Pore-scale assessment of Middle Bakken reservoir using centrifuge, mercury injection, nitrogen adsorption, NMR, and resistivity instruments

Abstract

To understand and decipher the pore-scale flow and transport mechanisms in the Bakken, and in similar low-permeability reservoirs, reliable data measured on cores is of great help. Thus, in this research a series of diverse experiments, which addressed specific issues, were conducted. The experiments included centrifuge, mercury intrusion capillary pressure (MICP), nitrogen adsorption, resistivity, and nuclear magnetic resonance (NMR) experiments on twelve Middle Bakken core plugs. The reason for such variety of experiments was the need to characterize the pores flow characteristics in addition to the rock-fluid interaction behavior of the pore space. As a result, capillary pressure characteristics of the drainage and imbibition cycles, residual saturations, mobile fluid saturation range, pore-size distribution, tortuosity, and fluid distribution within the pores were measured. From the core experiments, we were also able to determine how ultra-tight pore characteristics affect oil recovery. The cores used in the study were in three conditions: clean, preserved, and uncleaned exposed cores. Bakken oil, decane, formation brine, and several synthetic brines (with different salinities) were used in saturating and de-saturating the cores in an ultra-high-speed centrifuge. After saturating the cores with brine or oil, a set of drainage and imbibition experiments was performed. NMR measurement was conducted before and after each saturation/de-saturation step. Resistivity measurements on five of the brine-saturated cores were conducted to determine tortuosity. Centrifuge experiments yielded large water-oil capillary pressures (hundreds of psi) in the Bakken cores. The mobile fluid saturation range for water displacing oil was 8 to 12 percent. The saturation range for water displacing oil was much smaller than for gas displacing liquid. NMR evaluations indicated that, after every saturation/de-saturation step, brine resided in smaller pores while oil resided in larger pores. Resistivity measurements yielded large tortuosities. These large tortuosities indicate that fluids have great difficulty moving from one point in the reservoir to another. Tortuosity information is critical in assessing molecular mass transport by diffusion in reservoir pores. Because displacements involving liquid injection have difficulty with entering the pores in unconventional shale, gas diffusion could alleviate such problems as an alternative. In summary, drainage and imbibition experiments, followed by NMR measurements, provided a great amount of useful information for assessing EOR potential in Bakken and other low-permeability shale reservoirs.