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

Abstract

Summary 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-calciumcomplexes. 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.

Formation and Stability of Heterogeneous Organo–Ionic Surface Layers on Geological Carbonates

“…Crude oils exhibit remarkable physicochemical complexity, with the number of distilled organic components greatly exceeding 1000000 as well as the dynamic (re)­assembly of chemical constituents into emulsions, (nano)­aggregates and particles. − From this chemical haystack, certain molecules bind to mineral surfaces and form “organic layers”, − possibly leading to alterations in the geochemical properties of carbonates such as dissolution rates , and wettability. ,, However, mechanistic studies of organic molecules, undergoing surface deposition and desorption in carbonate rocks, are limited and those uncovering the chemical identity of primary sorbents report mixed observations.…”

“…Thus, substantial mineral dissolution can occur over the time scales of reservoir development and production, which are on the order of decades. Kinetics of carbonate dissolution is also affected by surface deposits of oil , and its long-chain fatty acid and polycyclic aromatic hydrocarbon ,− constituents. Recent studies even show that non-equilibrium fluids, under flow, substantially alter the surface potential and reactivity of minerals. , Thus, the displacement of oil in reservoir rocks and the out-of-equilibrium conditions innate to this soluble mineral appear closely connected.…”

“…Second, rock-bound organic matter is often reported as enriched in polar groups and polyaromatic hydrocarbons, ,,, compositionally distinct from the bulk oil. In this respect, reservoir oil is also spatially categorized as free, adsorbed, or inclusion oil.…”

“…Subsequently equilibrating calcite crystals in these fluids establishes a surface adlayer, rendering a more oil-wet mineral surface. Using this aging protocol, we will investigate the formation, composition, and stability of the carbonate-bound surface layer, also observed in reservoir core samples. ,, …”

Nonmonotonic Coupled Dissolution‐Precipitation Reactions at the Mineral–Water Interface

“…Based on this explanation, we propose that water compositions oversaturated with respect to underlying mineral phase and at the same time undersaturated with respect to the bound organo‐ionic layer are optimal for removing rock bound organo‐ionic matter. Consistent with the molecular characteristics of carbonate bound molecules as bulk aromatic hydrocarbons with polar modifications [ 61 ] and the physical nature of the rock bound organic matter as organo‐ionic surface phases, [ 69 ] this mechanism brings forth the rich interconnectivity between multiple bulk and surface equilibria involving organic and inorganic species in underground formations.…”