With the rapid development of unconventional oil and gas, the pore structure characterization of shale reservoirs has attracted an increasing attention. High pressure mercury intrusion porosimetry (HPMIP) has been widely used to quantitatively characterize the pore structure of tight shales. However, the pore structure obtained from HPMIP could be significantly affected by the sample particle size used for the analyses. This study mainly investigates the influence of shale sample particle size on the pore structure obtained from HPMIP, using Mississippian-aged Barnett Shale samples. The results show that the porosity of Barnett Shale samples with different particle sizes obtained from HPMIP has an exponentially increasing relation with the particle size, which is mainly caused by the new pores or fractures created during shale crushing process as well as the increasing exposure of blind or closed pores. The amount and proportion of mercury retention during mercury extrusion process increase with the decrease of shale particle size, which is closely related to the increased ink-bottle effect in shale sample with smaller particle size. In addition, the fractal dimension of Barnett Shale is positively related to the particle size, which indicates that the heterogeneity of pore structure is stronger in shale sample with larger particle size. Furthermore, the skeletal density of shale sample increases with the decrease of particle size, which is possibly caused by the differentiation of mineral composition during shale crushing process.
Spontaneous imbibition (SI) has been used widely to characterize shale wettability and pore structures, which can significantly affect the migration and accumulation processes of shale oil. However, the application of SI in the enrichment mechanism of shale oil is quite limited. Therefore, focusing on the Permian Fengcheng Formation shale deposited in an alkaline lake environment in the Mahu Sag of Junggar Basin, this study conducts systematic SI experiments with complementary experiments such as contact-angle measurement, optical microscopy, field emission scanning electron microscopy, micro-CT, high-pressure mercury intrusion porosimetry, and N 2 adsorption to clarify the controlling factors of SI behavior of different fluids in shale and attempts to evaluate the migration and accumulation potential of shale reservoirs using SI. The results show that the more hydrophobic the sample is, the stronger the absorption of n-decane is, and the SI of n-decane reaches equilibrium quickly. The development of an alkaline mineral layer that is several millimeters thick in Fengcheng Formation shale could promote hydrocarbon migration due to the large particles of alkaline minerals (200−400 μm) and more development of intergranular microfractures, which can be indicated by the higher SI slope and imbibed oil volume. Using SI parameters, the shale hydrocarbon migration−accumulation index H m was proposed in this study; the greater the H m of the shale reservoir is, the more conducive to the migration and accumulation of shale oil the shale reservoir is. Four migration−accumulation patterns were established for different lithofacies in the study area, and the migration and accumulation potential of different lithofacies of shale from strong to weak is in the order of siltstone, shale with an alkaline mineral layer, laminated shale, and then massive shale, which is generally in line with the order of shale oil content. The validity of the proposed shale hydrocarbon migration−accumulation index is also confirmed using data from the literature.
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