Pore Space Utilization Research Articles

Graph theory based estimation of probable CO2 plume spreading in siliciclastic reservoirs with lithological heterogeneity

Estimating plume spreading in geological CO2 storage reservoirs is critical for several reasons including the assessment of pore space utilization efficiency, preferential CO2 migration pathways and trapping. However, plume spreading critically depends on lithological heterogeneity of the reservoir and CO2 injection rate. It might require numerous high fidelity full physics numerical simulations to constrain the uncertainty in plume spreading for a given reservoir. This might not always be practical due to computational limitations. Hence, reduced physics approaches, such as invasion-percolation method and machine learning, could be useful to answer certain questions on plume spreading in the subsurface. This study presents a new reduced physics approach based on graph theory for estimating probable CO2 plume migration under very low and very high injection rates. The two end-member scenarios are assessed by performing random walk in the 3D reservoir space to constrain 20,000 possible paths of CO2 flow away from the injection well. The resistance to CO2 flow associated with each path is computed for viscous, capillary and gravity forces. The resistances are then transformed into the likelihood of CO2 migration along the path. The algorithm was applied to 45 reservoir models with varying degrees of lithological heterogeneity and the results were compared to those from full physics and invasion percolation simulations. The graph theory results showed a close match with the results from full physics approach for both flow regimes and with results from invasion-percolation approach for capillary-gravity dominated flow regime. The algorithm was further applied to answer key questions on reservoir screening such as pore space utilization potential. The graph theory approach was also integrated with machine learning to predict CO2 saturation. Testing suggested that the graph theory approach can be as much as 50 and 20 times faster than the full physics numerical simulations and invasion-percolation simulations, respectively.

Microscopic experiment on efficient construction of underground gas storages converted from water-invaded gas reservoirs

Based on the microfluidic technology, a microscopic visualization model was used to simulate the gas injection process in the initial construction stage and the bottom water invasion/gas injection process in the cyclical injection-production stage of the underground gas storage (UGS) rebuilt from water-invaded gas reservoirs. Through analysis of the gas-liquid contact stabilization mechanism, flow and occurrence, the optimal control method for lifecycle efficient operation of UGS was explored. The results show that in the initial construction stage of UGS, the action of gravity should be fully utilized by regulating the gas injection rate, so as to ensure the macroscopically stable migration of the gas-liquid contact, and greatly improve the gas sweeping capacity, providing a large pore space for gas storage in the subsequent cyclical injection-production stage. In the cyclical injection-production stage of UGS, a constant gas storage and production rate leads to a low pore space utilization. Gradually increasing the gas storage and production rate, that is, transitioning from small volume to large volume, can continuously break the hydraulic equilibrium of the remaining fluid in the porous media, which then expands the pore space and flow channels. This is conducive to the expansion of UGS capacity and efficiency for purpose of peak shaving and supply guarantee.

The role of injection method on residual trapping at the pore-scale in continuum-scale samples

The injection of CO2 into underground reservoirs provides a long term solution for anthropogenic emissions. A variable injection method (such as ramping the flow rate up or down) provides flexibility to injection sites, and could increase trapping at the pore-scale. However, the impact of a variable injection method on the connectivity of the gas, and subsequent trapping has not been explored at the pore-scale. Here, we conduct pore-scale imaging in a continuum-scale sample to observe the role of a variable flow rate on residual trapping. We show that the injection method influences how much of the pore space is accessible to the gas, even when total volumes injected, and total flow rates remain constant. Starting at a low flow rate led to a lower gas saturation at breakthrough. Once a pathway was established across the sample, increasing the flow rate did not improve gas saturation significantly, as the increase in flux was accommodated by the connected pathway across the sample. Starting at a high flow rate led to a higher pore space utilization, which is optimal for CO2 storage. Overall the high to low injection scenario led to more residual trapping.

