Fluid Permeability Research Articles

A Critical Review of Current States of Practice in Direct Shear Testing of Unfilled Rock Fractures Focused on Multi-Stage and Boundary Conditions

Direct shear (DS) is a common geotechnical laboratory test used to determine strength and deformation properties of rock discontinuities, such as normal and shear stiffness, peak and residual shear strength, and dilation. These are used as inputs for discontinuous geomechanical numerical models to simulate discontinuities discretely and shear strength is often expressed by Mohr–Coulomb, Patton, or Barton–Bandis constitutive models. This paper presents a critical review of the different boundary conditions and procedural techniques currently used in practice, summarizes previous contributions, addresses their impacts on interpreted results for rock engineering design, and introduces clarifying terminology for shear strength parameters. Based on the review, the authors advise that constant normal stress is best suited for discrete numerical-model-based rock engineering design in dry conditions, but constant normal stiffness should be considered where fluid permeability is of interest. Multi-stage testing should not be used to obtain peak shear strength values except for stage 1, because of accumulating asperity damage with successive shear stages. Nevertheless, if multi-stage testing must be employed due to limited budget or specimen availability, guidance is presented to improve shear strength results with limited displacement techniques.

Numerical modeling of near-wellbore diverter bridging in hydraulic fracturing

In this work, we used a particle transport model integrated into the fluid solver in FLAC to simulate the transport of particles in the near-wellbore region inside a hydraulic fracture. Complex flow behaviors, including divergent flow near the fracture entrance and detouring flow around diverter packs, can be captured in this coupling system. In the numerical implementation, the coupled fluid flow and particle transport are realized by updating the fracture permeability, particle concentration, and velocities of fluid and particles in each time step. Simulation results indicate that the final shape of the particle bridging zone at the near-wellbore region is determined by the flow-driven and settling-driven particle transport. A continuous bridging band initially forms at the bottom region of the fracture, then the fluid-driven particles move upwards to block the top region of the fracture. Lastly, a V-shaped bridging band is formed inside the fracture. It is observed that high initial particle concentration and large particle size result in the creation of a small-scale bridging band near the injection entrance, which promotes the rapid growth of the closed bridging band and high-pressure buildup.

A soil-brine retention model for wetting processes considering the hysteresis effects

Wetting hydrological processes are relevant to a considerable number of natural hazards, including landslides, dam breaks, surface runoff after floods, etc. The notable difference between wetting and drying soil–water retention curves (SWRC) has been attributed to the hysteresis phenomenon. Although recent studies confirm enhanced impurity levels of permeating fluid, most modeling approaches simply consider water retention along the drying path and neglect the impurity of pore water. Therefore, the main objective of this paper is to introduce a new water retention model along the wetting path by taking the osmotic potential into account, reflecting the influence of dissolved salts. The new model, referred to as the soil-brine retention curve (SBRC) was developed based on the 1980 van Genuchten model (VG). The results reveal the robustness of the model in predicting the experimental wetting retention data for different soil types exposed to both saline and distilled water. Statistical analysis confirms the capability of the new model with an average R2 of 0.97 and RMSE of 0.047. The wetting branch can be estimated based on an existing drying curve, assuming predefined values of entrapped air and air expulsion value. However, a more reliable prediction can be made if two extra parameters of entrapped air and air expulsion are calibrated for the pure water wetting curve. Furthermore, for the case of saline water, the wetting SBRC can be estimated using one more parameter than those used for pure water.

Modeling enhanced geothermal systems using a hybrid XFEM–ECM technique

Heat extraction from Enhanced Geothermal Systems (EGS) involves several multi-physics coupling processes, including the seepage through the fractured porous media, the thermal exchange between the working fluid and the matrix, and the deformation of fractured porous media, which play essential roles in exploiting the geothermal energy contained in hot dry rocks. In this study, a fully coupled numerical model for Thermo-Hydro-Mechanical simulation of EGS is presented based on the local thermal non-equilibrium using the eXtended Finite Element Method (XFEM) and Equivalent Continuum Method (ECM). The ECM can provide the equivalent tensors of not only fluid permeability but also solid compliance, which is an essential feature for coupled Thermo-Hydro-Mechanical simulation of fracture networks. In the model, the XFEM is employed for large-scale fractures to capture the mass and heat transfer between the fracture and matrix more accurately, while the ECM is applied on the network of small-scale fractures since considering their details explicitly is not usually cost-effective. Hence, the proposed model benefits from the advantages of both methods, and it allows for managing between accuracy and cost. The model is validated against the analytical solutions of heat transfer in porous media as well as single fracture problems. Moreover, its flexibility and applicability are shown by assessing the effects of the connectivity of fractures, characteristics of fracture networks, coupling conditions, and local thermal non-equilibrium assumption on the EGS simulation results. It is observed that the network connectivity and fracture alignment are both influential in the early thermal breakthrough; however, each can take dominance under different conditions. The results show the proposed computational model is a promising tool for estimation of the heat mining performance of EGS.

