A theoretical study of the occurrence state of shale oil based on the pore sizes of mixed Gaussian distribution

Abstract

The analysis of the occurrence state of crude oil in shale rock is crucial for the formation evaluation of shale oil reservoirs. A method based on Gaussian Mixture Model for estimating the fractions of physically adsorbed, immobile (trapped in the pore space resulting from van der Waals), and free oil while considering both organic and inorganic pores is established. In this work, the critical pore radius for immobile oil is assumed to be equal to the thickness of the adsorption region. A sensitivity analysis of the derived formulae is conducted to demonstrate the following: (a) physically adsorbed and immobile oil in shale rock are primarily distributed within organic pores; (b) the ratio of physically adsorbed or free oil is dominated by the organic porosity ratio; (c) the ratio of immobile oil is dominated by organic parameters, while inorganic factors only possess a minor impact; (d) the ratio of physically adsorbed oil is slightly underestimated using a pure circular pore model; and (e) the fractions of immobile, physically adsorbed and free oil are most likely to exist in the ranges [0, 10%], [20%, 30%], and [65%, 80%], respectively. A sensitivity analysis on the average pore radius demonstrates that the non-negligible, if not substantial, quantities of physically adsorbed and immobile oil in shale rock are very unique. Consequently, the efficient exploitation of immobile oil is probably a new fundamental issue in enhanced shale oil recovery research. Currently, the porosity related to immobile oil can be regarded as “dead porosity” and excluded from measured porosity for more accurate formation evaluation. Finally, comparisons are constructed to illustrate the superiority of the Gaussian Mixture Model in the depiction of the complicated and random porous structure of shale rock. This work can help with the formation evaluation of shale oil reservoirs and will provide insight toward the development of shale resources.