Pore Size Distribution of Unconventional Rocks with Dual-Wet Pore Network: A Sequential Spontaneous and Forced Imbibition Technique

Recent wettability studies indicate the dual-wet behavior of unconventional rocks that have hydrophobic pores within the organic matter with low wetting affinity to brine. In contrast, the hydrophilic pores bordered by inorganic minerals such as quartz, feldspar, calcite, and clays have strong wetting affinity to brine. The total pore network composed of hydrophobic and hydrophilic pores exhibits a dual-wet behavior. Conventional methods such as mercury injection capillary pressure (MICP) and nitrogen/CO2 sorption tests give the total pore size distribution (PSD_tot), regardless of pore wettability. Modeling two- phase transport mechanisms in such dual-wet media requires separate characterization of hydrophobic (PSD_HB) and hydrophilic (PSD_HL) pore size distributions.

We proposed a two-step experimental procedure for estimating PSD_HB and PSD_HL. In Step 1, we used reservoir brine and conducted co-current spontaneous imbibition (SI) tests on dry shale plugs from the Duvernay Formation. We considered the pore network of shale plugs as an idealized bundle of tortuous capillary tubes, and estimated PSD_HL using imbibition transient analysis (ITA) proposed in a previous study. The Lucas-Washburn equation was combined with a fractal model to develop ITA. In Step 2, we immersed partly brine-saturated plugs from SI test (Step 1) in brine and increased the pressure incrementally (forced imbibition or drainage process). We used incremental brine saturation at each pressure and estimated PSD_HB by the Young-Laplace (Y-L) equation. The results show that cumulative pore space filled by brine in spontaneous- and forced-imbibition tests under maximum pressure of 9,500 psig is more that 90% of pore volume (PV), while mercury in MICP test can fill less than 40% of PV under maximum pressure of 55,110 psig. Therefore, pore size distribution estimated by brine-imbibition tests is expected to be more representative compared with that estimated by MICP tests. The peak-pore throat size of hydrophobic pores (D_peak-HB) estimated by forced imbibition of brine is in the range of 5.8-14.6 nm, consistent with the two-dimensional visualization of organic pores using scanning electron microscopy (SEM) analysis. The minimum values of pore-throat diameters detected in mercury- and brine-injection tests are 3.8 nm and 1.2 nm, respectively. Therefore, smaller pore throats can be characterized by brine-injection test at a significantly lower pressure (9,500 psig) compared with that by mercury-injection test (55,110 psig). The results show that a part of the pore network with pore throats smaller than 3.8 nm is not accessible for mercury. However, brine can be injected into this part of the pore network.