Breakthrough pressure anisotropy and intra-source migration model of crude oil in shale

Abstract

With shale oil becoming an increasingly important resource in global oil and gas exploration and development, the breakthrough pressure of crude oil in shale series source rocks has become an important topic for research on hydrocarbon migration. Shale has long been recognized as an effective source and cap rock, with, up to now, the most common method used for breakthrough pressure testing in shale being the gas method. However, this paper sets out a new system for testing the breakthrough pressure of crude oil in siltstone, mudstone, and shale based on differences in electrical resistance between crude oil and formation water. The breakthrough pressure of crude oil with different lithology was tested in different directions using this method. The results show that breakthrough pressure anisotropy can be observed in mudstone, shale, and siltstone, and that the breakthrough pressure parallel to the bedding direction is one fourth or one seventh of the breakthrough pressure perpendicular to the bedding direction. The horizontal breakthrough pressure of shale is one twentieth to one third of that of mudstone and siltstone, and the vertical breakthrough pressure of siltstone is one seventh of that of mudstone and shale. The breakthrough pressure anisotropy of crude oil reveals that the main migration direction in shale series source rocks is in the direction of bedding unless faults or fractures are encountered. The main migration channels in shale series source rocks are in the horizontal direction, and hydrocarbon migration occurs more easily in laminar shale than in mudstone. Horizontal migration is also the principal mode of hydrocarbon expulsion in thick mudstone and shale. The breakthrough pressure experiments on crude oil in this study also reveal the migration directions and modes in intra-source rocks. This improves on the existing oil migration model and has great significance for understanding the migration of oil in shale source rocks.