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Asphaltenes, precipitated from oil due to pressure, temperature, or oil compositional changes, can affect not only the performance of CO2 enhanced oil recovery (EOR), but also carbon capture and storage (CCS) by reducing porosity and permeability and altering wettability. A comprehensive solid deposition model, which includes adsorption, pore throat plugging, and re-entrainment, was developed to predict the amount of precipitated asphaltene under dynamic conditions. In this study, a hysteretic three-phase fluid flow model was used to simulate residual and solubility trapping during the water alternating gas (WAG) process. Alteration of rock wettability during asphaltene deposition led to a decrease in oil and gas mobility and an increase in water mobility, increasing the amount of CO2 residual trapping to 39%, via a hysteresis effect. Residual trapped CO2 replaced liquids in the pore space; more water was substituted when asphaltene deposition was considered than that without it because of greater water mobility. Consequentially solubility and residual trapping were overestimated by 20% and underestimated by 39% when asphaltene deposition was not considered. Formation damage by asphaltene deposition increased water production by 14% and decreased oil recovery by 9% compared to a model without asphaltene deposition. These results indicate that asphaltene deposition affects CO2 trapping mechanisms and EOR performance during coupled CO2 EOR and storage processes.

Dimethyl ether (DME) is a widely used industrial compound, and Shell developed a chemical EOR technique called DME-enhanced waterflood (DEW). DME is applied as a miscible solvent for EOR application to enhance the performance of conventional waterflood. When DME is injected into the reservoir and contacts the oil, the first-contact miscibility process occurs, which leads to oil swelling and viscosity reduction. The reduction in oil density and viscosity improves oil mobility and reduces residual oil saturation, enhancing oil production. A numerical study based on compositional simulation has been developed to describe the phase behavior in the DEW model. An accurate compositional model is imperative because DME has a unique advantage of solubility in both oil and water. For DEW, oil recovery increased by 34% and 12% compared to conventional waterflood and CO2 flood, respectively. Compositional modeling and simulation of the DEW process indicated the unique solubility effect of DME on EOR performance.

Although research on CO2 injection in shale gas reservoirs has been focused for enhanced gas recovery (EGR) and CO2 storage, previous studies have not examined both multi-component transport and geomechanical effects. Therefore, this study presents new shale gas models for CO2 injection considering multi-component adsorption, dissolution, molecular diffusion, and stress-dependent compaction. Based on these mechanisms and data for Barnett shale field, a simulation model was constructed for CO2 flooding and huff and puff. The proposed model was used to examine the effects of CO2 injection to EGR and CO2 storage and various mechanisms. The results presented that CO2 flooding and huff and puff improve CH4 production by 24% and 6% respectively compared with no injection scenario. At the end of simulated time, the injected CO2 is stored as free, adsorbed, and dissolved states in proportions of 42%, 55%, and 3% respectively. To confirm these results, Marcellus and New Albany shale models, which have different reservoir properties, are generated and compared with Barnett shale model. However, in Marcellus and New Albany shale models, effects of CO2 injection are lower than that of Barnett shale model. Therefore, to investigate factors affecting to the efficiency of CO2 injection in shale gas reservoirs, extensive simulations were performed. Results of the simulation analyses show that natural fracture permeability, hydraulic fracture half-length, well spacing, and Langmuir constants are significant factors for EGR and CO2 storage. For the real applications, these parameters should be mainly considered. The investigations performed in this study present better understanding of CO2 injection processes to EGR and CO2 storage and they are important for optimizing the designs of CO2 injection in field applications.

Shale gas reservoirs have ultra-low matrix permeability and consist of matrix and natural fracture systems. Horizontal wells with hydraulic fractures are necessary to economically exploit shale gas reservoirs. Due to these complex features of shale gas reservoirs, understanding pressure transient characteristics is of importance. Through coupling of flow and geomechanical models for well/reservoir/hydraulic fracture systems, a new approach has been established to predict the pressure transient behavior and to generate type curves. To account for geomechanical effects with stress dependent permeability, linear elastic geomechanical models and two stress dependent permeability correlations, exponential and power law correlations, from previous experimental researches were implemented in the reservoir model. A number of cases considering a geomechanical model, exponential correlation, and power law correlation were compared. These processes showed roughly a 1–5% variation of productivity in shale gas reservoirs. Based on the extensive numerical simulations with the updated model, a series of type curves were developed in terms of dimensionless pseudopressure and derivative of dimensionless pseudopressure versus dimensionless time considering various reservoir and fracture properties. Using the type curves as a guide, we then presented flow regimes that are expected for different values of geomechanical properties, number of fractures, length of the fractures, spacing between fractures, and properties of stimulated reservoir volume (SRV). Using the characteristics of new dimensionless parameters and the developed type-curve set, a simple and practical procedure was presented to estimate reservoir properties in multi-fractured horizontal wells. Application of type curve matching to synthetic data was shown to be useful in estimating effective fracture and matrix permeability and porosity. A history match of a field example in Barnett Shale data indicates that the model with stress-dependent properties matches the actual production data better than the model with constant properties. This study provides insight into the characteristics of a hydraulically-fractured horizontal well, and the newly developed type curves yield more unique and accurate results.

