Page 40 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

32 Hybrid Enhanced Oil Recovery using Smart Waterflooding

materials from chalk surface (Fig. 2.4). In high temper- with the experimental observations of z-potential of
2
ature conditions, another mechanism is formulated calcite and the adsorption of SO 4 . Using the chemical
with the substitution of Ca 2þ by Mg 2þ onto chalk model, Hiorth et al. (2010) concluded that the change
surface. The substitution potentially displaces carbox- of z-potential by water chemistry hardly explains the
ylic groups reacted with Ca 2þ on the chalk surface observations of temperature-dependent oil recovery,
and increases the water-wetness of chalk surface. In which are observed in the studies (Zhang & Austad,
2
addition, the coadsorption of SO 4 lowers the electro- 2006; Zhang, Tweheyo, & Austad, 2006, 2007).
2
static repulsive force. In the excess of SO 4 on the chalk Therefore, the study proposed that the mechanism of
surface, the ionic interaction between Mg 2þ and SO 4 2 calcite mineral dissolution is accountable for the
increases the concentration of Mg 2þ close to the chalk increasing oil recovery and temperature dependency
surface (Fig. 2.4). The increasing concentration of of oil recovery during LSWF. The calcite mineral
Mg 2þ close to the surface potentially leads to the dissolution can explain the temperature-dependent
more substitution of Ca 2þ by Mg . Following the observations of experiments. At a lower temperature
2þ
mechanism, neither Ca 2þ nor Mg 2þ can remove nega- condition, seawater is equilibrium with calcite. When
tively charged carboxylic organic material from the the temperature increases, Ca 2þ is started to react
2
positively charged chalk surface without SO 4 in both with SO 4 2 and anhydrite is precipitated. The loss of
low and high temperatures. Ca 2þ is compensated from the calcite dissolution.
When the calcite dissolves, the adsorbed oil onto the
Mineral Dissolution calcite surface is released and wettability is changed
Hiorth, Cathles, and Madland (2010) explored (Fig. 2.5).
whether rock surface charge or mineral dissolution is
attributed to the wettability modification of LSWF. Surface Charge
Based on experimental results, the study numerically Alotaibi, Nasr-El-Din, and Fletcher (2011) investigated
constructed chemical model to determine the potential the electrokinetics of limestone and dolomite suspen-
mechanism. The chemical model predicts the z-poten- sions at different temperature conditions (25 C and

2
tial of calcite and the adsorption of SO 4 on surface. 50 C). Using the synthetic formation brine, seawater,

The predictions are compared with the experimental re- and aquifer water, the study measured the z-potentials
sults of the studies (Strand, Høgnesen, & Austad, 2006; of limestone and dolomite suspensions using phase
Thompson & Pownall, 1989; Zhang & Austad, 2006). analysis light scattering (PALS) techniques. The interfa-
The predictions by chemical model are comparable cial phenomena at the surface between rock and brine is

FIG. 2.4 The schematic descriptions of wettability modification by potential-determining ions at low and high
temperatures. (Credit: From Zhang, P., Tweheyo, M.T. & Austad, T. (2007). Wettability alteration and improved
2þ
oil recovery by Spontaneous Imbibition of seawater into chalk: Impact of the potential determining ions Ca ,
2þ
2
Mg , and SO 4 . Colloids and Surfaces A: Physicochemical and Engineering Aspects, 301(1), 199e208.
https://doi.org/10.1016/j.colsurfa.2006.12.058.)