Controls on Liquid Hydrocarbon Permeability of Tight Oil and Liquid- Rich Gas Reservoirs: Examples from Bakken and Montney Formations (Canada)

摘要

Quantification of absolute permeability to liquid hydrocarbons is critical for the evaluation of production potential of tight oil and liquid-rich gas reservoirs. However, due in part to the nanometer-sized pore throats in these low- permeability (tight) unconventional reservoirs, laboratory-based characterization of oil permeability is particularly challenging. Focusing on the Montney and Bakken formations in western Canada, the primary objectives of this work are therefore to 1) compare absolute (formation oil) and slip-corrected gas (N2) permeability values for selected core plug samples under similar experimental conditions, 2) compare steady-state and non-steady-state (i.e. pulse-decay) flow techniques for measuring absolute (oil) permeability and 3) examine controls (e.g. effective stress, porosity) on liquid hydrocarbon permeability in these tight rock samples. Using a customized liquid permeameter designed and built in-house, permeability measurements are conducted with formation oil on selected intact core plugs from the Montney and Bakken formations using steady-state and pulse- decay flow techniques at varying stress conditions. The experiments are performed on the same core plugs used previously for pulse-decay gas (N2) permeability tests at similar experimental conditions – therefore, the impact of heterogeneity on liquid/gas permeability comparisons is mitigated. For the core plugs analyzed in this study, the absolute (oil) permeability values range between 2.6. 10-4 and 3.5. 10-2 md, depending on the lithology/formation (Bakken, Montney), methodology (steady-state, pulse-decay), effective stress (500-2300 psi) and mean pore pressure (283-815 psi) conditions. Experimental observations suggest that 1) absolute (oil) permeability values are consistently (up to 20%) lower than the slip-corrected gas (N2) permeability values measured under similar experimental conditions, 2) for one of the analyzed core plugs, the oil permeability value measured using the steady- state technique is approximately 30% larger than that measured using the pulse-decay technique, 3) the measured oil permeability values increase consistently with increasing helium porosity (5.5-13.1 %), and 4) oil permeability values decrease up to 30% with increasing effective stress (500-2300 psi). The observed discrepancies between permeability values obtained from steady-state and pulse-decay techniques can be attributed to 1) non-uniform propagation of pressure gradients and stress regimes inherent to steady-state and pulse-decay permeability methods and 2) experimental/numerical errors associated with permeability determination. Absolute permeability, while an important control on oil/condensate flow in tight oil reservoirs, is difficult and time- consuming to measure for low-permeability rocks in the laboratory. As a result, liquid hydrocarbon permeability data are not commonly reported in the literature – for unconventional reservoirs, these data have been primarily measured for comparatively high-permeability rocks (permeabilities within the millidarcy range). Through measurement of absolute (formation oil) permeability values on selected tight rock samples with varying lithology and porosity, the current study provides critical data and insights of importance to the evaluation of primary and enhanced oil potential in western Canadian tight reservoirs with permeabilities down to the nanodarcy range.