Effect of kerogen maturity on organic shale wettability: A Duvernay case study

We reviewed and evaluated the wettability of organic-rich shale plugs from 6 wells drilled in the Duvernay Formation, which is a self-sourced reservoir. We divided our samples into Upper (UD) and Lower (LD) Duvernay based on the variations in depth and kerogen maturity levels. UD and LD samples belong to oil and gas windows, respectively. We compared spontaneous imbibition of oil and brine into 17 twin plugs with wide range of kerogen maturity level and total organic carbon (TOC) content. We also characterized the shale samples by analyzing the results of tight-rock analysis (TRA), x-ray diffraction (XRD), and rock-eval pyrolysis tests to identify the petrophysical properties, mineralogy, and organic-carbon properties, respectively. To investigate the size, abundance, and morphology of pores, we analyzed scanning electron microscopy (SEM) of the shale samples. Finally, we investigated the relationships between the results of spontaneous-imbibition tests and geochemical properties to understand the effects of kerogen maturity level on shale wettability. The results of wettability tests show higher normalized imbibed volume of oil (I-o) compared with that of brine (I-w), suggesting that the uncleaned dry plugs are strongly oil-wet. The positive correlations of TOC content with effective porosity and pressure-decay permeability indicate that a significant fraction of the pore space is within the organic matter (OM), supported by the SEM images. The abundance of organic pores may explain the strong wetting affinity of the shale samples to oil. We observed negative and positive correlations between TOC content and equilibrated I-o (I-o(eq)) of UD and LD samples, respectively. This discrepancy can be explained by the smaller number of organic pores of UD samples with lower thermal maturity (oil window), confirmed by the results of SEM analysis and pore size distribution (N-2 sorption) tests. Furthermore, the free hydrocarbons content (S-1 in rock-eval data) in the form of oil in the UD samples is higher than that in the LD samples. The existence of free hydrocarbons content may reduce the pore space available for oil imbibition, leading to lower I-o(eg) for the UD samples with higher TOC content.