Mechanistic Model Describes Wettability Change in Sandstones and Carbonates
Authors: Carpenter Chris 
Published: December 2018

This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 190236, “Modeling Wettability Change in Sandstones and Carbonates Using a Surface-Complexation-Based Method,” by Aboulghasem K.N. Korrani, SPE, and Gary R. Jerauld, SPE, BP, prepared for the 2018 SPE Improved Oil Recovery Conference, Tulsa, 14–18 April. The paper has not been peer reviewed.
Previously proposed models of wettability change have not been tied to the chemistry that controls wettability but instead were driven by simplistic criteria such as salinity level or concentration of an adsorbed species. In this paper, after testing proposed models in the literature on sandstones and carbonates, the authors propose a mechanistic surface-complexation-based model that describes observations quantitatively for ionically treated waterfloods. To the best of the authors’ knowledge, this is the first surface-complexation-based model that describes fully ionic compositional dependence observed in ionically treated waterfloods in both sandstones and carbonates.
Introduction
While some debate remains about the underlying mechanisms of ionically tuned waterflooding, the geochemical reactions that control the wetting of crude oil on the rock surface are likely to be central to a detailed description of the process. Models of wettability change often have been simplistic and not tied to the chemistry that controls wettability, instead driven by a simplistic criterion such as salinity level or concentration of an adsorbed species. Such models are inadequate for modeling the effect of compositional changes in brine, which is key to optimizing ionic design. One problem has been the lack of reservoir models that included geochemistry. The oilfield reactive-transport simulators are simplistic; in these, either aqueous reactions are assumed to be ideal (i.e., with aqueous activity coefficients of unity) or they lack important geochemical features such as kinetics or surface-complexation reactions.
In a previous work, the authors assumed that decrease in the total ionic strength is the driving mechanism for wettability change during low-salinity waterflooding. The reason was that the total ionic strength is the controlling parameter for the double-layer expansion. Double-layer thickness increases as the brine total ionic strength decreases. The thickness of the double layer is an inverse function of the square root of the total ionic strength. However, the proposed model was too simple and was unable to predict many low-salinity or modified-salinity waterflooding observations, such as the detrimental effect of divalents on oil recovery in sandstones, the positive effect of sulfate in chalks, the positive effect of high pH in sand-stones, and the detrimental effect of sodium chloride in chalks.
In this study, the authors identified and tested an alternative approach based on a stability analysis of thin water film, which yields a dimensionless group, a ratio of electrostatic-to-Van der Waals (VdW) forces called the stability number. The stability number can be used as a measure of wettability. The authors use simulation software to model surface reactions that can estimate surface charge and potential from surface-complexation models, demonstrating that this process can be used in a model to match a wide variety of data in sandstones and carbonates found in models in the literature. The complete paper provides details of these comparisons.