A Novel Characterization of Effective Permeability of Tight Reservoir - Based on the Flow Experiments in Microtubes

Abstract

In tight reservoirs, as throat radius decreases to micro-nano scale, pore structure becomes more complex. The effect of interaction between seepage fluids and rocks increases, which changes flow characteristics in micro scale. In order to realize efficient development of tight reservoirs, the throat distribution and permeability needs new understanding. In this paper, a device was established to measure water flow rate in microtubes, under different pressure gradient. The water was de-ionized and tubes were made of fused silica with radius of 1, 2.5, 5, 7.5, 10 and 15μm. The pressure gradient was set mainly 0.02~0.6MPa/m, close to that in oilfield, while previous published experiments focused on 1~80MPa/m. The results showed that experimental flow rate was lower than theoretical value calculated by Poiseuille equation. A stable boundary layer was formed near the solid wall, blocking the flow. Its thickness was calculated, ranging 29.6nm~ 1.08μm. As pressure gradient increased, the boundary layer declined and its effect vanished. This change led to nonlinear flow characteristics in micro scale. The thickness increased with throat radius increasing and reached a constant value when tube radius were larger than 15μm. Based on experimental results, a boundary layer thickness function was regressed. The independent variables were pressure gradient, throat radius and viscosity. Considering boundary layer, the effective throat radius distribution was characterized, taking normal distribution as the original distribution. The results showed that the range of effective throat distribution was narrower than the original one and the peak value was higher. The sensitivity of the function was analyzed. Based on the characterization of effective throat distribution, the formulas of Klinkenberg permeability and effective permeability were derived. The effective permeability formula was validated using data from Shiwu and Bohai oil reservoir. The deviation is lower than 6%. The effective permeability of tight reservoir under different conditions was calculated and investigated. The larger the median radius and the range of the throat distribution, the higher the effective permeability. The effective permeability of a core is not determined only by its pore structure. It is also effected by pressure gradient and the properties of the fluids. When the pressure gradient increases, the effective permeability increases as the boundary layer declines. When the fluid viscosity increases, the effective permeability decreases.