Study Of Pore-Dependent Petroleum Fluid Properties In Tight Shale Plays: Bakken Formation

In 2013, the United States Geological Survey (USGS) released an estimate of 7.38 billion barrels of oil held within the Bakken Formation, one of the most significant discoveries. However, the Bakken Formation, a tight shale play, is also known for its ultra-low permeability and nano-scale pore size (1 – 100 nm). A nano-scale pore size can impact the petroleum fluid properties when compared to its bulk properties. This study utilized a theory-based analytical method and compares the results with those from experiments conducted under the simulated conditions. Peng-Robinson Equation of State (PR-EOS) was modified to calculate the critical properties of individual hydrocarbon component in confined conditions. The modification was deduced from the Helmholtz free energy function with Lennard-Jones potential for particles interaction. The modified PR-EOS became a function of molecule-to-pore-size ratio. The value of pore size was obtained using nuclear magnetic resonance (NMR) and from related experiments. NMR measurements were conducted on five-Middle Bakken core samples and provided the pore size distribution and porosity. The results from the analytical method and experiments were compared with the data from the literature to examine the theory and for comparison purposes. Suppression effect on critical temperature was calculated on eight hydrocarbons (from methane to decane) and compared with the experimental results. The total porosity from five- NMR experiments, is within the range of 2.73 to 9.38 % with an average of 5.4 %. A close match was found between the model and experiments with a deviation of 14% for pore sizes within 1 – 3 nm and of 1.02% for pore sizes larger than 3 nm. The pore size of 3 nm seems to work as a cutoff due to the adsorption layers on the vicinity of the pore wall. Also, the bulk critical properties were estimated using the equations and presented similar results to the literature, with a maximum deviation of 0.03%