Micro-nano scale pore structure and fractal dimension of ultra-high performance cementitious composites modified with nanofillers

The development of ultra-high performance cementitious composite (UHPCC) represents a significant advancement in the field of concrete science and technology, but insufficient hydration and high autogenous shrinkage relatively increase the pores inside UHPCC, in turn, affecting the macro-performance of UHPCC. This paper, initially, optimized the pore structure of UHPCC using different types and dimensions of nanofillers. Subsequently, the pore structure characteristics of nano-modified UHPCC were investigated by the mercury intrusion porosimeter method and fractal theory. Finally, the fluid permeability of nano-modified UHPCC was estimated by applying the Katz-Thompson equation. Experimental results showed that all incorporated nanofillers can refine the pore structure of UHPCC, but nanofillers with different types and dimensions have various effects on the pore structure of UHPCC. Specifically, CNTs, especially the thin-short one, can significantly reduce the porosity of UHPCC, whereas nanoparticles, especially nano-SiO2, are more conducive to refine the pore size. Among all nanofillers, nano-SiO2 has the most obvious effect on pore structure, reducing the porosity, specific pore volume and most probable pore radius of UHPCC by 31.9%, 35.1% and 40.9%, respectively. Additionally, the pore size distribution of nano-modified UHPCC ranges from 10−1nm–105nm, and the gel pores and fine capillary pores in the range of 3–50 nm account for more than 70% of the total pore content, confirming nanofillers incorporation can effectively weaken pore connectivity and induce pore distribution to concentrate at nanoscale. Fractal results indicated the provision of nanofillers reduces the structural heterogeneity of gel pores and fine capillary pores, and induces homogenization and densification of UHPCC matrix, in turn, decreasing the UHPCC fluid permeability by 15.7%–79.2%.

Leakage reduction mechanism of supercritical CO2 scallop damper seal: Vortex structure and turbulence dissipation

The scallop damper seal (SDS) is a new sealing solution utilized in supercritical CO2(S-CO2) turbomachinery, and its sealing performance is of great interest. Analyzing the energy dissipation of fluid in the seal is critical for understanding the leakage characteristics of SDS. In this paper, we develop a high-order compressible flow solver with real gas thermophysical modeling. The vortex structure and turbulence dissipation of leakage flow are investigated to uncover the leakage reduction mechanism of SDS. By comparing the flow fields within a labyrinth seal (LABY) and a full-partition pocket damper seal (FPDS), we demonstrate that SDS has better leakage reduction performance for S-CO2. The results indicate that S-CO2 fluid flows into the SDS cavity and expands significantly. The formed vortex dissipates sufficiently more energy, reducing the leakage flow rate (LFR). The increase in the Mach number of the fluid flowing through the SDS gap is limited. Still, the gas permeability phenomenon caused by the throttling effect is observed in the clearance of LABY and FPDS, resulting in an increased LFR. At the differential pressure of 5 MPa, the LFR of SDS is 36.6% and 54.4% lower than that of LABY and FPDS, respectively. Although the rotor rotation leads to an asymmetric distribution of vortex in the SDS cavity, the vortex develops rapidly and occupies the entire space. For the seal design of S-CO2 turbomachinery, enhancing the turbulence dissipation of fluid in the cavity and reducing the gas permeability of fluid in clearance should be the focus of attention.

Laboratory investigation of hydraulic fracturing in granitic rocks using active and passive seismic monitoring

