Page 38 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

30 Hybrid Enhanced Oil Recovery using Smart Waterflooding

C
O O C
R 2 R
R 1 3
Ca O - O
R N+ 2+
Na+ Na+ Na+ Na+ Ca
O- O- O- O- O- O O-

Van der Waals Cation exchange Ligand bridging
FIG. 2.2 Schematic description of diverse adhesion mechanism between clay surface and crude oil. (Credit:
From Lager, A., Webb, K. J., Black, C. J. J., Singleton, M., & Sorbie, K. S. (2008). Low salinity oil recovery - an
experimental investigation. Petrophysics, 49(1), 28e35. https://doi.org/SPWLA-2008-v49n1a2.)

bonds around the hydrophobic part. Inorganic cations concentration of the cations screens the negative charge
2þ
of Ca 2þ ,Mg , and Na þ destroy the water structure of both clay and crude oil and suppresses the electrostatic
around the organic material and decrease the solubility repulsive force between clay and crude oil. This leads to
of organic material in water. The divalent cations have the low level of the negative z-potential. Therefore,
the higher energy to break the water structure over oil enables to react with these clay particles forming
monovalent cations. Therefore, the divalent cations organometallic complexes, leading to the local
have the higher effect on the solubility of organic mate- oil-wetness of clay surface. In the low salinity condition,
rial in water. Following the theory, the solubility of i.e., lower level of ionic strength, the screening potential
organic component drastically increases as salinity decreases with a reduction of concentration of the multi-
decreases, that is, salting-in effect. Following this valent cations and increases the z-potential. It expands
description, the salting-in mechanism is formulated in the electrical double layers increasing the electrostatic
the application of LSWF in sandstone. During LSWF, repulsion between clay and crude oil. It is believed that
the adsorbed organic component onto clay surface the repulsive forces exceed the binding forces via the
might be detached from clay and dissolves into water. multivalent cation bridge, and then, the oil particles
Initially, the adsorbed organic material should bond can be detached from the clay surfaces. It leads to the
weakly to the surface. Then, the increasing solubility wettability modification toward water-wetness. This
of organic material in water leads to the desorption of study also explained the observation of fines migration
organic material from the surface improving the (Tang & Morrow, 1999) with the EDL expansion.
water-wetness of reservoirs. In addition, it is also When the ionic strength is reduced further, the mutual
explained that the release of cations from the clay repulsive electrostatic forces within the clay minerals
surface increases the pH as Eq. (2.3). potentially exceed binding forces and yield stripping of
oil-bearing fines from the pore wall.
þ
Clay-Ca 2þ þ H 2 O % Clay-H þ Ca 2þ þ OH (2.3)
pH Increase
Electrical Double Layer Expansion Whereas McGuire et al. (2005) explained the pH increase
Ligthelm et al. (2009) proposed the mechanism of to trigger the in situ generation of surfactant, Austad,
electrical double layer (EDL) expansion to explain the Rezaeidoust, and Puntervold (2010) suggested the pH
observations of LSWF experiments. In sandstone increase as the mechanism of LSWF. Austad et al.
reservoirs, clay minerals often have the negative electrical (2010) explained the physical process in terms of pH
charge owing to crystal lattice. The crude oil also exhibits and basic and acidic organic components, not in situ
the negative charge in nature. The multivalent cations of saponification. In an actual reservoir condition, the basic
Mg 2þ and Ca 2þ in formation water are believed to link and acidic organic components of oil can adsorb onto
between clay and crude oil as explained by Lager et al. the clay representing oil-wet system. The clay mineral is
(2008). In high salinity condition, the sufficient a cation exchanger and has a relatively large surface