Experimental Investigation of the Role of Temperature on the Threshold Gradient of non-Darcian Flow in Clay/Sand Mixtures

Abstract:

A threshold pressure gradient is needed to trigger fluid flow in low-permeability porous media, causing a nonlinear relationship between the pressure gradient and flow velocity at low-pressure gradient levels (i.e., the non-Darcian flow behavior) and thus has important implications in shale oil and gas recovery and high-level nuclear waste disposal in geologic repositories. Despite the importance of non-Darcian flow in many natural and engineering processes, it is still unclear how temperature influences the threshold gradient. To answer this fundamental question, a customized core flooding system designed for high-precision measurements of hydraulic gradients and permeability in swelling bentonite clay was used. The measurements were conducted under steady-state flow conditions using a NaCl solution having 0.1 M ionic strength and clay/sand mixtures. The temperatures range from 20℃ to 90℃ and the mass fractions of clay in the mixtures range from 10% to 50%. The correlation curves between Darcy flow velocity and pressure gradient were measured to demonstrate the nonlinear behaviors in the low-pressure-gradient regimes. Experimental results indicated that a higher temperature led to a lower threshold gradient. As the clay mass fraction increased, the measured permeability decreased whereas the threshold gradient increased. The relationship between permeability and threshold gradient followed a power-law empirical correlation, which was consistent with the finding in a previous study. The results also showed that higher temperatures shifted the correlation curve in the bottom-left direction, and the variations of the two fitting coefficients in the power-law correlation were well predicted by a continuum-scale two-parameter model proposed in our previous study, which provides mechanistic insights into the role of temperature on non-Darcian flow in saturated low-permeability porous media. Our experimental results validated the proposed continuum-scale model and will have valuable applications in unconventional hydrocarbon recovery and geologic disposal of high-level radioactive nuclear waste.