Feasibility assessment of solution mining and gas storage in salt caverns: a case study of the Sanshui salt mine

The Sanshui salt mine is the sole location in the Guangdong province of South China with the potential to construct a salt cavern gas storage (SCGS) facility. Nevertheless, the gas storage construction of this mine faces significant challenges due to the presence of low‒grade salt deposits and numerous interlayers. To demonstrate the feasibility and calculate the gas storage capacity in this specific mining area, two representative salt caverns within this salt mine were simulated using a self-developed cavern-building simulation program, enabling us to accurately determine their respective volumes and shapes. Herein, the findings indicate that the combined caverns possess a total mining space volume of 1,157,000 m3, with the brine space accounting for merely 291,800 m3 (representing 25.22% of the overall mining space), and an extensive sedimentary volume of 865,200 m3 is also observed (constituting approximately 74.78% of the total mining capacity). Fortunately, this study has revealed that the sediments exhibit a porosity exceeding 40% and possess favorable permeability; consequently, countermeasures have been proposed to enhance the gas storage capacity within the pore space of these caverns, and we also utilized FLAC3D software for numerical simulation calculations to compare the stability of the cavern under different conditions of sediment pore utilization by calculating the volume loss rate, cavern wall displacement deformation, and plastic zone distribution. Moreover, the proposed method is anticipated to double the caverns’ working gas volume, increasing it from 40 million m3 to nearly 80 million m3. On the other hand, the long-term stability of caverns is numerically assessed under different pore space utilization rates of the sediments. The results also indicate that the caverns’ volume shrinkage, plastic zones, and surrounding rock displacement remain within allowable limits during 30 years of gas storage operation. The primary problem in the subsequent phase lies in effectively achieving gas injection and brine removal from the pore space of sediments while devising a methodology to extend this method to other salt caverns within similar salt mine areas. Thus, this study provides theoretical and technical guidance for the establishment of gas storage in existing salt caverns in the Sanshui salt mine and in salt mines worldwide that share similar geological conditions.

The feasibility of enhanced pore space utilization in CO2 storage reservoirs using an artificially emplaced Si-gel flow barrier

Carbon capture and storage is a key technology to abate CO2 emissions. One of the challenges towards ensuring the efficiency and the security of CO2 storage in reservoirs, such as open saline aquifers, is the low pore space utilization. This study investigates the feasibility of using an artificial Si-gel barrier to enhance pore space utilisation in such reservoirs under variable geological conditions. Conceptually, enhanced CO2 capillary trapping is achieved by emplacing a disk-shaped, low-permeability barrier above the CO2 injection point forcing the injected CO2 to migrate laterally underneath the barrier before transitioning to buoyancy-controlled migration. Multiphase fluid flow simulations were conducted to test the feasibility of this concept. Sensitivity analysis revealed that the barrier exhibits a strong control on CO2 plume geometry. Specifically, the relative impact of the barrier diameter on increasing the CO2 plume width, reducing the plume height and enhancing trapping varied between 67 and 86%. Capillary trapping was enhanced by 40–60% with a 20 m increase in barrier diameter in low permeability reservoirs. Additionally, the results indicate that the barrier can enhance the security of trapping CO2 in high permeability reservoirs. Results were tested for the South-West Hub reservoir, a case study area in Western Australia.

Study of the impact of various porous media on pore space utilization and CO2 storage by injection of microbubbles into oil reservoirs

CO2 capture and storage technology have the potential to help reach net zero emissions. CO2-enhanced oil recovery (CO2 EOR) is a favourable method. Limited pilot testing has provided preliminary evidence that the unique physical properties of gas microbubbles (MBs) can be used to improve pore utilization efficiency. In this study, supercritical CO2 injection at 0.05 mL/min into sand samples with different grain sizes under reservoir conditions was investigated by nuclear magnetic resonance (NMR). The spatial distribution of the CO2 and oil and the dynamic pore occupancy mechanism were determined for the first time, and the results from normal bubbles (NBs) and MBs were compared. The results show a more significant left shift of the transverse relaxation time in the case of MB injection, which means that CO2 can easily occupy a wide range of large pore spaces. Dynamic changes in the oil volume in the four types of pores classified by the transverse relaxation time indicate that the oil in large pores is flushed earlier when MB is injected and that the CO2 saturation in minimal pores (T2 relaxation time range from 5 ms to 100 ms) and small pores (T2 relaxation time range from 100 ms to 500 ms) increases. The cumulative oil volume fraction decreases by 9.0% when injecting MBs. From the perspective of carbon storage, the gas storage potential is significantly enhanced within low-permeability sand when injected with MBs. The maximum volume of free CO2 was enhanced by 16.9% in this case. In addition, the storage efficiency is enhanced up to 5.1% for low-permeability sand and 1.6% for medium-permeability sand. However, bubble flow is not as advantageous in high-permeability sand. Considering dissolution trapping, the storage efficiency reaches 26.3%.

