Drying of porous materials at pore scale using lattice Boltzmann and pore network models

Drying at macroscale shows a first drying period with constant drying rate followed by second drying period showing a receding moisture front, phenomena that can be tailored upon need. In order to study the drying of materials, we present a new hybrid computational method, where the dynamics of the liquid-vapor interfaces is modelled by lattice Boltzmann modelling (LBM) in the two-phase pores, while the single-phase flow in the pores filled solely by vapor or liquid is solved by pore network model (PNM). This hybrid method is validated by comparison with reference full LBM simulations. The hybrid method combines the advantages of both methods, i.e., accuracy and computational efficiency. LBM and the hybrid LBM-PNM method are used to study the drying of porous media at pore scale. We analyse two different pore structures and consider how capillary pumping effect can maximize the drying rate. Finally, we indicate how optimized drying rates are relevant when designing facade or pavement solutions that can mitigate higher surface temperatures in urban environments by evaporative cooling.

Pore-scale investigation of dynamic effects on the moisture retention curve during spontaneous imbibition

The moisture retention curve of porous materials is often assumed to be independent of the process dynamics, i.e., of the drying/wetting rate. Experimental outcomes and pore-scale simulations put this assumption into question though. It has been shown that dynamic effects can significantly affect the moisture retention curve, which presents different behaviours, depending on whether it is determined at transient or steady-state conditions. The cause of this phenomenon is addressed as “dynamic effects” in the literature. While dynamic effects of the drainage process have been widely studied, the data concerning spontaneous imbibition are still quite limited. We attempt at reducing this lack of knowledge by modelling spontaneous imbibition in an artificial material sample represented by a pore network model. In our model, the liquid flow is described via the Hagen-Poiseuille equation, while a percolation algorithm controls the dynamics of liquid-gas interfaces through the network junctions. A dynamic contact angle between liquid water and pore surface is considered, depending on the velocity of the meniscus. Dynamic states are determined by linking the local capillary pressure to the local moisture content in the artificial material sample subject to spontaneous imbibition. Our investigation demonstrates that dynamic effects due to contact angle variations may have a major impact on the imbibition process.

A New Method of Central Axis Extracting for Pore Network Modeling in Rock Engineering

Characterizing internal microscopic structures of porous media is of vital importance to simulate fluid and electric current flow. Compared to traditional rock mechanics and geophysical experiments, digital core and pore network modeling is attracting more interests as it can provide more details on rock microstructure with much less time needed. The axis extraction algorithm, which has been widely applied for pore network modeling, mainly consists of a reduction and burning algorithm. However, the commonly used methods in an axis extraction algorithm have the disadvantages of complex judgment conditions and relatively low operating efficiency, thus losing the practicality in application to large-scale pore structure simulation. In this paper, the updated algorithm proposed by Palágyi and Kuba was used to perform digital core and pore network modeling. Firstly, digital core was reconstructed by using the Markov Chain Monte Carlo (MCMC) method based on the binary images of a rock cutting plane taken from heavy oil reservoir sandstone. The digital core accuracy was verified by comparing porosity and autocorrelation function. Then, we extracted the central axis of the digital pore space and characterize structural parameters through geometric transformation technology and maximal sphere method. The obtained geometric parameters were further assigned to the corresponding nodes of pore and throat on the central axis of the constructed model. Moreover, the accuracy of the new developed pore network model was measured by comparing pore/throat parameters, curves of mercury injection, and oil-water relative permeability. The modeling results showed that the new developed method is generally effective for digital core and pore network simulation. Meanwhile, the more homogeneity of the rock, which means the stronger “representative” of binary map the rock cutting plane, the more accurate simulated results can be obtained.

Impact of de-ionized water on changes in porosity and permeability of shales mineralogy due to clay-swelling

Hydraulic fracturing is widely applied for economical gas production from shale reservoirs. Still, the swelling of the clay micro/nano pores due to retained fluid from hydraulic fracturing causes a gradual reduction of gas production. Four different gas-bearing shale samples with different mineralogical characteristics were investigated to study the expected shale swelling and reduction in gas permeability due to hydraulic fracturing. To simulate shale softening, these shale samples were immersed in deionized (DI) water heated to 100 °C temperature and subjected to 8 MPa pressure in a laboratory reactor for 72 hours to simulate shale softening. The low-temperature nitrogen adsorption and density measurements were performed on the original and treated shale to determine the changes in micro and nano pore structure. The micro and nano pore structures changed, and the porosity decreased after shale treatment. The porosity decreased by 4% for clayey shale, while for well-cemented shale the porosity only decreased by 0.52%. The findings showed that the initial mineralogical composition of shale plays a significant role in the change of micro and nano pores and the pore structure alteration due to retained fluid from hydraulic fracturing. A pore network model is used to simulate the permeability of shale used in this study. To define pore structure properties, specific factors such as porosity, pore size, pore throat distribution, and coordination number were used. Furthermore, the anisotropy characteristics of shale were integrated into the model via a coordination number ratio. Finally, the change in permeability due to shale softening was determined and compared with untreated with the progress of shale softening. The simulation showed that the permeability of Longmaxi shale could decrease from 3.82E–16 m2 to 4.71E–17 m2 after treatment.

