Page 39 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

CHAPTER 2 Mechanisms of Low-Salinity and Smart Waterflood 31

area. Initially, the basic and acidic organic materials appropriate to the carbonate reservoirs. In addition,
2þ
bond to the clay with inorganic cation, mainly Ca , the structure and mineralogy of the carbonate reservoirs
from the formation brine. When the low-salinity water are different to the sandstone reservoirs. Therefore,
is injected into the reservoir, it dilutes the ionic concen- other mechanisms of LSWF or smart waterflood in
tration close to clay surface and an equilibrium associ- carbonate reservoirs have been proposed, and they are
ated with the geochemical interaction of brine-rock is briefly described in this section.
disturbed. As a result, it causes the desorption of Ca 2þ
and the coadsorption of H þ replacing the desorption Potential-Determining Ions
of Ca 2þ as shown in Eq. (2.3).Thisexchange causes a
Zhang, Tweheyo, and Austad (2007) proposed the
local increase in pH close to clay. Successively, the
mechanism that the adsorptions of potential-
increasing OH close to the clay surface leads to the 2þ 2þ 2
determining ions (Ca ,Mg , and SO 4 ) onto the
reactions with the adsorbed basic and acidic materials
chalk surface contribute to the wettability modification
as in an ordinary acid-base proton transfer reactions of
of chalk surface toward water-wetness. Initially, the
Eqs. (2.4) and (2.5). These reactions result in the desorp-
negatively charged organic component of crude oil
tion of basic and acidic organic materials from the clay
adheres to the positively charged surface of chalk.
modifying wettability toward water-wetness (Fig. 2.3). 2
Generally, the formation water has negligible SO 4 ,
þ

Clay-NHR þ OH % Clay þ R 3 N þ H 2 O (2.4) but the seawater has considerable concentration of
3
2
SO 4 . When the seawater is injected into the oil-wet

Clay-RCOOH þ OH % Clay þ RCOO þ H 2 O (2.5) 2
chalk reservoir, the SO 4 in injecting seawater poten-
tially adsorbs onto the positively charged water-wet
2
sites on the chalk. The adhesion of SO 4 on the chalk
MECHANISMS IN CARBONATE surface lowers the degree of positive charge density of
RESERVOIRS chalk surface. The reduction in the positive surface
Previous experiments of coreflooding and a pilot test charge density of chalk surface will promote to decrease
have indicated that improved oil recovery by LSWF is the electrostatic repulsion between cation and chalk
mainly attributed to the wettability modification in surface. The less electrostatic repulsion, the more Ca 2þ
mixed- to oil-wet sandstone. A number of mechanisms enables to adsorb onto the chalk and the more Ca 2þ
are suggested to describe the enhanced oil recovery of remains close to the chalk surface. It also accelerates
LSWF in sandstone reservoirs, and the clay has the key the coadsorption of SO 4 2 on the surface. The Ca 2þ
role to explain the mechanism. Because carbonates potentially reacts with the adsorbed carboxylic groups,
have negligible clay mineral content, the proposed which are the negatively charged organic components
mechanisms of LSWF in sandstone reservoirs are not of crude oil. The reaction releases the organic carboxylic

FIG. 2.3 The schematic description of pH increase mechanism. (Credit: From Austad, T., Rezaeidoust, A., &
Puntervold, T. (2010). Chemical mechanism of low salinity water flooding in sandstone reservoirs. In Paper
presented at the SPE improved oil recovery Symposium, Tulsa, Oklahoma, USA, 2428 April. https://doi.org/10.
2118/129767-MS.)