Page 66 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

48 David A. Wood and Bin Yuan

divalent cations by the monovalent cations (Lager et al., 2008). This
makes the system more water-wet and thereby increases the oil recov-
ery factor. If the MIE mechanism is dominant, there should be an
inverse correlation between oil recovery factors and divalent cation
adsorption. The higher the divalent adsorption, the lower the oil
recovery should be.
By injecting cores with solutions of different concentrations of both
NaCl and CaCl 2 , Pouryousefy et al. (2016) showed that the results did
not match with those predicted by MIE. The expectations for MIE are
that high concentration of NaCl should lead to higher oil recoveries than
lower concentrations, and that all concentrations of CaCl 2 should lead to
lower recoveries than NaCl with little variation in oil recovery due to dif-
ferent CaCl 2 concentrations. Experimental results showed that lower con-
centration of Ca 21 achieved higher oil recoveries than higher
concentrations of Ca 21 . Also, although the oil recoveries with CaCl 2
were lower than for NaCl (consistent with MIE), the highest overall oil
recovery was achieved with the lowest NaCl concentrations.
Based on these results, Pouryousefy et al. (2016) conclude that MIE
can only be one of the mechanisms at play with LSWF and suggest that
DLE, induced by LSWF (and perhaps in part by MIE) is also playing a
role. The exact mechanism(s) by which this might occur remain unclear.
Several possible chemical bonding mechanisms are likely involved, where
polar components of crude oil are directly attached to the surface of the
rock, or at least the thin water film covering its pore linings and its associ-
ated electrical double layers (Fig. 2.1). These include cation bridging,
cation exchange, ligand bonding, anion bridging, which are all impacted
by the electrolyte concentrations in the formation fluids. Myint and
Firoozabadi (2015) have shown how some of the chemical mechanisms
proposed by Austad et al. (2010) (e.g., organometallic bridges, hydrogen
bonding, acid/base interactions) impact the expansion of the electrical
double layer in sandstones and carbonates during LSFW.

2.8 RELATIONSHIPS BETWEEN OIL RECOVERY,
SALINITY, AND WETTABILITY VARIABLES

The wettability of a porous rock refers to the ability of one fluid to
spread across or adhere to the solid surface in presence of another