Mineral replacement in long-term flooded porous carbonate rocks

Abstract

This study reports mineralogical and physical property changes linked to geo-chemical alterations processes during three ultra-long-term tri-axial tests on outcrop-chalk from Liège (Belgium). The test core plugs were flooded with MgCl2-brines for approximately one and a half, two and three years, mimicking effective reservoir stresses (9.5–12.5 MPa) and temperature (130 °C) of important hydrocarbon deposits at the Norwegian Continental Shelf. The flooded cores were studied using electron microscopy, whole-rock and stable isotope geochemical analyses, and ion chromatography of the effluent water.

All tests show altered textures and mineralogy at the flow-inlet side of the approximately 7 cm long cores. With longer duration of flooding, these alterations moved further into the cores, and for the three-year-test, the entire core was altered. When studied at nano-scale, the newly formed crystals were found to be magnesite containing minor calcium impurities, together with clay-minerals. On the outlet side of the alteration-fronts in the two shorter tests, the mineralogy still mainly consists of calcite and primary clay-minerals, together with newly formed magnesite and secondary clay-minerals. Dolomite or low- and high-Mg-calcite are not observed. The textures of larger micro-fossils are often preserved, but the mineralogy of their shells is altered.

A sharp, only 4 mm narrow transition zone at the border of the alteration front towards the less altered area for the two shorter tests, shows the highest porosity in the cores. This pattern resembles what is observed in single-crystal experiments, where the alterations are driven by phase dissolution and subsequent precipitation, the progression of high porosity zones and the state of equilibrium at the boundary between the primary and new mineral phase. This is also in line with observations in nature and models for transport driven mineral replacement in porous media, where differences in dissolution and precipitation rates may cause high porosity transitions zones.

During the experiments, all cores underwent severe overall compaction between 10.1% and 18.2%. However, in the two- and three-year long test-cores, the permeability, and calculated porosity, started to increase after a primary phase of reduction. As magnesite precipitates at the expense of calcite, the density increase, but the solid volume decrease. As the bulk volume is constant, porosity and permeability are increased. The changes in ion-concentration of effluents, monitored throughout the experiments, balance the changes in mineralogy, compaction and permeability within the cores. Compositional variations of the injection fluid effectively control the amount of chemical reaction in chalk. This allows for control and predicting changes in geo-mechanical parameters induced by mineralogical replacement, which has significant impact on reservoir conditions.