Impact of Geochemical Properties on Wettability of Kerogen and Organic-Rich Mudrocks

Abstract

Summary The wettability of organic-rich mudrocks has a significant effect on multiphase-fluid flow and hydrocarbon recovery. This important rock property has still not been well-quantified in organic-rich mudrocks. Kerogen constitutes a significant fraction of mudrocks and can considerably affect their wettability. Recent publications suggested that kerogen wettability is affected by the thermal maturity of rocks and can influence the wettability of mudrocks. In this paper, we experimentally quantify the influence of geochemistry and thermal maturity of kerogen on the wettability of organic-rich mudrocks, and the influence of thermal maturity and chemical bonding on the wettability of kerogen. The wettability of organic-rich-mudrock samples at different experimental thermal-maturity levels was measured using the sessile-drop method, and also qualitatively estimated using a Flotation test and spontaneous-imbibition experiments on crushed-organic-rich-mudrock samples. The concentration of minerals in the mudrock samples was quantified using X-ray diffraction (XRD) at different experimental maturity levels. We then isolated kerogen samples from an organic-rich-mudrock formation and experimentally matured them. The variation in the chemical-bonding state of carbon present in kerogen at different levels of natural and experimental thermal maturity was determined using X-ray-photoelectron-spectroscopy (XPS) measurements. Finally, the wettability of pure-kerogen samples at different thermal-maturity levels was quantified using the sessile-drop method and the effect of aromatic carbon content on the wettability of the kerogen samples was determined. The sessile-drop test performed on the organic-rich-mudrock-rock samples showed a 5° increase in contact angle with a 96% decrease in the hydrogen index (HI). The Flotation test showed that the oil-wet fraction of the mudrock samples increases by 81% as the heat-treatment temperature increases from nonheated to 650°C. The water-imbibition measurements in crushed-mudrock samples suggest that the volume of water imbibed was higher by 22 cm3 at lower thermal maturity [i.e., HI of 328 mg hydrocarbon/g organic carbon (mg HC/g OC)] compared with mudrock samples at higher thermal maturity (i.e., HI of 10 mg HC/g OC). Results indicate that the thermal maturity of kerogen could potentially affect the wettability of mudrocks and that the mudrock has higher water wettability at lower thermal maturity of kerogen. The experimental results also demonstrated that the wettability of kerogen changes from waterwet to hydrocarbon-wet with an increase in the aromatic carbon content. The contact angle of the water droplet on the kerogen samples from Formation A increased by 78° when the aromatic carbon concentration increased by 19%. The results contribute to a better understanding of the effects of kerogen wettability and thermal maturity on the wettability of organic-rich mudrocks. The outcomes can also have potential future contributions in understanding flow mechanisms in organic-rich mudrocks as well as in developing reliable rockphysics models for the interpretation of borehole geophysical measurements [e.g., electromagnetic and nuclear-magnetic-resonance (NMR) measurements] in organic-rich mudrocks.

Breakthrough pressure of oil displacement by water through the ultra-narrow kerogen pore throat from the Young–Laplace equation and molecular dynamic simulations

“…This indicates that the Type II-C kerogen is generally nC 8 -wet, which is in line with the experimental finding for kerogen with high maturity. 68 In Fig. 6(c), we present the H 2 O density profile along the z -direction at X = 0.…”

Complexities Driving Wettability Evaluation of Shales toward Unconventional Approaches: A Comprehensive Review

“…They expressed that the lack of OM pores in oil-mature samples is due to the bitumen infilling of pores. Even a low concentration of kerogen (e.g., 4 wt %) can significantly impact the water adsorption capacity and wettability of the organic-rich mudrocks . Therefore, the maturity (on the basis of the vitrinite reflectance) of OM affects the wetting affinity, as discussed in the following section.…”

“…Many researchers observed substantial contact angle variation within a small shale sample only a few millimeters apart. ,− Whereas the pores at the surface of the rock are artificially accessible and can drastically influence the observed contact angle, this method is not sufficient for characterizing the wettability of tight/shale rocks . Furthermore, the contact angle measurement may not realistically represent the actual reservoir ( in situ ) wettability in heterogeneous organic-rich rocks. , …”

“…Recently, Jagadisan and Heidari prepared synthetic organic-rich mudrock samples by mixing known quantities of clay minerals, non-clay inorganic minerals, and kerogen. They observed that the water adsorption capacity of heat-treated kerogen samples decreased with increasing thermal maturity.…”

Use of Outcrop as Substitute for Subsurface Shale: Current Understanding of Similarities, Discrepancies, and Associated Challenges

“…In some recent studies, Jagadisan and Heidari − observed that thermal maturation induces variations in the kerogen’s chemical structure and modifies its oxygen and hydrogen content, which affects its wettability. Although the maturities of outcrop and subsurface shales are different, the variations in the measured wettabilities of outcrop and equivalent subsurface samples might be unexpected.…”