Quantification of Kerogen Wettability Using Adsorption Isotherms

Assessment of fluid production in organic-rich mudrocks can be affected by different rock components, fluid mobility, and geochemistry. Kerogen wettability can significantly affect the preferential movement of fluids in organic-rich mudrocks as it constitutes a significant fraction of mudrock volume. In previous publications, the determination of the wettability of kerogen and organic-rich mudrocks is typically achieved using contact angle measurement through the sessile drop method, which might not be considered as a ground truth quantitative measure of wettability. This method also requires pellets of kerogen to create a surface for the contact angle to be measured. No standardized procedure exists for making pellets under stress conditions and saturating fluid to simulate reservoir conditions. In this paper, we introduce a novel method for quantifying the wettability of kerogen as a function of thermal maturity using adsorption isotherms. We start by crushing organic-rich mudrock samples and sieving them at 170 mesh to obtain a uniformly crushed sample of approximately 90 µm in particle size. We then use a chemical extraction process using hydrochloric and hydrofluoric acid to demineralize the crushed mudrock samples and to obtain pure kerogen. The thermal maturity of the samples is estimated using pyrolysis and vitrinite reflectance measurements. We use the extracted kerogen samples with different natural and synthetically altered thermal maturity levels to perform adsorption isotherm experiments. These measurements are used to quantify the amount of adsorbate adsorbed on the surface of a sample with varying pressure keeping the temperature of the setup constant, which can be converted to a quantitative measure of wettability. We successfully applied the aforementioned method to several samples covering a wide range of thermal maturity levels collected from a challenging organic-rich mudrock formation. The adsorption test on pure extracted kerogen samples showed a 40% relative decrease in water adsorbed at 98% relative humidity level with an increase in the natural thermal maturity of the samples from a hydrogen index (HI) of 198 (Sample A, the lowest thermal maturity) to 130 (Sample B, the highest thermal maturity) mg-hydrocarbon/g-organic-carbon (mg-HC/g-OC). When Sample A was heat treated at 450°C, the HI was reduced to 32 mg-HC/g-OC, and the adsorption test showed a 70% relative decrease in water absorbed compared to the sample in its natural thermal maturity state. The decrease in the amount of water absorbed from Sample A to Sample B correlated with the decrease in the amount of water produced from the locations/wells the samples were collected from. We compared the results from the adsorption isotherm experiments with contact angle measurements. Sample A formed a 15° air/water contact angle compared to Sample B, forming a 109° air/water contact angle. The novelties of this workflow include (a) estimation of the wettability of kerogen quantitatively using adsorption isotherms, (b) eliminating the challenges of making pellets and errors associated with contact angle measurements for wettability assessment, (c) quantifying the influence of thermal maturity on the wettability of kerogen and organic-rich mudrocks, which in turn affect hydrocarbon/water production, and (d) the possibility of enhancing prediction of water/hydrocarbon production by taking into account geochemistry and thermal maturity of organic-rich mudrocks.