Page 65 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

Low-Salinity Water Flooding: from Novel to Mature Technology 47

10% and 50% of the oil phase within the interstices or pores of the rock.
They identified using Eq. (2.15) that ultra-low values of the oil-water
IFT are required to achieve improved recovery:

Viscous forces vμ
N ca 5 5 (2.15)
Capillary forces σ cos θ
The combined effects of IFT (capillary forces) and viscosity ratio (vis-
cous forces) on oil recovery are related to the capillary number (N ca )
(Melrose and Brander, 1974; Ayirala and Rao, 2004) where: N ca is capil-
lary number; v is interstitial velocity of the displacing fluid (cm/s); μ is
viscosity of the displacing fluid (water), σ is the IFT between oil and
water (dyne/cm), and θ is the contact angle at the crude oil-formation
water-porous rock interface.
As the capillary number (N ca ) increases, oil saturation (S or ) should
decrease. This can be achieved by either lowering IFT and/or θ or
increasing μ. Many studies that evaluate capillary number, essentially

ignore the contact angle term Eq. (2.1), by setting cos θ 5 1.0 (θ 5 0 ),
which simplistically assumes a perfect water-wet system. This was the
assumption made by Salehi et al. (2017), observing in their experiments
that S or did indeed decrease with increasing capillary number and with
increasing water salinity. Their capillary pressure curves also moved
toward higher water saturations as salinity increased, indicating that S or
decreased with decreasing IFT. Plotting capillary pressure versus oil
saturation can determine the irreducible water saturation and residual oil
saturation of a formation.
With respect to the chemical mechanisms at play during LSWF, there
has been some debate regarding the roles of MIE (e.g., Lager et al., 2008;
Seccombe et al., 2010) and DLE (e.g., Nasralla and Nasr-El-Din, 2012,
2014; Xie et al., 2014). Charged formation surfaces in contact with
formation water have an excess of ions close to that surface, which is
referred to as the double layer. Pouryousefy et al. (2016) investigated, by
combining simulation and experimental results, the relative roles in wetta-
bility change of MIE and DLE in sandstone reservoirs. They concluded
that both mechanisms contribute to the oil recovery impacts of LSWF,
which agree with the concept illustrated here in Fig. 2.1.
In MIE, different electrolyte concentrations to the formation brine are
introduced by LSWF. This disturbs the equilibrium of the oil/water/rock
system and variations in ionic concentration result in the substitution of