An experimental study of supercritical CO2 flow in pipes and porous micro-models for carbon sequestration applications.

摘要

The flow of high-pressure, near-critical CO2 in configurations relevant to CO2 sequestration was investigated. The first configuration was CO2 flow in pipes and orifices at pressures and temperatures close to the critical point of CO2 (74 bar, 31°C). A 60-cm-long stainless steel pipe with 2.1 mm inner diameter was used in order to study near-critical CO2 pipe flow. In terms of raw flow data, the results indicated high sensitivity of pressure drop to mass flow rate as well as to inlet conditions; i.e. pressure and temperature. Remarkably though, when friction factor and Reynolds number were defined in terms of the inlet conditions, it was established that the classical Moody chart described the flow with satisfactory accuracy. This was rationalized using shadowgraphs that visualized the process of transition from a supercritical state to a two-phase subcritical state. During this transition, the two phases were separated due to density mismatch and an interface was established that traveled in the direction of the flow. This interface separated the flow in two regions of essentially single-phase flow, which explained the effective validity of the classical Moody chart. Also, Joule-Thomson throttling was studied using a 0.36-mm-diameter orifice. For conditions relevant to carbon capture and sequestration, the fluid underwent Joule-Thompson cooling of approximately 0.5°C/bar. The temperature difference during the cooling increased with increasing inlet enthalpy. Discrepancies with previous computed and experimentally measured values of Joule-Thompson throttling were discussed in detail. In a second configuration, liquid/supercritical CO2 was injected into two-dimensional porous micro-models saturated with water, which mimicked the process of injection and flow into saline aquifers. This flow configuration was studied using fluorescent microscopy and micro-PIV by seeding the water phase with fluorescent tracer particles, and dyeing CO2 with a fluorescent dye. This technique allowed for measurement of the velocity field in the water phase, and tracking the CO2 phase in the porous medium. The results revealed the nature of the flow field during the initial invasion and migration of the CO2 front. In particular, it was established that the front developed growing dendritic features called fingers. During that growth process, velocities 20--25 times the bulk velocity were measured, which occurred in both the flow direction and opposite to it. These velocity jumps support the notion of pressure bursts and Haines jump during pore drainage events. In addition, the variations of the interfacial curvature with time and their connection with water flow field during the growth of fingers were studied. The results revealed the existence of high-momentum pathways in water ahead of growing CO2 fingers. After the passage of the CO 2 front, shear-induced flow was detected in the trapped water ganglia in the form of circulation zones near the CO2-water interfaces. The shear from CO2 flow also induced motion in the thin water films covering the surfaces of the micro-model.