Page 48 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

30 David A. Wood and Bin Yuan

2.4 CLAY SWELLING, DETACHMENT, AND PORE
BLOCKING LEADING TO REDUCTIONS IN PERMEABILITY
AND POROSITY

In addition to fines migration, particle swelling and swelling-
induced migration are the other main formation damage mechanisms
which impact sandstone reservoirs subjected to LSWF. Mohan et al.
(1993) conducted experiments on cores from the Upper Miocene Stevens
sandstone from the Elk Hills oil field (California, USA) and compared the
results with those from the Berea sandstone. The Stevens sandstone has
similar porosity (18 20%) to the Berea sandstone, but lower permeability
(,10 mD compared to .100 mD). Whereas, both sandstones have simi-
lar total clay contents (B8%), the Stevens sandstone contains a range of
swelling (i.e., smectite, mixed-layer) and nonswelling clays (i.e., kaolinite
and illite); whereas, the Berea sandstone clays are dominated by kaolinite
and some illite (nonswelling).
Their LSWF experiments (Mohan et al.,1993) showed that the critical
salt concentrations (CSCs) of chlorides of sodium, potassium, and calcium
required to prevent formation damage in the form of permeability loss
were higher than those required to prevent that effect in the Berea sand-
stone. They attributed this difference to the expansion of swelling clays in
the Stevens sandstone. In contrast to the Berea sandstone, lowering the
pH of the injection fluid had a limited effect in inhibiting the permeabil-
ity reduction process in the Stevens sandstone (Fig. 2.5). These results
suggest that water sensitivity of various sandstones to LSWF is dependent
on the composition of the clays they contain, in addition to the total clay
content and distribution of clays grains within the formation.
Clay swelling is a widely studied phenomenon in relation to formation
damage and can have significant negative impacts on hydraulic fracture
conductivity (Sanaei et al., 2016), as well as on the effectiveness of LSWF.
The structural layers of clay minerals are deficient, to varying degrees, in
positive charges due to cation substitution (Zhou et al. (1996). This
means that interlayer cations are required to balance the negative charges.
The distance between two structural layers in clay minerals (d-spacing)
varies according to the exchangeable cation, the formation water compo-
sition, and the type of clay mineral. Salts that keep the d-spacing of clays
low are considered to be good clay stabilizers or swelling inhibiters.
Amorim et al. (2007) studied the effects of various salts to inhibit clay