Migration characteristics of supercritical CO2 microbubble flow in the Berea sandstone revealed by voxel-based X-ray computed tomography imaging analysis

In this study, two supercritical CO2 (scCO2) injection experiments (microbubble flow injection (MBI) and conventional flow injection (CI)) were performed using the same Berea sandstone sample to investigate the unique transport characteristics of microbubble flow. The four-dimensional characteristics of spatial CO2 saturation (SCO2) and the relationship to the spatial porosity were investigated by utilizing the X-ray computed tomography (CT) imaging. The results show that the microbubble flow has a stronger sweep efficiency than that of the conventional flow, ultimately resulting in higher SCO2. According to the statistics of saturation distribution, MBI can improve the pore space utilization, particularly in the medium-sized porosity space. Several CO2 preferential flows, migrated along with the high-permeability layers, were identified. Comparing to CI, the accumulation of CO2 in pore space was enhanced in many local macro-regions during MBI, which produced a discrete distribution manner. These flows not only improve the displacement efficiency of CO2 in occupied pore space but also further enhance the subsequent vertical CO2 infiltration from the high-to low-permeability layers. The final experimental result shows that the dual advantages of CO2 microbubble flow in sweeping and displacement can effectively improve CO2 storage capacity.

Cathode Local Curvature Affects Lithium Peroxide Growth in Li-O2 Batteries.

The growth of lithium peroxide (Li2O2) in cathodes determines the performance of lithium-oxygen batteries (LOBs). The factor affecting the Li2O2 growth position is of great importance. Here, three hollow carbon spheres with diameters of 200 nm, 500 nm, and 2 μm, corresponding to different curvatures of 10, 4, and 1, respectively, are prepared as LOB cathodes. It is found that the larger the curvature, the more difficult it is for Li2O2 to grow inside the hollow sphere. Increasing the discharge current density can promote the growth of Li2O2 onto a highly curved concave substrate. Therefore, to maximize the battery performance, the applied current density and the local curvature of the porous cathode need to match to optimize the pore space utilization and meanwhile to enhance the interface charge transfer between Li2O2 and electrode. The revealed relationship among the local curvature of the porous electrode, Li2O2 deposition position, and battery performance is valuable to the topography design of the LOB cathode.

Enhancement of CO2 dissolution and sweep efficiency in saline aquifer by micro bubble CO2 injection

Due to the favorable physical properties of micro bubbles (MBs), MB CO2 injection into saline aquifers was proposed as a novel, economic, and leak-free underground injection method in the purpose of sequestration. In this study, supercritical CO2 injection into a Berea sandstone sample at 0.05 mL/min under reservoir conditions was investigated using X-ray CT. Experiments with micro bubble (MB) and normal bubble (NB) CO2 injection were conducted. The dependence of CO2 migration on pore space shows macroscopic bypassing via preferential flow paths, as the distribution and saturation of CO2 were largely influenced by the heterogeneous pore structure. Most CO2 migrated forwards via three flow paths that formed along the bedding layers with high porosity. The CO2 distribution was discontinuous, and isolated CO2 clusters appeared due to the heterogeneity of the pore structure and tortuosity of the flow paths. MB CO2 migrated into the areas adjacent to major flow paths, forming irregular channels, while NB CO2 migrated through smooth channels. MB CO2 breakthrough occurred at 0.18 pore volume (PV), which was larger than NB CO2. A long breakthrough time reflects high pore space utilization. A significant differences of MB and NB CO2 in the existing duration of flow paths in the whole displacement period was observed, which reflected higher stability of MB CO2 in the sample. Additionally, little MB CO2 was observed near the inlet, suggesting that MBs accelerated dissolution in the early stage of injection (0.02 PV). Smaller saturation and larger volume differences were observed for MB CO2 compared to NB CO2 in the early stage because of dissolution. The enhanced dissolution efficiency of MB CO2 reached 18.7% at 0.02 PV. Moreover, pore space utilization improved because MB CO2 permeated into regions of low porosity and low flow ability. The improvement in the utilization efficiency reached 4.9% in low-porosity regions. And the storage efficiency was increased up to 23.6%.The results of this study suggest that MB CO2 injection has higher dissolution and improved pore space utilization, making it an efficient method of CO2 sequestration in low-porosity regions.

