Impacts of Competitive Water Adsorption of Kerogen and Clay Minerals on Wettability of Organic-Rich Mudrocks

Abstract

Summary Quantifying wettability of organic-rich mudrocks is important for reliable formation evaluation, optimizing production, predicting water/hydrocarbon production, and selection of appropriate fracture fluids. Recent publications suggest that kerogen wettability can vary as a function of thermal maturity, ranging from water- to hydrocarbon-wet at low to high thermal maturities, respectively. However, clay minerals tend to preferentially be water-wet. It is therefore important to determine which of these constituents have a dominant contribution to overall wettability of the rock. To answer this question, we introduce methods to quantify the relative water-adsorption capacities of clay minerals, kerogen, and organic-rich mudrocks at different thermal-maturity levels. We started with isolating kerogen from organic-rich mudrock samples using chemical and physical separation methods and synthetically matured them to different thermal-maturity levels. We then prepared synthetic organic-rich mudrock samples by mixing known quantities of clay minerals, nonclay inorganic minerals, and kerogen. We then performed water-vapor adsorption measurements on pure clay minerals, pure kerogen samples, and synthetic organic-rich mudrock samples under controlled humidity conditions. Nuclear magnetic resonance (NMR) measurements were then used to quantify the volume of water adsorbed on clay minerals and organic-rich mudrock samples. We used the flotation test to qualitatively assess the wettability of the synthetic organic-rich mudrocks. Water-vapor adsorption experiments showed that the volume of water adsorbed on the surface of nonheated kerogen samples at low thermal maturities is 5.31 mL/100 g, which decreases significantly to 0.09 mL/100 g when the kerogen sample is heat-treated to 450°C. The results can be attributed to strong attraction between the oxygen content in kerogen and water at low thermal maturities. We quantified the water-adsorption capacity of kerogen samples heat-treated at 450°C and found that volume of water adsorbed decreases with an increase in thermal maturity both in the presence and absence of bitumen. In the case of synthetic organic-rich mudrock samples, we found that the volume of water adsorbed in samples at higher thermal maturity decreases by 16% compared with organic-rich mudrocks at low thermal maturity at the same concentration of nonswelling clay minerals. Results from the flotation test showed that the oil-wettability of the synthetic organic-rich mudrock samples increases as its thermal maturity decreases, with a hydrogen index (HI) of 328 to 54 mg hydrocarbon/g organic carbon (mg-HC/g-OC). Results confirmed that kerogen and its geochemistry can have a significant influence on the overall wettability of organic-rich mudrocks even at low concentrations of 4 wt%. The outcomes of this paper can contribute to a better understanding of the parameters affecting wettability of organic-rich mudrocks and are promising for in-situ assessment of their wettability. This can potentially contribute to improved understanding of flow mechanisms in organic-rich mudrocks, which control hydrocarbon/water production.