Enhanced Learning of Fundamental Petrophysical Concepts Through Image Processing and 3D Printing

We designed a multidimensional visual learning project with the primary goal of helping undergraduate students better understand fundamental concepts in petrophysics through a set of exercises centered around an analysis of flooding experiment images. More specifically, we focused on concepts related to the trapped fluid within a rock’s pores in this project. To do this, eight different pore networks with unique internal structures were used and then 3D printed. The models were printed using a transparent resin to showcase the movement of fluids inside the rock model. The fluid’s displacement within the 3D-printed rock model was recorded using a high-definition camera, and still images were taken. Undergraduate petroleum engineering students were then assigned a set of exercises to guide them through an analysis of the pore network model images. Students conducted the analysis through an open-source image analysis software (Fiji) to help explore and better understand fundamental petrophysical properties: porosity, fluid saturation, wettability, grain-size distribution, and displacement efficiency. A survey was given to the students to gauge the effectiveness of the exercise in improving their understanding of these concepts. Survey results illustrated that the project-based learning exercises were effective in helping students to better understand difficult-to-grasp petrophysical concepts as they could be more easily visualized through the captured flooding experiment images and the accompanying analysis. An additional benefit to this unique visual learning experience is the ease at which it can be delivered remotely to adhere to safety measures as a result of the global COVID-19 pandemic.

Permeability Analysis of Hydrate-Bearing Porous Media Considering the Effect of Phase Transition and Mechanical Strain during the Shear Process

Summary The permeability characteristics of hydrate-bearing reservoirs are critical factors governing gas and water flow during gas hydrate exploitation. Herein, X-ray microcomputed tomography (CT) and the pore network model (PNM) are applied to study the dynamic gas and water relative permeabilities (krg and krw) of hydrate-bearing porous media during the shear process. As such, the dynamic region extraction method of hydrate-bearing porous media under continuous shear is adopted by considering deformation in the vertical direction. The results show that krw and krg of hydrate-bearing porous media are influenced by the effect of disordered sand particle movement under axial strain. Declines in the critical pore structure factors (pore space connectivity, pore size, and throat size) contribute to the reduction in krw and the increase in krg. However, krg decreases during the shear process at a high water saturation (Sw) because of the high threshold pressure and flow channel blockage. In addition, the connate water saturation (Swc) continuously increases during the shear process. Swc is influenced by pore size, throat size, and flow channel blockage. Moreover, the preferential flow direction of krg and krw changes during the continuous shear process. The results of dynamic permeability evolution during the continuous shear process under triaxial stress provide a reference for pore-scale gas and water flow regulation analysis.

Pore-throat structure characterization of carbon fiber reinforced resin matrix composites: Employing Micro-CT and Avizo technique

Micro-CT technique poses significant applications in characterizing the microstructure of materials. Based on the CT three-dimensional(3D) reconstruction technology and “Avizo” 3D visualization software, the microscopic pore-throat structure of porous media can be quantitatively characterized. This paper takes the carbon fiber reinforced resin matrix composites as an example to introduce the operation process of “Avizo” in details, which mainly covers the following modules: Volume Edit, Interactive Thresholding, Fill Holes, Mask, Separate Objects and Generate Pore Network Model, then further discuss the difficult problems when the “Avizo” is employed to analyze. The microstructures of carbon fiber reinforced resin matrix composites illustrate that pores in the upper part of sample are dramatically dispersed, and mainly concentrated in the lower part of sample. The porosity of adopted cuboid is 3.6%, accordingly the numbers of pores and throats reach 268 and 7, respectively. The equivalent radius of pores seems mainly distributed in the range of 0.7–0.8μm, accounting for 28.73% of the total pore number. The surface area of pore ranges from 5 to 10μm2, accounting for 14.16% of the total pore number. The pore volume concentrates in the range of 1–20μm3, accounting for 57.46% of the total pore number. In addition, the equivalent radius of throat mainly concentrates in the range of 1–5μm, the overall length of throat is distributed in the range of 37–60μm, and the equivalent area of throat is distributed non-uniformly in the range of 5–75μm2. This work provides a basis for the further investigation of fluid migration mechanism and law in the composite materials by the numerical simulation methodology.

Pore Network Modeling of Oil–Water Flow in Jimsar Shale Oil Reservoir

The oil–water two-phase flow mechanism is the critical issue for producing shale oil reservoirs after huge-volume hydraulic fracturing treatment. Due to the extremely low permeability of the shale matrix, the two-phase experimental measurement is impossible for shale samples. In this work, a pore network model is proposed to simulate steady-state oil–water flow with mixed wettability under consideration. The model is first applied in Berea sandstone, and the calculated relative permeabilities are validated with experimental studies for different wettability scenarios. Then, the three-dimensional FIB-SEM imaging of the Jimsar shale sample is used to extract a representative shale pore network with 13,419 pores and 31,393 throats. The mean values of pores and throats are 29.75 and 19.13 nm, and the calculated absolute permeability is 0.005 mD. With our proposed model, the calculated relative permeability curves show a high residual oil saturation for all the wettability conditions. Specifically, the oil-wet and mixed-wet conditions yield lower residual oil compared with the water-wet condition. For 50–50 mixed-wet conditions, the water phase relative permeability is much higher for smaller pores being oil-wet than the larger pores being oil-wet.