Experimental Quantification of the Effect of Thermal Maturity of Kerogen on Its Wettability

Abstract

Summary Kerogen is often considered to be fully hydrocarbon–wet in reservoir characterization. However, wettability of kerogen is not well–understood and quantified. Thermal maturation induces changes in the chemical structure of kerogen and alters its oxygen (O) and hydrogen (H) content. This process affects the surface properties of kerogen and can influence its wettability. Assumptions made regarding the wettability of kerogen affect the interpretation of borehole geophysical measurements such as electromagnetic measurements. Therefore, it is important to quantify the wettability of kerogen as a function of its thermal maturity. The objectives of this research are to experimentally quantify the wettability of kerogen at different thermal–maturity levels and to quantify the influence of chemical composition of kerogen on its wettability. To achieve these objectives, kerogen was first isolated from organic–rich mudrock samples from two different formations at different thermal–maturity levels. The extracted kerogen samples were then synthetically matured. Variations in the composition and chemical–bonding state of carbon (C) present in kerogen at different levels of natural and synthetic thermal maturity were determined using X–ray photoelectron spectroscopy (XPS). The sessile drop method was used to measure the contact angle to quantify the wettability of kerogen. We then investigated the effects of thermal maturity and chemical composition/bonding of kerogen on its wettability. Kerogen samples from two organic–rich mudrock formations (Formations A and B) were tested, and it was demonstrated experimentally that the wettability of kerogen varies with thermal maturity. Kerogen from Formation A at low thermal maturity formed a 44° air/water–contact angle and 110° air/oil–contact angle. However, at higher thermal maturities (heat treated at 650°C), the air/water–contact angle increased to 122°, and the oil droplet completely spreads on the kerogen sample. The results suggest that kerogen is oleophilic and hydrophobic at high thermal maturity and hydrophilic at low thermal maturity. The air/water–contact angles in kerogen samples were also recorded after the removal of bitumen generated during synthetic maturation of kerogen using chloroform. The air/water–contact angle was shown to increase from 44 to 90° and from 111 to 125° with an increase in thermal maturity in Formations A and B, respectively, in the absence of bitumen. Thus, kerogen becomes hydrophobic with increasing thermal maturity in both the presence and absence of bitumen. The outcomes of this study can potentially improve the formation evaluation of organic–rich mudrocks, in addition to improving our understanding of fluid–flow mechanisms in unconventional reservoirs.