Page 37 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

CHAPTER 2 Mechanisms of Low-Salinity and Smart Waterflood 29

increases. Surfactants are in situ generated from the TABLE 2.1
residual oil at elevated pH. The study formulated a hy- Mechanisms of Association Between Organic
pothetical mechanism of in situ generation of surfactant Functional Groups and Soil Minerals (Sposito,
by pH increase and validated the mechanism with 1989)
the coreflooding observations. The coreflooding obser-
vations of Tang and Morrow (1999) are referred to sup- Organic Functional Group
plement the proposed mechanism. In the observations, Mechanisms Involved
the pH increases from 8 to 10 when injecting brine is Cation exchange Amino, ring NH, heterocyclic N
changed from the formation brine with 15,150 ppm (aromatic ring)
TDS to the low-salinity water with 1515 ppm TDS. It is
Protonation Amino, heterocyclic N, carbonyl
concluded that the acid or polar components in the carboxylate
crude oil are saponified by the generalized reactions of
Anion exchange Carboxylate
Eqs. (2.1) and (2.2), and then, surfactants are in situ
generated from the reactions. The in situ generated sur- Water bridging Amino, carboxylate, carbonyl,
alcoholic OH
factant alters wettability and reduces IFT between water
and oil. In addition, they act as emulsifying agents to Cation bridging Carboxylate, amines, carbonyl,
disperse oil into the water. In the study, it is documented alcoholic OH
that an additional advantage of low salinity prevents Ligand bridging Carboxylate
a precipitation of the surfactant because of low concen- Hydrogen bonding Amino, carbonyl, carboxyl,
tration of divalent cations. The study summarized the phenolic OH
following prerequisites for the successful LSWF: (1) Van der Waals Uncharged organic units
acidic components in crude oil; (2) water-sensitive interaction
minerals; (3) initial water saturation; and (4) injecting
low-salinity water with less than 5000 ppm.
ðRCOOÞ C 3 H 5 þ 3NaOH/3ðRCOONaÞþ C 3 H 5 ðOHÞ 3
3
(2.1) during LSWF and it influences the four adsorption mech-
anisms of organic materials. The multivalent cations at a
2ðRCOONaÞþ CaðHCO 3 Þ /ðRCOOÞ Ca þ 2NaHCO 3 clay surface bond to polar compounds of crude oil (resin
2
2
(2.2)
and asphaltene) forming organometallic complexes. The
adsorbed complexes on the clay surface lead to
Multicomponent Ionic Exchange oil-wetness of rock surface. Simultaneously, some
Lager, Webb, Black, Singleton, and Sorbie (2008) organic polar compounds substitute the most labile
reviewed the previous mechanisms and devised a new cations and directly bond to the mineral surface. This
mechanism to explain the experimental observations of substitution also promotes the initial oil-wetness of the
LSWF in sandstone reservoirs. The study carried out an clay surface. When LSWF is applied to the oil-wet
effluent analysis of coreflood injecting low-salinity and sandstone rocks, the MIE occurs and replaces both
high-salinity waters and investigated water chemistry. It organic polar compounds and organometallic
showed a drop of Ca 2þ and Mg 2þ concentrations being complexes by uncomplexed cations on the clay surface.
lower than their concentrations in injecting brines (Fig. This study concluded that the replacement by MIE
1.7). These results indicate the strong adsorption of modifies the wettability of sandstone reservoirs toward
Ca 2þ and Mg 2þ onto the rock matrix. Based on these ob- water-wet and increases oil recovery.
servations, a multicomponent ionic exchange (MIE)
mechanism is proposed in sandstone. According to Salting-In Effect
Arnarson and Keil (2000) and Sposito (1989),organic Salting-out or salting-in effects have been used to
matters are possible to adhere onto clay minerals describe the solubility of polar organic material in water
depending on the organic function of the organic matter as a function of salinity or ionic composition in the
and the clay surface conditions. The eight potential area of chemistry. RezaeiDoust, Puntervold, Strand,
mechanisms of the adsorption exist as described in and Austad (2009) applied this theory to explain the
Table 2.1. The four mechanisms including cation observations of LSWF experiments and proposed
exchange, ligand bonding, cation bridging, and water the salting-in effect as another mechanism of LSWF.
briding are strongly affected by the cation exchange In the theory, organic compounds in water are solvated
(Fig. 2.2). It is explained that the cation exchange occurs by the water structure, which is made by hydrogen