SUMMARY Knowledge of the fracturing processes can be important for the optimization of pressurized fluid injection operations in the deep underground rock mass. Active and passive seismic monitoring techniques have been used in the field for tracking or mapping the propagating hydraulic fracture. Although both these monitoring techniques provide valuable information about the generated fracture network, it is difficult for either technique to comprehensibly identify the different processes associated with hydraulic fracturing. The combined active and passive monitoring has the potential for better characterization of the complex hydraulic fracturing phenomena. In this study, laboratory hydraulic fracturing experiments with combined active and passive seismic monitoring were conducted on true triaxially loaded Barre granite cubes with different fluid injection rates. The seismic inelastic fracturing was detected by 16 passive acoustic emission sensors, where 3678 and 2370 seismic source events were detected for the high and low injection rate experiments, respectively. For active monitoring, strong variations in the attributes of signals were observed which were transmitted through four source–receiver pairs, placed both perpendicular and parallel to the generated hydraulic fracture. Positive velocity changes were observed for active sensor pairs with ray paths passing through the generated hydraulic fracture indicating fluid permeation, whereas isolated dry deformation was characterized by a slight but permanent velocity decrease. Compared to velocity, the energy of the active signals was 1–2 orders of magnitude more sensitive to different hydraulic fracturing processes. However, the sensitivity and signatures of the active signal attributes were found to be dependent on the frequency range and direction of ray path with respect to the location of the generated fracture network. Using the coupled evaluation of the active and passive signals we were able to systematically identify various hydraulic fracturing processes including: (1) aseismic deformation, (2) fracture initiation and fluid permeation, (3) pressure build-up, (4) fracture propagation and (5) pressure release and leak-off. The results of this study showed that combining the respective advantages of active and passive seismic techniques and using both of them to monitor the failure processes can facilitate a more comprehensive understanding and better control of the hydraulic stimulations in subsurface operations.

Synthesis and Plugging Performance of Nano-Micron Polymeric Gel Microsphere Plugging Agents for Oil-Based Drilling Fluids

As shale gas recovery progresses to deep layers, the wellbore instability during drilling in applications of oil-based drilling fluids (OBFs) becomes increasingly severe. This research developed a plugging agent of nano-micron polymeric microspheres based on inverse emulsion polymerization. Through the single-factor analysis with respect to the permeability plugging apparatus (PPA) fluid loss of drilling fluids, the optimal synthesis conditions of polymeric microspheres (AMN) were determined. Specifically, the optimal synthesis conditions are as follows: the monomer ratio of 2-acrylamido-2-methylpropanesulfonic acid (AMPS): Acrylamide (AM): N-vinylpyrrolidone (NVP) were 2:3:5; the total monomer concentration was 30%; the concentrations and HLB values of emulsifier (Span 80: Tween 60) were 10% and 5.1, respectively; the oil-water ratio of the reaction system was 1:1; the cross-linker concentration was 0.4%. The polymeric microsphere (AMN) produced via the optimal synthesis formula had the corresponding functional groups and good thermal stability. The size distribution of AMN ranged mainly from 0.5 to 10 μm. The introduction of AMND in OBFs can increase the viscosity and yield point of oil-based drilling fluids and slightly decrease the demulsification voltage but significantly reduce high temperature and high pressure (HTHP) fluid loss and permeability plugging apparatus (PPA) fluid loss. The OBFs with 3% polymeric microsphere dispersion (AMND) reduced the HTHP and PPA fluid loss by 42% and 50% at 130 °C, respectively. In addition, The AMND maintained good plugging performance at 180 °C. The AMN particles can block leakoff channels of artificial cores, effectively prevent the invasion of oil-based drilling fluids into formations and suppress pressure transfer. OBFs with 3% AMND enabled the corresponding equilibrium pressure to decrease by 69%, compared with that of the OBFs. The polymeric microspheres had a wide particle size distribution. Thus, they can well match leakage channels at various scales and form plugging layers via compression-deformation and packed accumulation, so as to prevent oil-based drilling fluid from invading formations and improve wellbore stability.

Physical, chemical, and biological characterization of biodegradable chitosan dressing for biomedical applications: Could sodium bicarbonate act as a crosslinking agent?

Chitosan, a biomaterial with properties that allows the elaboration of biocompatible and biodegradable systems, was employed to prepare dressings - 2% (w/v) in acetic acid - followed by freezing and lyophilization. Some samples were kept chemically unchanged and used as control, while others were cross-linked with Epichlorohydrin 0.01 mol L−1 for 24 h being washed, frozen and lyophilized. The neutralization procedure was performed with sodium bicarbonate, which ended up leading to a crosslinking by ionic interactions in the polymer network, and characteristics as promising as those of the chemically cross-linked with Epichlorohydrin. SEM showed that the crosslinking leads to a controlled formation of the polymeric network, conferring suitable steam permeation capacity 107 g mm.day−1.m−2.kPa−1, simulated fluid permeation between 2000 and 2500 g m−2.day-1 and porosity with interconnection which allows fluid absorption capacity above 900%. The crosslinking processes favored a better handling mechanical resistance confirmed by biodegradation assays. The dressing blocked micro-organisms permeation, forming an efficient barrier against contamination, is safe and biocompatible, allowing satisfactory cell adhesion and proliferation. These results showed that both dressings present statistically and satisfactory equivalent characteristics to be used as a functional bio dressing suitable for the treatment of different pathologies.

