Low porosity and permeability liquid hydrocarbon reservoirs that require hydraulic fracturing are commonly known as "Tight Liquid Reservoirs." These reservoirs have recently received significant attention as a resource for producing oil due to the success of new horizontal well drilling and completion practices, as well as recent market economics. Characterization of these resources, particularly with respect to matrix permeability, is an important factor in determining the producibility and profitability of these resources. Current industry practices to measure permeability on these oil bearing rock samples are similar to those used on gas shale samples, which involves crushing the rocks and conducting measuritalicents on the crushed samples. Such crushed-rock methods have been investigated for gas shales, and a number of publications have indicated the limitations of such methods in characterizing reservoir rocks.
In this paper, we present results using a steady-state method to measure liquid permeability on intact tight oil rock samples from the Vaca Muerta formation in Argentina, under reservoir net confining stress. Our experiments indicate that factors such as stress creep, overburden stress, and type of fluid utilized for permeability measuritalicents could have a significant impact on the measured permeability. Knowledge of these effects enables the development of best practices for tight oil permeability measuritalicents, leading to results that are more representative of the reservoir and enabling more accurate business decisions.
Our results indicate that stress creep is significant and slow. Large permeability declines occur during the first several days after stress is applied, and further declines can continue for a month or more. Results also show that the steady-state permeability is highly stress-sensitive, with a strong hysteresis effect. Large permeability declines are observed when the applied stress is increased, and little recovery occurs as the stress is relieved. Klinkenberg corrected steady state gas permeability measured at reservoir conditions was found to be within a factor of two with steady state liquid permeability measured on the same samples at same net confining stress.