Evaluation of the compositional changes during flooding of reactive fluids using scanning electron microscopy, nano-secondary ion mass spectrometry, x-ray diffraction and whole rock geochemistry

Abstract

Outcrop chalk of late Campanian age (Gulpen Formation) from Liège (Belgium) was flooded with MgCl 2 in a triaxial cell for 516 days under reservoir conditions to understand how the nonequilibrium nature of the fluids altered the chalks. The study is motivated by enhanced oil recovery (EOR) processes because dissolution and precipitation change the way in which oils are trapped in chalk reservoirs. Relative to initial composition, the first centimeter of the flooded chalk sample shows an increase in MgO by approximately 100, from a weight percent of 0.33% to 33.03% and a corresponding depletion of CaO by more than 70% from 52.22 to 14.43 wt.%. Except for Sr, other major or trace elements do not show a significant change in concentration. Magnesite was identified as the major newly grown mineral phase. At the same time, porosity was reduced by approximately 20%. The amount of Cl − in the effluent brine remained unchanged, whereas Mg 2+ was depleted and Ca 2+ enriched. The loss of Ca 2+ and gain in Mg 2+ are attributed to precipitation of new minerals and leaching the tested core by approximately 20%, respectively. Dramatic mineralogical and geochemical changes are observed with scanning electron microscopy-energy-dispersive x-ray spectroscopy, nano secondary ion mass spectrometry, x-ray diffraction, and whole-rock geochemistry techniques. The understanding of how fluids interact with rocks is important to, for example, EOR, because textural changes in the pore space affect how water will imbibe and expel oil from the rock. The mechanisms of dissolution and mineralization of fine-grained chalk can be described and quantified and, when understood, offer numerous possibilities in the engineering of carbonate reservoirs.

How Stress and Temperature Conditions Affect Rock-Fluid Chemistry and Mechanical Deformation

“…When dissolution of pre-existing quartz is taken into account, the geochemical calculations show that magnesium-bearing silicates such as tremolite and talc are also formed. These predictions have been supported by direct XRD and SEM observations that were published recently by Zimmermann et al [21]. As such, calculations and direct rock-observations have shown that chalk is prone to chemical rock-fluid reactions at high temperatures [17].…”

“…The concentration of CaCO 3 for these chalks has been reported to be ranging from approximately 90 to 95% (see e.g., [16,21,26]). Cylindrical plugs were drilled from a chalk block, before being radially adjusted to diameters D 0 and cut to lengths L 0 .…”

“…All the SEM-EDS that are reported have been performed approximately 0.8 cm into the core from the inlet and approximately 0.2 cm from the center axis. This is because Zimmermann et al [21] has shown how the replacement reaction varied throughout the core. The microstructural morphology varies spatially and heterogeneities can be observed on a variety of scales, see also [17].…”

“…In both [17,21] the compositional changes have been documented displaying how MgCl 2 flooding of chalk leads to calcite-magnesite replacement. This again leads to density increase and mass loss which changes the solid volume.…”

“…The solid-volume changes during flow of MgCl 2 brines in chalks have previously been shown to be related to the dissolution of CaCO 3 and the precipitation of Mg 2+ -bearing minerals such as dolomite and magnesite (see e.g., [17,21]). These minerals all have a density larger than calcium carbonate, which is in agreement with the gas adsorption measurements reported.…”