The role of flow rates on flow patterns and saturation in high-permeability porous media

This study aimed to investigate dynamic CO2 drainage using high-resolution magnetic resonance imaging (MRI) technology. Gaseous and supercritical CO2 were injected downward in brine-saturated porous media at different flow rates at 40 °C/6 MPa and 40 °C/8 MPa. These flow rates (0.015, 0.03 and 0.1 mL/min, under 10 Mt/year), as reflected in, were chosen according to the distance (1 km–100 m) from the injection well. Three stages were found from the change of signal intensity during CO2 drainage: before the CO2 front reached the field of view (FOV), breakthrough, and steady state. Channelling or drainage fronts immediately established through the large pores, and CO2 travelled vertically through these channels until breakthrough. The breakthrough time decreased with increasing flow rates and was longer for ScCO2 than gCO2 at the same flow rate, resulting in a longer residence time for ScCO2 in the sample. At low flow rates, the fingers first established along the larger pore spaces (especially at 0.015 mL/min) and then gradually extended into adjacent regions, resulting in a relatively flat interface. However, at high flow rates, the front moved along the larger pore spaces until breakthrough. The flow patterns for ScCO2 drainage were more uniform than those for gCO2 drainage. The pore volume fraction occupied by CO2, as a quantitative parameter of the flow pattern, reflected that the sweep efficiency and pore space utilization were optimized at 0.03 mL/min (Ca = 4.35 × 10−9 for gCO2 and Ca = 1.06 × 10-8 for ScCO2). The effect of the flow rate on the CO2 saturation and distribution was analysed. At low flow rates, the saturation gradient along the porous media gradually reduced, but trend to be stable at high flow rates. Additionally, the saturation at breakthrough and steady state were observed to be linearly related to the maximum rate of change in saturation during CO2 injection. Overly fast drainage results in relatively low saturation and an inhomogeneous distribution. The results can be applied to provide information for enhancing pore space utilization and improving sweep efficiencies during field storage.

Oxygen- and nitrogen-co-doped activated carbon from waste particleboard for potential application in high-performance capacitance

Oxygen- and nitrogen-co-doped activated carbons were obtained from phosphoric acid treated nitrogen-doped activated carbons which were prepared from waste particleboard bonded with urea-formaldehyde resin adhesives. The activated carbon samples obtained were tested as supercapacitors in two-electrode cell and extensive wetting 7M KOH electrolytes. Their structural properties and surface chemistry, before the electrical testing, were investigated using elemental analysis, X-ray photoelectron spectroscopy, scanning electron microscopy, X-ray diffraction, Raman spectra, and adsorption of nitrogen. Activated carbon treated by 4M phosphoric acid of the highest capacitance (235F/g) was measured in spite of a relatively lower surface (1360m2/g) than that of the activated carbon treated by 2M phosphoric acid (1433m2/g). The surface chemistry, and especially oxygen- and nitrogen-containing functional groups, was found of paramount importance for the capacitive behavior and for the effective pore space utilization by the electrolyte ions.

Effect of visible light and electrode wetting on the capacitive performance of S- and N-doped nanoporous carbons: Importance of surface chemistry

Nanoporous carbons with graphitic domains were synthesized from a polymer containing sulfur and nitrogen. The materials were characterized using adsorption of nitrogen, potentiometric titration TA/MS, XPS, TEM and XRD. Then they were tested as supercapacitors in three-electrode cell and under visible light irradiation after extensive wetting either in water or a sulfuric acid electrolyte. The capacitance up to 450 F/g was measured in spite of a relatively low surface (<850 m 2 /g). The surface chemistry, and especially sulfur and nitrogen containing functional groups, were found of paramount importance for the capacitive behavior and for the effective pore space utilization by the electrolyte ions. Photocurrent measured in light also affects the capacitance. Its generation is linked to the excitation of sulfonic/sulfoxide chromophores-like moieties decorating the surface of the polymer-derived carbons.

Complexity of CO2 adsorption on nanoporous sulfur-doped carbons – Is surface chemistry an important factor?

