Artificial Diagenesis of Carbonates: Temperature-Dependent Inorganic and Organic Modifications in Reservoir Mimetic Fluids

Within reservoirs, spatial variations related to mineralogy and fluid chemistry determine the success of improved oil recovery (IOR) technologies. However, the composition and structure of mineral-adsorbent/fluid interfaces, which fundamentally determine the wettability of reservoir rocks and crude oil (CRO) displacement, are unclear. Replicating the diagenetic alterations of carbonates, this study addresses the temperature dependence of the inorganic and organic modifications of calcite by reservoir pertinent fluids as well as its consequences on mineral wettability and reactivity.

We apply a suite of characterization methods, namely confocal Raman, scanning electron, and atomic force microscopy (AFM) as well as infrared spectroscopy, to investigate the modifications of carbonates on aging in formation water (FW), CRO-equilibrated FW, and FW-equilibrated CRO. The microscopic modifications of carbonates show a strong dependence on the aging temperature and are varied, encompassing topographical alterations, substitution of lattice Ca2+ ions by Mg2+ ions and the deposition of particles enriched with polyaromatic hydrocarbons (PAHs) as organic adlayers. Aging in the FWs leads to substantial reconstruction of calcite surfaces, with the deposition of magnesium calcite layers at elevated temperatures. Subsequent aging in FW-equilibrated CRO produces an organic coating on the mineral surfaces, which is composed of PAH-enriched particles. Deposited most strongly at high temperature, these organic layers render contact angles more “oil-wet.” In addition, these layers present a limited permeability for ionic species and substantially reduce the dissolution rates of calcite. The multilayer deposition of organic particles, which thus turns out as a key factor for wettability alteration, is attributed to the interconnected bulk and surface reactions for interfacially active constituents of CRO and the surface precipitation of organo-calcium complexes.

Results of this study are relevant to multiple aspects of reservoir development and maintenance, including laboratory scale wettability and coreflooding experiments, and in-silico modeling. The observed nano- and microscopic surface alterations of carbonates within reservoir mimetic environments improve our understanding of the physicochemical relations between mineralogy and fluid chemistry at the mineral-sorbent/fluid interfaces within reservoirs and thereby provide a starting point for the development of novel advanced IOR strategies.