Shuyang Liu

In Situ Local Contact Angle Measurement in a CO2–Brine–Sand System Using Microfocused X-ray CT

The wettability of porous media is of major interest in a broad range of natural and engineering applications. The wettability of a fluid on a solid surface is usually evaluated by the contact angle between them. While in situ local contact angle measurements are complicated by the topology of porous media, which can make it difficult to use traditional methods, recent advances in microfocused X-ray computed tomography (micro-CT) and image processing techniques have made it possible to measure contact angles on the scale of the pore sizes in such media. However, the effects of ionic strength, CO2 phase, and flow pattern (drainage or imbibition) on pore-scale contact angle distribution are still not clear and have not been reported in detail in previous studies. In this study, we employed a micro-CT scanner for in situ investigation of local contact angles in a CO2-brine-sand system under various conditions. The effects of ionic strength, CO2 phase, and flow pattern on the local contact-angle distribution were examined in detail. The results showed that the local contact angles vary over a wide range as a result of the interaction of surface contaminants, roughness, pore topology, and capillarity. The wettability of a porous surface could thus slowly weaken with increasing ionic strength, and the average contact angle could significantly increase when gaseous CO2 (gCO2) turns into supercritical CO2 (scCO2). Contact angle hysteresis also occurred between drainage and imbibition procedures, and the hysteresis was more significant under gCO2 condition.

Pure methane, carbon dioxide, and nitrogen adsorption on anthracite from China over a wide range of pressures and temperatures: experiments and modeling

The adsorption isotherms and kinetics characteristics were investigated at 294 K, 311 K, 333 K, and 353 K with pressures up to 18 MPa for CH4 and N2 and 5.5 MPa for CO2 on anthracite from China using a volumetric method. The excess adsorption for N2 belongs to the type I isotherm, while the adsorption capacity for CH4 and CO2 initially increased, followed by a sequence decrease with increasing pressure. The preferential ratio of the maximum in the excess adsorption for N2:CH4:CO2 are 1:1.1:1.6, 1:1.3:1.8, 1:1.4:1.9, and 1:1.2:1.6 at 294 K, 311 K, 333 K, and 353 K, respectively. In addition, the excess adsorption capacity was predicted using the Langmuir + k and simplified Ono–Kondo lattice models. The Langmuir + k monolayer model has higher accuracy in modeling pure gas adsorption on coal, especially for CH4 and N2. Moreover, the pressure decay method was used to analyse the adsorption kinetics of gases on coal. It is observed from the kinetics data that the adsorption rate and the effective diffusivity increase with the increasing pressure for CH4 at 0.55–6.97 MPa and N2 at 0.63–10.05 MPa. However, for CO2, an increase in pressure reduces the adsorption rate and the effective diffusivity at 0.11–3.96 MPa due to intensive gas molecule–molecule collisions and the strong coal matrix adsorption swelling. The adsorption rate and the effective diffusivity with temperature are similar for the three gases, which increase with increasing temperature. Through verification of the experimental data, the diffusion model can be used to model the kinetics data of gases on coal under low and medium pressure conditions.

CO2/water two-phase flow in a two-dimensional micromodel of heterogeneous pores and throats

Gas–liquid two-phase flow in porous media is highly relevant to numerous geological engineering processes. Pore network micromodeling is able to provide an effective way to experimentally observe the gas–liquid displacement phenomena. However, micromodel experiments were rarely conducted in heterogeneous conditions, which may significantly affect the displacement process. In this study, CO2/water displacement experiments were conducted at 25 °C and ambient pressure conditions in an etched glass micromodel with heterogeneous pores and throats. The experiments were performed in both vertical and horizontal directions with the CO2 injection rates ranging from 0.2 ml h−1 to 6.0 ml h−1. Dynamic displacements were detected in real time by a digital single lens reflex camera. Based on the experimental results, a detailed discussion about the instability of CO2 front and CO2 saturation variation was conducted. It is found that the displacements become more and more unstable with an advancing CO2 front. Small fingerings can be collapsed by capillary pressure. Micro-scale heterogeneity significantly influenced the flow pattern at both the microscale and macroscale. Moreover, we created a new evaluation parameter Seval to characterise CO2 saturation variations and the transformation of Seval agrees well with our experimental results of CO2 saturation.

