Electrostatic Origins of CO2-Increased Hydrophilicity in Carbonate Reservoirs

Abstract

Injecting CO2 into oil reservoirs appears to be cost-effective and environmentally friendly due to decreasing the use of chemicals and cutting back on the greenhouse gas emission released. However, there is a pressing need for new algorithms to characterize oil/brine/rock system wettability, thus better predict and manage CO2 geological storage and enhanced oil recovery in oil reservoirs. We coupled surface complexation/CO2 and calcite dissolution model, and accurately predicted measured oil-on-calcite contact angles in NaCl and CaCl2 solutions with and without CO2. Contact angles decreased in carbonated water indicating increased hydrophilicity under carbonation. Lowered salinity increased hydrophilicity as did Ca2+. Hydrophilicity correlates with independently calculated oil-calcite electrostatic bridging. The link between the two may be used to better implement CO2 EOR in fields.

Molecular-scale origins of wettability at petroleum–brine–carbonate interfaces

“…Presence of a water wetting layer. We do not see any evidence for the presence of an aqueous brine film at the calcite-petroleum interface [2][3][4]13,14,19,20 . This conclusion derives from two observations.…”

“…Relevance for oil production. The present results suggest that the intrinsic carbonate-petroleum reactions are best described by a conceptual picture that is based on carbonate surface reactivity 16,[27][28][29][30] rather than doublelayer expansion (DLE) [2][3][4]13,14,19,20 . That is, our results directly reveal the presence of an interfacial layer that is seen only after interaction of calcite with petroleum, which is formed whether or not the calcite was previously in contact with an aqueous brine, and which is not displaced either by brines or the synthetic oil mixtures.…”

“…The mechanisms to explain this behavior generally fall within two distinct concepts: the double layer expansion (DLE) model 13,14 posits the presence of an aqueous wetting layer between the carbonate and petroleum phases, whose thickness is controlled by changes in the diffuse layer screening in low ionic strength brines. This concept implicitly assumes that the carbonate surface has a pH-dependent surface charge (e.g., due to protonation reactions of the surface functional groups), which is screened by counter ions, either adsorbed on the surface or distributed in a diffuse ion layer (with a screening length that is controlled by the ionic strength of a solution) [15][16][17][18][19][20][21] . This picture is well-established for oxide-water interfaces, such as the rutile (TiO 2 ) (110) surface 22-24 . In contrast, molecular scale measurements of the well-defined calcite (CaCO 3 ) (104) cleavage surface have found that its speciation does not follow this prediction 25 , and separate measurements of ion adsorption suggest that the net charge of a calcite surface is quite small 26 .…”

Response of Non-Polar Oil Component on Low Salinity Effect in Carbonate Reservoirs: Adhesion Force Measurement Using Atomic Force Microscopy

“…In general, our results show that increases of pH during low-salinity flooding in carbonate reservoirs lead to detachment of the non-polar oil components (the major oil components) from carbonate surface. This physicochemical process may not be modelled using the existing geochemical reactions because only the polar functional groups are captured in the geochemical models [22,28,75]. However, our results show that the pH increase triggered by low-salinity waterflooding may lead to the detachment of non-polar component.…”

Variation of CO2-Brine Contact Angles on Natural Rocks of Different Compositions

“…The impact of pressure and temperature has also been investigated using common minerals, such as quartz and calcite [28,29] and coal [30]. Other studies have focused on the effect of salt type, brine salinities, and surface charges [31][32][33] as well as surface contamination [34] and capillary pressure as a function of saturation [35,36].…”