Once asphaltenes have been precipitated from oil due to pressure, temperature, or oil composition changes, they can affect reservoir performance by reducing porosity and permeability and altering wettability. Moreover, they can cause plugging of wellbore and surface production facilities. The addition of liquefied petroleum gas (LPG) into the CO 2 stream during CO 2 flooding reduces oil viscosity, interfacial tension (IFT), and oil swelling, resulting in improved displacement efficiency, but also causes asphaltene precipitation because of a change in oil composition. When toluene is introduced into the LPG and CO 2 stream to overcome the limitations of a CO 2 -LPG flood, it acts as an inhibitor of asphaltene precipitation. This research systematically examines the mechanism of asphaltene precipitation and how to mitigate problems associated with asphaltene deposition based on compositional simulations. Solid deposition model that included adsorption, pore throat plugging, and re-entrainment was considered. A compositional equation of state (EOS) model was developed to analyze the effects of LPG and toluene on asphaltene deposition and formation damage. At the center of reservoir, the amount of asphaltene deposition reached the maximum for 10% LPG-added CO 2 -LPG flooding, which was 12% higher than that from CO 2 flooding. When 1% toluene was added into the CO 2 -LPG flood, the amount of asphaltene deposited declined by 2%. Oil resistance factor had the same tendency because of porosity reduction due to decreased deposition of asphaltene. Oil recovery for the CO 2 -LPG flood was enhanced by 4% compared to the CO 2 flood, despite an increment in asphaltene deposition, and by 9% for the CO 2 -LPG-toluene flood. Results from this study indicate that LPG addition causes increased asphaltene deposition, which can be mitigated by using toluene as an inhibitor.

We report on the current transport mechanism in Au/GaN nanowire–Schottky diodes at high temperature using current–voltage characterization in the temperature range of 300–573 K. We observed that the ideality factor for the GaN nano-Schottky diodes decreased from 11.2 at 323 K and reached its lowest value of 7.9 at 500 K. Then, it started to increase with increasing temperatures up to 573 K. On the other hand, the Schottky barrier height increased throughout all of the temperature ranges up to 573 K. We found that the abnormal behaviors of the barrier heights and the ideality factors in the current transport by varying the temperature of 300–500 K can be well explained by a combination of the thermionic field emission (TFE) mechanism and a model with a Gaussian distribution of the barrier heights that were predominant at the interface between the Schottky metal (Au) and the semiconductor (GaN nanowire). We also suggested that the increase of the ideality factor at a temperature higher than 500 K could be attributed to a trap-assisted tunneling current in a TFE mechanism.

To maximize dye absorption and improve energy conversion efficiency, we adopted a branched structure for single crystalline ZnO nanowires in dye-sensitized solar cells (DSSCs). The high-density branched structures of these ZnO nanowires were incorporated by a two-step seeding with zinc acetate dihydrate seeds. The ZnO nanowire DSSCs exhibited an overall efficiency of ∼0.46%. This represented a 35% improvement compared to the ZnO nanowire DSSCs with pillar-shaped ZnO nanowires. Our current density–voltage (J–V) characterizations suggest that the current densities and power conversion efficiencies were improved by increasing the surface area with these branched structures of ZnO nanowires in DSSCs.

This is a report on the electrical characterization of gallium nitride (GaN) nanowire (NW) p–n junction diodes. These diodes were formed by assembling n-GaN NWs on p-Si (1 0 0) substrates using alternating current (AC) dielectrophoresis (DEP). The AC DEP was optimized with a bias voltage of 15 V p−p at a frequency of 1 kHz. The hetero-junction single GaN nanowire p-n diode (n-GaN NW/p-Si) showed well-defined current rectifying behavior with a forward voltage drop of 1.2–2.0 V at a current density of 10–60 A/cm 2 . The GaN nanowire p–n diodes had a high parasite resistance in the range of >470 kΩ. We observed that these high resistances were mostly the result of the metal contacts to the n-GaN NWs. We also found that these parasite resistances were reduced by the formation of an additional capping layer on the top of the n-GaN NW as well as high temperature annealing.

We quantitatively characterized the ac dielectrophoresis force on semiconducting gallium nitride (GaN) nanowires in suspension with variations of the ac electric field and frequency. The yield of aligned GaN nanowires increased with increasing ac electric field (up to 20 Vp–p). The yield results indicate that the GaN nanowires were well aligned with a high yield of ∼80% over the entire array in a chip at the frequencies of 10 kHz and 20 MHz. In addition, the electrical properties of GaN nanowires prepared by dielectrophoresis were investigated using conventional three-probe schemes in field-effect transistor structures.

Electric field assisted assembly of single walled carbon nanotubes (SWCNTs) on lithographically patterned electrodes has been studied using both dc and ac electric fields. The nanotube alignment is strongly dependent on the magnitude and the frequency of the applied electric field. An improved carbon nanotube (CNT) orientation is achieved with ac electric field of high frequency due to alternating force exerted rapidly on field-induced dipoles of the nanotubes and the nanotubes orient nearly at right angle to the metal electrodes. The purification of as-prepared CNTs from background nanoparticles appears possible by aligning nanotubes with an ac electric field.

Single walled carbon nanotubes (SWCNTs) have been aligned across the metal electrodes using a dc electric field. Effects of electric field strength, nanotube concentration in the suspension, and the solvents used for CNT dispersion were examined on the aligning nature of nanotubes. An improved dispersion of CNTs has been found in dimethylformamide solution compared to ethanol. CNTs mostly moved towards anode for the applied electric field indicating the negative charge of the nanotubes. Experimental results exhibit the possibilities of precise positioning of nanotubes on pre-patterned electrodes by controlling the magnitude of electric field as well as the concentration of CNT suspension.