Evaluating Wettability of Calcareous Shales from the Duvernay Formation: Effect of Petrophysical Properties, Thermal Maturity, and Natural Fractures
Creators: Xu, Shiyu
Publisher: name: University of Alberta Library
Year: 2022

<p>Recent advances in horizontal drilling and multi-stage hydraulic fracturing technologies have led to rapid development of unconventional oil and gas resources to meet the increasing energy demand. In particular, organic-rich shales are considered important resource plays worldwide. In order to design successful hydraulic fracturing and enhanced oil recovery operations, one should have a detailed understanding of rock properties of these organic-rich shales and their hydrocarbon recovery potentials.<br>
Spontaneous imbibition (SI) tests have been used to evaluate hydrocarbon recovery potential in unconventional reservoirs. In this study, we conducted SI tests on calcareous shale plugs from 5 wells drilled in the Early-oil (EOW) and oil windows (OW) in East Shale Basin (ESB) of the Duvernay Formation. The results were compared with those of siliceous samples from the West Shale Basin (WSB) of Duvernay formation to investigate effects of kerogen maturity on oil and brine uptake. We characterized EOW plugs based on the results of tight-rock analysis (TRA), x-ray diffraction (XRD), and rock-eval pyrolysis. We compared the SI results and petrophysical properties with those of highly-mature and siliceous plugs in the oil and gas windows in the West Shale Basin (WSB) of the Duvernay. We investigated the relationships between SI results and petrophysical properties of ESB plugs to understand the effects of kerogen maturity and rock mineralogy on SI and pore structure of organic-rich shales. We also investigated the effects of organic-pore connectivity and fractures on wettability of the EOW plugs. By using CT scan images, we divided EOW plugs into highly-fractured (HF), slightly-fractured (SF), and non-fractured (NF) plugs. We used reservoir oil and conducted comparative imbibition tests on selected core plugs to investigate effects of fracture intensity on imbibition profiles.<br>
The ESB plugs are categorized as calcite-rich shale (calcareous shale) that are rich in organic matter (average total organic carbon (TOC) of 5.5 wt%) with significantly high value of Hydrogen Index (HI &gt;600). The results of SI tests show that oil wettability index (WIo) of ESB plugs is positively correlated with production index (PI) and Tmax, but not with TOC content. We hypothesize that the low oil imbibition in plugs with high TOC content is due to poor connectivity of their organic pores, and we test this hypothesis by analyzing SEM images and conducting nitrogen sorption tests. The lack of well-developed organic pores within organic matter of the EOW plugs is confirmed with SEM images. The results of nitrogen sorption tests show that micro and fine mesopores (&lt; 10 nm) within organic matter are more abundant in OW plugs than in EOW plugs. The poor connectivity of pores can also be confirmed by observing a large portion of pore space not being accessed by oil when comparing the normalized imbibed volume of oil with the corresponding porosity value. We also observed that the normalized Imbibed volume of oil is much higher in the HF and SF plugs compared with that in the NF plugs. The results suggest that the fractures enhance accessibility of isolated pores, leading to more connected pore network for oil imbibition. This observation suggests that fracture porosity plays a significant role in wetting behavior of the EOW plugs.  We also concluded that the porosity measured by Boyle’s law helium-porosimetry using crushed EOW samples can be overestimated due to enhanced accessibility of isolated pores. This is because crushing the samples enhances accessibility of isolated pores considered as ineffective porosity under intact conditions. Combined analyses of imbibition profiles and core images of the fractured plugs show that oil rapidly imbibes into the fracture system, and then gradually imbibes from fractures into rock matrix.</p>