Surface Interaction of Crude Oil, Maltenes, and Asphaltenes with Calcite. An Atomic Force Microscopy Perspective.

The study of surface-active compounds in crude oil is important because these molecules have the ability to change the mineral surface wettability, and therefore, influence important aspects of oil production and recovery from porous and fractured reservoirs. Crude oil, however, is a highly complex compound, and deciphering its molecular structure with enough precision to establish a direct relation to function is an ongoing effort. Ancillary methods, such as image analyses of adsorbates on mineral surfaces exposed to different fractions of crude oil, have the potential to clarify the behavior of surface active compounds as they relate to molecular structure and architecture.

Our previous work shows that short-term exposure of freshly cleaved calcite to crude oil produces adsorbates that tend to follow surface features (e.g., steps) along specific crystallographic directions, and that the surface potential associated to these features influences the observed behavior. Because the bulk of the surface-active compounds in crude oil exists in the asphaltenes fraction, adsorption controlled by surface features along crystallographic directions was also expected to occur from exposure to isolated asphaltenes, which was not the case. Here, we explore the cause of this discrepancy using atomic force microscopy (AFM) imaging of adsorbates resulting from brief exposure of calcite to (1) Kuwait crude oil UG8, (2) its separated asphaltenes re-dissolved in toluene at about 5 % concentration to match its original abundance in the crude, and (3) the remainder asphaltenes-free oil fraction (i.e., maltenes). The goal is to understand to what extent the adsorption patterns obtained after interaction of these fractions with calcite surfaces confirm or contradict what is understood about asphaltenes molecular structure and why.

Using AFM to monitor the behavior of different solubility- and polarity-defined fractions of oil is a straightforward technique to characterize and better understand interactions that modify rock wettability in the absence of detailed knowledge of the molecular structure and architecture of crude oil.