Page 276 - Enhanced Oil Recovery in Shale and Tight Reservoirs

EOR mechanisms of wettability alteration and its comparison with IFT 253

Table 9.5 A guideline to select surfactants.
Sandstone Carbonate

Rock surface charge e þ
Adsorbed oil Weakly acidic, e.g., Weakly basic, e.g., organic
compounds alkylated quinolones carboxylic acids
pyridines
Preferred surfactants Anionic Cationic

The above discussions may be summarized in Table 9.5.
Fluorocarbon surfactants are used to alter the wettability to less liquid-
wet or more gas-wet in gas condensate reservoirs. This has been discussed
in Chapter 4.

9.8 Determination of wettability

Since wettability alteration is very important in shale and tight reser-
voirs, the wettability must be correctly measured or determined. Before
discussing the methods to determine the wettability of a shale or tight core,
the methods including those used in conventional cores are reviewed.

9.8.1 Commonly used methods
First clarify the terminology about intermediate wet, fractional wet, and mixed
wet. Anderson (1986, 1987) define the system as neutrally wet, if the contact

angles (measured through the water phase) are 60e75 to 105e120 ,it is

water-wet below 60e75 , and it is oil-wet above 105e120 .Itis implied

that neutrally wet was synonymous to intermediate wet. Strictly speaking,
being neutrally wet, the contact angle should be close to 90 (Dandekar,

2013). Fractional wettability refers to the system where some of the pores are
water-wet, while others are oil-wet. Jerauld and Rathmell (1997) state that
fractional wettability is when there are oil-wet and water-wet regions in the
same pore. Although mixed wettability was proposed by Salathiel in 1973,
referring to a special type of fractional wettability in which the oil-wet surfaces
form continuous paths through the larger pores, mixed wettability is widely
used, and it actually refers to fractional wettability. In the modern literature,
mixed wettability is more often used than fractional wettability.
Typical methods to determine wettability are contact angle measurement,
Amott method (often called AmotteHarvey method), U.S. Bureau of Mines
(USBM) method, and combined Amott-USBM method. The method to
measure contact angles is affected by the rock heterogeneity in wettability