A comparative investigation of shale wettability: The significance of pore connectivity

• Contact angle measurements do not necessarily represent the true wettability of shales.
• Wettability of unconventional tight rocks is affected by the connectivity of hydrophobic and hydrophilic pores.
• The presence of degraded bitumen results in strong affinity of tight rocks to oil.

We conduct spontaneous imbibition experiments, using oil and water, to compare the wetting behavior of rock samples from the Montney (MT) and the Horn River (HR), two massive unconventional gas plays in the Western Canadian Sedimentary Basin. The MT samples imbibe more oil than water, while, the HR samples imbibe more water than oil. For the two cases, the difference between oil and water imbibition is even more pronounced when plotted versus dimensionless scaling time. The observed imbibition behavior cannot be fully explained by the contact-angle results showing complete spreading of oil and partial spreading of water, for the two rock cases.We hypothesize that the significant oil uptake of the MT samples is because the majority of connected pores are within or coated by degraded bitumen, and we test this hypothesis by analyzing thin sections, SEM images, micrograph and TOC of the corresponding rock samples. Analogously, we hypothesize that the relatively lower oil uptake of the HR samples is due to the poor connectivity of organic pores, and we test this hypothesis by analyzing SEM images and conducting complementary imbibition tests using crushed HR samples. The presence of solid bitumen in well-connected MT pore network is confirmed by photomicrographs. SEM images of the HR samples reveal that organic regions of the HR samples are poorly connected. However, high oil imbibition into the crushed HR samples suggests that the majority of rock mass is oil-wet. It is the affinity of the connected pore network to the fluid that determines the imbibition behavior of oil and water. Hydrophobicity of the connected pore network in the intact MT samples results in higher oil imbibition. In contrast, hydrophilicity of the connected pores, mainly due to presence of clay mineral, in the intact HR samples results in strong water uptake and weak oil uptake.