In situ measurement of the dispersion coefficient of liquid/supercritical CO2–CH4 in a sandpack using CT

Dispersion exists in many scientific and engineering applications, especially for CO2 enhanced gas recovery, which is a vital factor for controlling the contamination of remaining natural gas and gas recovery. In this paper, an in situ method for the dispersion coefficient measurement of liquid/supercritical CO2–CH4 in a sandpack using CT was proposed. The dispersion coefficient in the sandpack was obtained directly from the CO2 mole fraction profiles translated from a CT greyvalue image, which eliminate the deviation caused by the entry/exit effect. The finite difference method, Crank–Nicolson method, was applied to solve the advection dispersion equation for obtaining the dispersion coefficient. The breakthrough profile of the effluent gas was also analyzed and the apparent dispersion coefficient containing the entry/exit effect was measured using the dynamic column breakthrough method. The entry/exit effect enlarged the dispersion coefficient in the range of 14–23% under a water-free experiment according to the deviation between the two methods. And the dispersion coefficient with the sandpack containing residual water was smaller than that of the water-free condition, which was probably caused by the dissolution of CO2 in the displacing frontier into residual water. The dissolution stabilized the dispersion in the displacing frontier and resulted in the reduction of the dispersion coefficient.

Competitive adsorption/desorption of CO2/CH4 mixtures on anthracite from China over a wide range of pressures and temperatures

The adsorption/desorption of CO2/CH4 mixtures with three different volume fractions was investigated at 294 K, 311 K, 333 K, and 353 K with pressures of up to 70 bar on anthracite from China using a high-pressure volumetric analyzer (HPVA II-200). For the mixtures, the total excess adsorbed amount decreased as the temperature rose. In addition, it displayed an upward tendency with an increase in the CO2 fraction in the feed gas. The excess adsorbed amounts of the component gases were calculated on the basis of the composition of the gas phase measured by gas chromatography. For a mixture with a CO2 fraction of 50%, relatively good adsorptivity for CO2 was displayed at low pressures (<30 bar), whereas better adsorptivity for CH4 was displayed at high pressures. When the CO2 fraction in the feed gas increased from 20% to 50%, the excess adsorbed amount of CO2 increased dramatically, whereas the excess adsorbed amount of CH4 decreased slightly (6.3%). When the CO2 fraction increased from 50% to 80%, the excess adsorbed amount of CO2 increased substantially, whereas the excess adsorbed amount of CH4 decreased drastically (42.4%). On the basis of the experimental data, the total excess adsorbed amount can be well simulated by the Ono–Kondo (OK) lattice thermodynamic model with an average deviation of 6.4%. Moreover, the excess adsorbed amounts of the individual components have also been predicted using the OK model.

Surface decoration and dispersibility of carbon nanofibers in aqueous surfactant solution

As a novel functional nanomaterial, the dispersion effect of carbon nanofibers (CNFs) has a significant influence on the application of CNFs in the composites. Two effective surfactants, methylcellulose (MC) and polycarboxylate superplasticizer, were used to analyze the dispersion of CNFs in aqueous solution. A method utilizing ultrasonic processing was employed to achieve a homogenous CNF suspension, and the dispersion effect was further characterized by the method of measuring ultraviolet absorbency (UV absorbency), zeta potential, surface tension and transmission electron microscopy (TEM) micrographs. The results show that the zeta potential and surface tension reach the saturation plateau at MC concentration and polycarboxylate superplasticizer concentration of about 0.4 and 0.8 g/L, respectively, which reflects that the optimum concentration ratio of MC to CNFs is 2: 1, and the optimum dispersing polycarboxylate superplasticizer to CNFs ratio of 4: 1 is required to achieve dispersions with maximum achievable dispersion of CNFs.

Pore‐Scale Imaging and Analysis of Phase Topologies and Displacement Mechanisms for CO2‐Brine Two‐Phase Flow in Unconsolidated Sand Packs

CO2 storage in saline aquifers is considered a potential solution for CO2 mitigation, owing to its significant capacity and worldwide distribution capability. It is therefore becoming more important to understand the underground CO2/brine flow mechanisms. CO2 migration is primarily controlled by the pore-scale subsurface flows in different saline aquifer sites with variable reservoir formation compositions and reservoir conditions. Variations occur in the state of CO2 phase (gas versus supercritical), brine salinity, and rock wettability, under different reservoir conditions, and may result in different subsurface CO2/brine migration phenomena. In this study, we investigate the drainage and imbibition procedures of CO2 and brine by injecting fluids into unconsolidated sand packs under different conditions of CO2 phase states, brine salinity, and wettability of sand packs. The pore-scale fluid distribution is visualized using micro X-ray computed tomography (micro-CT). It is found that the phase states of CO2, brine salinity, and wettability have low impacts on CO2 distribution during drainage. However, the increase in brine salinity significantly damages the connectedness of the water phase in pore structures and further decreases the CO2-brine interfacial areas. In addition, a pore-scale event called the droplet fragmentation of nonwetting phase is found to occur in the imbibition procedure, which is considered to be beneficial to the dissolution trapping in CO2 geological storages. It is experimentally demonstrated that the pore structure of rock cores is a factor that significantly contributes to droplet fragmentation.