Page 41 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

CHAPTER 2 Mechanisms of Low-Salinity and Smart Waterflood 33

FIG. 2.5 The schematic descriptions of mineral dissolution mechanism. (Credit: From Hiorth, A., Cathles, L.
M., & Madland, M. V. (2010). The impact of pore water chemistry on carbonate surface charge and oil
wettability. Transport in Porous Media, 85(1), 121. https://doi.org/10.1007/s11242-010-9543-6.)
controlled by the EDL forces. The z-potential is the key configuration (Fig. 2.6A). It is a reversible process.
indicator to state EDL. Generally, z-potential is a strong The degree of oil sorption is a function of the distance
function of pH and ionic strength, and the crude oil of separation between the oil and mineral, i.e., thick-
tends to have negative surface charge because of the ness of the water layer. The thickness of water layer de-
carboxylate groups of the crude oil. In the experimental pends on the net charge present at the two interfaces of
study, it is observed that low-salinity water results in rock/water and oil/water. The charge is sensitive to the
more negative surface charge on the limestone and ionic composition of water layer. In addition, a frac-
dolomite particles. Because both oil and limestone/ tion of oil can directly adhere to the minerals without
dolomite have the negative surface charges, oil hardly water layer (Fig. 2.6B). When the attractive force is
adheres to the rock surface resulting in water-wetness. greater than the disjoining pressure, the direct adsorption
Therefore, this study hypothesized that the change of of oil occurs (Buckley & Liu, 1998; Hirasaki, 1991). The
surface potential in carbonate rock from positive to disjoining pressure indicates the force that tends to
negative alters the wettability of carbonate rock toward disjoin or separate the two interfaces. This study assumed
water-wetness. that the directly adhered oil can be removed only by the
external treatment such as surfactant injection, alkaline
Adhesion Energy of Crude Oil/Brine/Rock injection, etc. In addition, there is an incomplete or
A number of studies have exploited the adhesion extremely slow desorption of species, i.e., ions and mac-
energy of the crude oil/brine/rock system to explain romolecules, from the mineral surfaces. This is because
the experimental observations in carbonate reser- the directly sorbed species forms the much stronger
voirs. The studies (Brady, Krumhansl, & Mariner, anhydrous bonds by linking directly to the high energy
2012; Brady and Thyne 2016) have proposed the sites on the mineral surfaces. The concept of the func-
concept of fractional wettability, which describes the tional wettability distinguishes the indirect and direct ad-
process of indirect and direct adhesion of oil to car- hesions separately to identify the causes of wettability
bonate reservoir. The balance of forces (van der Waals alteration in carbonate rocks. The indirect adhesion of
force, structural force of hydration or hydrogen- oil on the rock surface is the main target for the wetta-
bonding forces, and electrostatic force) determines bility modification by LSWF. Using the concept, a predic-
the status of oil in the system of mineral surface and tive model is developed to bridge the experimental
fluids. The oil can be free phase or directly and indi- observations and the modifications of oil and mineral
rectly adhered to the mineral surface in the reservoir. surface charges by changes in water chemistry. The
The indirect adherence of oil on the mineral surface model requires the modeling of both calcite and oil sur-
is accomplished through a three-layer oil/water/rock face charges.