Understanding enhanced oil recovery (EOR) in sandstone reservoirs: the role of redox changes in clay minerals on wettability

Abstract

A great body of research has been focused on understanding enhanced oil recovery in mature
sandstone reservoirs. The benefit of further producing such mature fields is indisputable since the
natural-driven oil recovery of the oil initially in place can vary from <5% to 50%, in the best-case
scenario. The enhanced oil recovery methods, such as CO2 injection, steam injection, surfactant
injection etc., have been established through the years, with researchers proposing mechanisms
that can explain the additional oil recovery. The following pages of this thesis explore in more
detail the low salinity water
flooding (LSWF), a method that has gained significant ground the
recent years, due to the low costs of implementation. Many mechanisms have been proposed
since the 1950's, when first observations were made, with more light being shed since the late
1990's, continuously to present day.
The experimental work of this PhD project focused on examining reduction-oxidation (redox)
processes during oil recovery upon EOR implementation. This was approached by using
iron-bearing clay minerals, with various iron content, as proxies of iron phases present in the
reservoir rock. First, the wettability of those clay minerals, such as natural occurring nontronite
and illite, was explored via clay mineral films, measuring the contact angle of crude oil and DI
water, under reduced and oxidised conditions, with reduced clay films, exhibiting more water-wet
surfaces. Then, the hydration and structural changes of a nontronite clay mineral was established
with infrared spectroscopy (IR). At these experiments, the saturating cation was manipulated by
clay mineral treatment, acquiring homoionic Na+, Ca2+ and K+ samples of nontronite, allowing
the isolation of hydration effects and other clay mineral / cation interactions. Those IR measurements
revealed a more hydrated state under partial reduction, and stronger clay mineral/
cation interaction under (partially) reduced conditions (N-IR, M-IR). Significant spectral alterations
were also observed at the F-IR range, upon clay mineral reduction, with minimum effects
due to cation saturation and relative humidity induced. Lastly, the thermodynamics of cation
exchange reactions, using two Na+-saturated nontronites and a Na+-saturated montmorillonite,
was attempted to be quantified. Two different reactions were considered for all three minerals:
clay mineral- Na-->Ca and Na-->K. These experiments, conducted under fully reduced conditions,
showed that the inverse of the Na-->Ca reaction is favoured (Ca favoured with ΔG <0), which
supports the basic theory of LSWF, as sodium is considered a key factor for LSWF positive
effect, but also how cation exchange, under such redox conditions, are exhibiting hysteresis, a
key observation for better understanding such processes on clay minerals, across disciplines.