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

Abstract

Abstract To understand the flow and transport mechanisms in shale reservoirs, in 2014 we needed reliable core-measured data that were not available to us. Thus, we conducted a series of diverse experiments to characterize pores and determine the flow properties of twelve Middle Bakken cores 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, nitrogen adsorption, and NMR, we determined pore-size distribution. 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 enhanced oil recovery technique. The cores were in three conditions: clean, preserved, and un-cleaned; 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, pore size distribution 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: 1. Centrifuge capillary pressure in Bakken cores was on the order of hundreds of psi—both in positive and negative range. 2. Mobile oil saturation range for water-displacing-oil was very narrow (around12 % PV) and much wider (around 40 % PV) for gas-displacing-oil. 3. In Bakken cores, oil production by spontaneous imbibition of high-salinity brine was small unless low-salinity brine was used for spontaneous imbibition. 4. 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.