Page 45 - Formation Damage during Improved Oil Recovery Fundamentals and Applications

Low-Salinity Water Flooding: from Novel to Mature Technology 27

reducing permeability. They identified that this was due to the release
then trapping in pore throats of clay particles. The release of the particles
(fines) to form a colloidal solution was shown to be sensitive to salinity
falling below certain threshold levels. Mechanism initiating the release of
fines and how to control it continue to be a major focus of LSWF
research.
Much of the early work on the water-composition sensitivity of fines
migration was based on core-flooding experiments conducted on the
Berea sandstone (Devonian, Ohio / West Virginia, USA). Kia et al.
(1987) established that pH of the injected fluids played a role in the
process of releasing fines from the pore walls. They demonstrated that
significant permeability reductions occurred if LSW was injected with pH
. 6, but that formation damage decreased as pH of the fluids decreased.
Injecting LSW with pH less than about 4.8 resulted in no permeability
reduction. They identified that the LSW pH was influencing the multiva-
lent ion-exchange process at work, which was related to the surface
charges of the mineral grains (clays and sand particles). The surface
charges were very low at low pH, but increased as pH increased causing
the electrostatic repulsion forces between the clay particles and the pore
walls to increase. Kia et al. (1987) conceived a double layer model to
predict the release of fines particles.
Ochi and Vernoux (1998) demonstrated that the combined effects of
salinity and flow rate influenced formation damage in the Berea sand-
stone. Their core-flooding experiments’ results identified a critical flow
rate above which the permeability reduced related to hydrodynamic fines
release. The critical flow rate increased with the fluid salinity. The hydro-
dynamic release of fines could, alone, result in a 50% reduction in perme-
ability; but, the formation damage impacts related to salinity were shown
to be greater. Increase in flow rate leads to a less significant drop in
permeability than salinity decline, because the hydrodynamic changes
cause just limited fines to mobilize with those particles quickly deposited
in the pore throats.
Tang and Morrow (1999) established with core-flooding experiments
on the Berea sandstone that adsorption of crude oil to the pore walls,
mobile fines, and a water-oil formation fluid were all required for
LSWF to increase oil recovery (Fig. 2.3). Their results suggested that oil
recovery and wettability trends during LSWF required the heavy polar
components of oil adsorb onto matrix grains on pore walls to generate
mixed-wet fines.