Nanoporous S-doped carbon and its composites with graphite oxide were tested as adsorbents of CO2 (1MPa at 0°C) after degassing either at 120°C or 350°C. The adsorption capacities were over 4mmol/g at ambient pressure and 8mmol/g at 0.9MPa in spite of a low volume of micropores. The nitrogen adsorption experiments showed an increase in porosity upon an increase in the degassing temperature. The extent of this effect depends on the stability of surface groups. Interestingly, the CO2 adsorption, especially at low pressure, was not affected. The good performance is due to the presence of ultramicropores similar in sizes to CO2 molecule and to sulfur in various functionalities. Sulfur incorporated to aromatic rings enhances CO2 adsorption via acid–base interactions in micropores. Moreover, sulfonic acids, sulfoxides and sulfones attract CO2 via polar interactions. Hydrogen bonding of CO2 with acidic groups on the surface can also play an important role in the CO2 retention. These carbons have high potential for application as CO2 removal media owing to their high degree of pore space utilization. The results obtained also show that high degassing temperatures might result in the decomposition of surface groups and thus in changes in surface interactions.

Carbon Dioxide-in-Water Foams Stabilized with a Mixture of Nanoparticles and Surfactant for CO2 Storage and Utilization Applications

Synergistic interactions between appropriately designed surface-modified nanoparticles and surfactants are shown to stabilize foams of CO2 bubbles/droplets dispersed in water at elevated temperature and pressure typical of subsurface formations for enhanced oil recovery or geologic storage of CO2. The foams are sufficiently viscous to mitigate or eliminate the instability associated with CO2 displacement of fluids resident in the oil reservoir or brine aquifer. This technology therefore has the potential to increase the efficiency of oil recovery and the efficiency of pore space utilization for storage.

Active pore space utilization in nanoporous carbon-based supercapacitors: Effects of conductivity and pore accessibility

Composites of commercial graphene and nanoporous sodium-salt-polymer-derived carbons were prepared with 5 or 20 weight% graphene. The materials were characterized using the adsorption of nitrogen, SEM/EDX, thermal analysis, Raman spectroscopy and potentiometric titration. The samples' conductivity was also measured. The performance of the carbon composites in energy storage was linked to their porosity and electronic conductivity. The small pores (<0.7) were found as very active for double layer capacitance. It was demonstrated that when double layer capacitance is a predominant mechanism of charge storage, the degree of the pore space utilization for that storage can be increased by increasing the conductivity of the carbons. That active pore space utilization is defined as gravimetric capacitance per unit pore volume in pores smaller than 0.7 nm. Its magnitude is affected by conductivity of the carbon materials. The functional groups, besides pseudocapacitive contribution, increased the wettability and thus the degree of the pore space utilization. Graphene phase, owing to its conductivity, also took part in an insitu increase of the small pore accessibility and thus the capacitance of the composites via enhancing an electron transfer to small pores and thus imposing the reduction of groups blocking the pores for electrolyte ions.

Displacement and mass transfer between saturated and unsaturated CO2–brine systems in sandstone

AbstractThe process of displacement and mass transfer between CO2 and brine, which are relevant for the prediction of plume migration and pore-space utilization during CO2 injection in saline aquifers, were studied by conducting unsteady-state core flood experiments in nearly homogeneous Berea sandstone rock. Mutually saturated and unsaturated CO2 and brine phases were injected in the rock under realistic sequestration conditions.Relative permeability and capillary pressure curves were extracted by history matching the unsteady state experiments conducted with mutually saturated CO2 and brine. As a reference and for comparison, decane–brine primary drainage was conducted on the same sample. The CO2–brine relative permeability was found to be different from the decane–brine relative permeability (which had been validated against steady-state experiments on twin-samples), reflecting the change in the wetting state from water-wet decane–brine/Berea to the rather intermediate-wet behavior of CO2–brine/Berea, which is in agreement with literature data on contact-angles for the two cases. However, the CO2 brine data are somewhat different from data on the same rock type as reported by Perrin and Benson (2010) which is likely a consequence of sample heterogeneity.Aspects of the mass transfer between the CO2 and the brine phase were studied by drainage and imbibition with unsaturated phases. When comparing saturated and unsaturated CO2–brine primary drainage, the mass transfer due to mutual solubility leads to two effects: (1) evaporation near the inlet due to water dissolving in CO2 and (2) a diminished displacement of brine by CO2 due to CO2 dissolving in brine. In addition, an imbibition experiment was conducted where unsaturated brine was injected into rock filled with mutually saturated CO2 and brine phase at near-residual CO2 saturation. After the CO2-saturated brine had been miscibly displaced by unsaturated brine, dissolution of the trapped CO2 in the injected brine was subsequently observed. These experiments represent the transition from residual trapping to solubility trapping and indicate the time and length scales involved.