Nanopetrophysics Characterization Of The Bakken Formation, Williston Basin, North Dakota, USA
Anyanwu, Joseph Ikechukwu
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The recent oil boom in the US has been attributed to the result of hydraulic fracturing and horizontal drilling of shale plays. However despite this boom, production and maximum recovery is still limited to be only few percentage of the original oil in place. There have been many studies attempting to enhance the oil recovery of the Bakken Shale, however one area that has not been addressed is the structure of the nanopores storing and transporting the hydrocarbons. The pore geometry and connectivity of these nanopore structures affect the fluid flow and mass transport, which is linked to overall oil recovery. This work focuses on the connectivity of the nanopore structure within the Bakken shale using cores of all three Bakken members obtained from a producing well in North Dakota. For these tight rock samples, we study pore structure and edge-accessible porosity, using the following complementary tests: 1) mercury injection capillary pressure (MICP); 2) fluid and tracer imbibition into initially-dry sample; and 3) tracer diffusion into fluid-saturated sample. Tracer imbibition and diffusion is measured by elemental laser ablation-ICP-MS mapping.Results from the MICP show that the Bakken members have nano-sized pores with low permeability and tortuous flow pathways. Low connectivity of small pore produce anomalous imbibition behavior indicated by the imbibition slope, which is consistent with percolation theory. Limited connected pathways for the tracers were also observed from tracer imbibition and saturated diffusion tests.The findings from these innovative approaches provide information on pore structure and connectivity that can be used in fluid dynamics to estimate overall oil recovery.