About the Course
SPWLA Distinguished Speaker Series. Kerogen is often considered to be fully hydrocarbon-wet in reservoir characterization. Wettability of kerogen is, however, not well understood and quantified. Thermal maturation induces changes in the chemical structure of kerogen and alters its oxygen and hydrogen content. This process affects surface properties of kerogen and can influence its wettability. Assumptions made about wettability of kerogen affects interpretation of borehole geophysical measurements such as electromagnetic measurements. It is therefore important to quantify wettability of kerogen as a function of its thermal maturity. The objectives of this paper are to
experimentally quantify wettability of kerogen at different thermal maturity levels and to quantify the influence of chemical composition of kerogen on its wettability.
In order to achieve the aforementioned objectives, we first isolated kerogen from organic-rich mudrock samples from two different formations at different thermal maturity levels. The extracted kerogen samples are then synthetically matured and the chemical composition of kerogen is quantified using XPS (X-ray Photoelectron Spectrometer) and pyrolysis. We used the sessile drop method to measure the contact angle, to quantify wettability of kerogen. We then investigated the impacts of thermal maturity and chemical composition of kerogen on its wettability.
We tested kerogen samples from two organic-rich mudrock formations (named A and B) and demonstrated experimentally that the wettability of kerogen varies with thermal maturity. Kerogen from formation A at low thermal maturity formed a 44° contact angle of water. However, at higher thermal maturities (heat-treated at 650°C) this contact angle of water increased to 122°. We show that kerogen samples from two organic-rich mudrock formations, heat-treated at 650°C (high thermal maturity), become oil-wet. The results suggest that kerogen is hydrocarbon-wet at high thermal maturity and remains water-wet at low thermal maturity. We also recorded the contact angle in kerogen samples after removal of bitumen generated during synthetic maturation of kerogen using chloroform. We demonstrated that contact angle increases from 44° to 90° and from in 111° to 125° with increase in thermal maturity in formations A and B, respectively. Thus, kerogen becomes hydrocarbon-wet with increasing thermal maturity both with and without the presence of bitumen. The outcomes of this paper improve formation evaluation of organic-rich mudrocks by providing quantitative information on wettability of kerogen. It can also contribute to correcting the assumptions made in reservoir simulators developed for organic-rich mudrocks and can significantly improve our understanding of fluid flow mechanisms in unconventional reservoirs.