Page 100 - Hybrid Enhanced Oil Recovery Using Smart Waterflooding

92 Hybrid Enhanced Oil Recovery using Smart Waterflooding

salinity condition of microemulsion. If the phase
behavior test determines the surfactant to have ten-
dency of decreasing optimum salinity as the surfactant
concentration decreases, the injecting surfactant solu-
tion with decreasing salinity might maintain the opti-
mum condition during the transport in porous media.
It is termed as negative salinity gradient, which indicates
the salinities of preflush water, surfactant slug, and post-
flush are in descending order. Nelson (1982) published
experimental works to demonstrate the negative salinity
gradient achieving the optimum salinity condition.
Whether optimum salinity gradient is positive or nega-
tive depends on the system considering the surfactant,
cosolvent, salinity, divalent cations, etc. Therefore, it is
FIG. 4.22 Schematic description of ternary diagram. (From clear that the achievement of ultralow IFT at optimal
Sheng, J. (2011). Modern chemical enhanced oil recovery: salinity increases the oil production but does not guar-
Theory and practice. Amsterdam, Boston, MA: Gulf antee the highest oil recovery in real field applications.
Professional Pub.)
The comprehensive investigations of surfactant for a
specific condition are necessary for successful applica-
phase behavior of microemulsion and solubilization ra- tion of surfactant EOR.
tios of water and oil. A couple of explanations are pro-
posed for the IFT reduction. The adsorption of Low SalinityeAugmented Surfactant Flood
surfactant molecules on the oil/water interface and the Experiment
formation of micelles reaching CMC potentially achieve Alagic and Skauge (2010) proposed the hybrid EOR
the ultralow IFT overcoming the capillary forces holding process when the LSWF is combined with surfactant
the oil in reservoirs. The concept of capillary number is
flood. The study investigated the tertiary low salinitye
discussed to explain the relationship between IFT reduc- augmented surfactant flood (LSSF) following secondary
tion and oil recovery increase. The capillary number is LSWF or secondary conventional waterflood. In addi-
the ratio of viscous force to capillary force. The reduc- tion, it quantified the role of high pH condition and
tion in IFT decreasing capillary force increases the capil- alkaline effect on the performance of tertiary LSSF.
lary number. The increasing capillary number implies The low salinity condition expects to bring a couple of
the relatively higher viscous force over capillary force advantages including the improving solubility of surfac-
and leads to the residual oil to be mobilized. Another tant and reduction of retention to surfactant EOR pro-
important mechanism of surfactant flood is to improve cess. In the experiments, the synthetic seawater of
the initial wettability of reservoirs when the surfactant 36,321 ppm TDS and low-salinity water of 0.5% NaCl
interacts with the rock surface. Incorporating the mech- brine are investigated. The anionic surfactant is pre-
anisms, the surfactant EOR process, conventionally, can pared and will form the Winsor type 1 microemulsion
be categorized into diluted surfactant flood and with the low-salinity water. The study carried out the
micellar flood. Because of the expensive cost of surfac- IFT measurement, coreflooding, ion analysis using
tant additives, it is practically the immiscible process inductively coupled plasma (ICP), etc. The viscosities
in the field application. Fully miscible process might of the surfactant solutions/brines and the IFTs between
be achieved in the early stage of the process, but the crude oil and surfactant solutions/brines are measured.
immiscible process quickly follows forming the multi- The viscosity of low-saline surfactant solution is slightly
ple phases. higher than surfactant-free brines. The IFT between
crude oil and seawater shows the 23.5 dyne/cm. Moder-
Optimum salinity gradient ate IFT reduction by 9 dyne/cm is observed for the low-
During the transport of surfactant solution in porous salinity water. When the pH of low-salinity water
media, the retention of surfactant decreases the surfac- increases by the addition of NaOH, the IFT decreases
tant concentration in bulk solution. In addition, the to 1.8 dyne/cm. The low-saline surfactant solution
surfactant concentration decreases as the surfactant so- achieves the low IFT on the order of 10 2 dyne/cm at
lution moves forward in porous media. The change in moderate and high pH conditions. The higher pH
the surfactant concentration varies the in situ optimum shows the lower IFT for the low-saline surfactant