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

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.