Phase behavior of nanoconfined continental shale oil and reservoir fluid phases in the Gulong shale of the Songliao basin

Abstract

Abstract The shale oil phase state is essential for assessing shale oil production, establishing development plans, and enhancing oil recovery. Previous studies on shale oil phase state mainly focused on bulk fluid phase based on fluid composition retrieved from drill holes, while nano-confined shale oil phase state based on subsurface in-situ fluid compositions is rarely discussed. In this work, we established a new workflow that used pressure-preserved shale cores and pyrolysis gas chromatogram (Py-GC) to retrieve in-situ shale oil compositions from various shales with different thermal maturities. The workflow includes retrieving pressure-preserved shale cores, cutting, transporting, and then preparing samples under the protection of liquid nitrogen; the new process makes the evaporation loss of lightweight hydrocarbons the least. By comparison, fluid samples from wellheads and wellbores were also retrieved from Gulong shale oil reservoirs in the Songliao Basin. Both bulk and nano-confined shale oil phases were then analyzed, and the results showed that methane content in the Gulong shale oil increased from 10.11 to 23.39% with increasing thermal maturity; by contrast, C7+ hydrocarbons decreased from 64.73 to 41.13%. As for bulk fluid phases, Gulong shale oils are black oil phases, while in terms of nano-confined fluid phases, their phases are controlled by thermal maturity. They are black oil phases at lower thermal maturity with vitrinite reflectance (Ro) less than 1.4% and are condensate phases at higher thermal maturity with Ro greater than 1.4%. Shale oil production data showed that nano-confined phase analysis using in-situ shale oil compositions from pressure-preserved cores can best predict shale oil production in this study.

Keywords

• Pressure-preserved cores
• Bulk hydrocarbon composition
• Shale oil phase
• Nano-confined effect
• Gulong shale oil