(s) - disjoining pressure , kaolinite , calcite , chemistry , mineralogy , saturation (graph theory) , porosity , adsorption , surface charge , ionic radius , ion , materials science , composite material , wetting , mathematics , organic chemistry , combinatorics
Electrostatic forces acting at the particle scale can be an important drive behind water weakening of chalk. Upon the replacement of oil with brine, ions present in the imbibing brine can exchange with ions already adsorbed to the calcite surface, leading to a change in the surface potential. This can cause an increase in the disjoining pressure between particles, either reducing the cohesion of particles connected via contact cement or decreasing friction between free particles. We have assessed the effect of electrostatic forces by measuring pore-water effects on porosity in sediment columns using nuclear magnetic resonance relaxometry. Samples of calcite, quartz, or kaolinite powder were saturated with brines containing ions found in seawater ([Formula: see text], [Formula: see text], [Formula: see text], [Formula: see text], and [Formula: see text]) at varying ionic strengths and as a nonpolar reference, with ethylene glycol. The difference in porosity between samples saturated with glycol and with brines reflects the disjoining pressure. For calcite samples, saturation with solutions containing divalent cations ([Formula: see text] and [Formula: see text]) lead to higher repulsive forces between the grains, whereas adsorption of [Formula: see text] counteracts the initially positive surface charge, lowering the repulsive forces. Calcium-based brines induced the highest repulsion, probably due to higher surface coverage of [Formula: see text] than that of [Formula: see text] due to its smaller hydrated radius. For kaolinite, differences in potential between the silica and alumina faces as well as the edges can either lead to repulsion between particles or to flocculation, depending on the ionic strength and ionic species of the fluid. Our results indicate that low-salinity water flooding may lead to kaolinite mobilization within reservoirs. A comparison of the results from our calcite powder experiments with results from mechanical tests performed on chalk samples indicates that electrical double layer-related forces can contribute to the weakening of chalk. Saturating brines for which the repulsion between grains in powder experiments was larger corresponds to weaker chalk samples.