--> Evaluation on the Adsorbed and Free Oil in Lacustrine Shales

AAPG ACE 2018

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Evaluation on the Adsorbed and Free Oil in Lacustrine Shales

Abstract

Free oil, rather than adsorbed oil, is potentially the most producible component of tight nanoporous shale reservoirs using existing technologies. To date, a variety of geochemical parameters, mainly related to the retention, migration and expulsion of generated oil, have been developed to estimate the free oil in shale. However, there is still a lack of theoretical models accounting for the mechanism of oil accumulation in nanoporous shale, which would help evaluate adsorbed and free oil. In this study, we presented a set of theoretical models for quantitatively evaluating the amounts of adsorbed and free oil confined in nanopous shale and mainly in adsorbed and free states. The models comprehensively considered the multilayer adsorption of oil and the microstructure of shale pore system.

Microscopically, the amounts of shale oil in different states are comprehensively impacted by the microstructure (size, morphology and volume) of pores and the state of the oil (densities and adsorption thickness). With respect to the studied shales, main findings are as follows: (1) With increasing mean pore diameter (dm), the amount of adsorbed (free) oil gradually decreases. (2) The effect of pore morphology on both the adsorbed and free oils is remarkable when dm is < ~100 nm. (3) When pore diameter is > ~100 nm, the size and morphology of pores have a minor impact on the oil in an adsorbed and free amounts, which primarily depends on the volume of oil-bearing pores. (4) For regular spherical (representative of a typical pore), columnar (throat) and slit-shaped (fracture) pores, the adsorbed amount is greatest (smallest) in spherical (slit-shaped) pores, and the corresponding free amount is smallest (greastest) in spherical (slit-shaped) pores. (5) Compared with the organic-bearing and -rich shales, the weight ratio of the adsorbed phase is relatively higher in organic-lean shales.