Characterizing Pores and Pore-Scale Flow Properties in Middle Bakken Cores

Abstract

Summary To understand the flow and transport mechanisms in shale reservoirs, we needed reliable core-measured data that were not available to us. Thus, in 2014, we conducted a series of diverse experiments to characterize pores and determine the flow properties of 12 Middle Bakken cores that served as representatives for unconventional low-permeability reservoirs. The experiments included centrifuge, mercury-intrusion capillary pressure (MICP), nitrogen adsorption, nuclear magnetic resonance (NMR), and resistivity. From the centrifuge measurements, we determined the mobile-fluid-saturation range for water displacing oil and gas displacing oil in addition to irreducible fluid saturations. From MICP and nitrogen adsorption, we determined pore-size distribution (PSD). Finally, from resistivity measurements, we determined tortuosity. In addition to flow characterization, these data provided key parameters that shed light on the mechanisms involved in primary production and the enhanced-oil-recovery (EOR) technique. The cores were in three conditions: clean, preserved, and uncleaned. The hydrocarbon included Bakken dead oil and decane, and the brine included Bakken produced water and synthetic brine. After saturating the cores with brine or oil, a set of drainage and imbibition experiments was performed. NMR measurements were conducted before and after each saturation/desaturation step. After cleaning, PSD was determined for four cores using MICP and nitrogen-adsorption tests. Finally, resistivity was measured for five of the brine-saturated cores. The most significant results include the following: Centrifuge capillary pressure in Bakken cores was on the order of hundreds of psi, both in positive and negative range. Mobile-oil-saturation range for water displacing oil was very narrow [approximately 12% pore volume (PV)] and much wider (approximately 40% PV) for gas displacing oil. In Bakken cores, oil production by spontaneous imbibition of high-salinity brine was small unless low-salinity brine was used for spontaneous imbibition. Resistivity measurements yielded unexpectedly large tortuosity values (12 to 19), indicating that molecules and bulk fluids have great difficulty to travel from one point to another in shale reservoirs.