Page 212 - Enhanced Oil Recovery in Shale and Tight Reservoirs

Fluid-rock interactions 195

However, water adsorption by clay minerals generates swelling pressure
that can be extremely high if water equilibrium inside the rock is disturbed
by its subsequent contact with different fluids (Chenevert, 1969; Santos
et al., 1997b). Liquid water or some reactive species in the crack tip
environment can facilitate crack propagation by promoting weakening
reactions. For the quartz/water system, reactions of the form

ð Si O Si Þ þ H 2 O/ð Si OH $ HO Si Þ
may occur. The strong silicon-oxygen bonds are replaced with much
weaker hydrogen bonds (Scholz, 1972; Martin, 1972; Swain et al., 1973;
Atkinson, 1979; Atkinson and Meredith, 1981). This phenomenon is
termed “stress corrosion” in the literature (Atkinson, 1982). To prevent this
problem, nonaqueous fluids based on diesel oil or mineral oil is used (Mehtar
et al., 2010). All of the rock-chemical solution combinations from Karfakis
and Akram (1993) show a statistically significant decrease in fracture
toughness, as well as a decrease in the work of fracture required in crack
initiation when compared to dry samples. If the chemical environment
contains species which can undergo ion exchange with species in the solid
phase, lattice strains may result from ion exchange which can facilitate crack

þ
extension, for example, exchange of H for Na in silicate glasses (Wie-
derhorn, 1978). Other studies (e.g., Dunning et al., 1980) show that sur-
factants cause a reduction in the bonding forces across the crack or fracture,
and that zeta potential of the fluid environment and chemical interaction
between a fluid and rock surface, such as ion exchange, also affected
microfracture propagation and growth.
Some solids contain dissolved chemical impurities, such as structurally
bound water in quartz, which if present in sufficient quantities can have a
degrading effect on strength. During crack propagation, stress-directed
diffusion of these chemical impurities to crack tips may occur, resulting in
weakening reactions and facilitating crack extension (Schwart and Mukher-
jee, 1974). On the other hand, Abousleiman et al. (2010) found that the
shale strength was increased when the shale was exposed to the high salinity
oil-based mud, confirming that shale strengthening by appropriate fluid
chemistry was possible (Hemphill, 2008). Bol et al. (1994) concluded that
salt and certain organic solvents were effective inhibitors of shale instability
if the concentrations were high enough. Carminati et al. (1999) studied the
effect of anions on shale stability, and Lu (1988) studied the effect of poly-
meric drilling fluid on shale stability.
Inducing fractures in shale depends upon whether the swelling stress can be
larger than the fracture closure stress. Smectite clays formed in a “T O T”