Novel Designed Surgical Drapes Reducing Fluid Permeability in the Surgical Critical Area of a Sterile Operation Interface: A Randomized Controlled Trial

Aim. To compare the impact and cost effects of medical long fiber polyester drapes and cotton fabric drapes on operative sterile operation interfaces. Background. The comparison of the properties of the commonly used surgical drapes materials in terms of leakage, device slip, and prevention of intraoperative adverse events is not clear. Method. A prospective randomized controlled study was conducted in the operating room of a tertiary hospital in Chengdu, China. A total of 400 patients who underwent urology surgery were enrolled and randomly divided into two groups by computer, the study group (200 cases) selected the new long-fiber polyester cloth, while the control group (200 cases) selected conventional cotton fabric surgical drapes during the operation to maintain a sterile operating interface. The impermeability and water absorption of surgical drapes, the rate of device slip and skin scald in surgical patients, and the cost effect of the two kinds of surgical drapes were compared. Results. The long fiber polyester surgical drapes were superior to conventional cotton cloth in water absorption (g/m2) (835 ± 15.8 VS 225 ± 21.0, t = 328.261, P < 0.001 ), preventing surgical site infections (2.5% VS 8.0%, χ2 = 6.081, P = 0.014 ), device slip (7.5% VS 17.0%, χ2 = 8.396, P = 0.004 ), patients from burning (0 VS 1, Fisher P = 1.0 ), and total cost per use ($) (0.83 VS 0.96–1.09). Conclusion. Long fiber polyester fabric has a stronger antipenetration ability of fluid and microorganisms thus forming an effective protective barrier. It also has strong hygroscopicity, and its special design can prevent the occurrence of sliding of surface instruments and skin scald in patients. In addition, its cost effect is superior. Implications for Nursing Management. Operating room nursing managers can introduce long fiber polyester drapes into the selection of medical textiles to construct aseptic surgical barriers and prevent surgical site infection.

Theoretical and Experimental Study on the Performance of Photovoltaic using Porous Media Cooling under Indoor Condition

This paper presents the theoretical and experimental investigation on performance of a photovoltaic (PV) panel cooled by porous media under indoor condition. Porous media offer a large exterior surface area and a high fluid permeability, making them ideal for PV cells cooling. The photovoltaic panel was cooled using 5 cm thick cooling channel filled with porous media (gravel). Several sizes of porosity (0.35, 0.4, 0.48, and 0.5) at different volume flow rates (1, 1.5, 2, 3, and 4 L/min) were tested to obtain the best cooling process. The theoretical analysis was performed at the optimum case found experimentally, which has a porosity of 0.35 and a volume flow rate of 2 L/min, to test various experimental results of the PV hot surface temperature, related power output, efficiency and I-V characteristic curve. The enhancement obtained in PV power output and efficiency is compared against the case without cooling and the case using water alone without porous media. Results showed that cooling using small size porous media and moderate flow rate is more efficient which reduces the average PV hot surface temperature of about 55.87% and increases the efficiency by 2.13% than uncooled PV. The optimum case reduced the PV hot surface temperature to 38.7°C, and increased the power output to 19 W, efficiency to 6.26%, and the open voltage to 22.77 V. The results showed that the presence of small porous media of 0.35 in the PV cooling process displayed the maximum effectiveness compared to the other two scenarios, because the heat loss from PV surface through porous media layer have developed a homogenous heat diffusion removed much quicker at high flow rate (2 L/min). A good agreement was obtained between experimental and theoretical results for different cases with a standard deviation from 3.2% to 5.6%.

An innovative design of integrative polyaniline/carbon foam flexible electrode material with improved electrochemical performance

The core task of high performance flexible supercapacitor is to generate flexible electrode material with high performance, light weight, and easy machining. Supercritical fluid technology is introduced, going beyond electrochemical deposition method, to solve the problems of low specific capacity and poor interface compatibility. An integrated polyaniline/carbon foam electrode material with improved electrochemical performance is constructed, by using supercritical fluid with high diffusivity, strong permeability, and superior solvation ability, to realize the controllable construction of composite microstructure and active interface regulation. The specific capacitance reaches 856.07 F/g at 1 A/g and high specific energy density of 96.3 Wh/kg at a power density of 3600 W/kg. This work makes possible the innovative experiment design of polymer and carbon-based materials integration with higher specific capacity and better interface compatibility. Therefore, the flexible electrode with high specific and high stability can be constructed by using the high diffusivity, strong permeability, and superior solvation ability of supercritical fluid.

The Investigation of Paraffin Handling with Parasol and Xylene Chemical Treatment: A Case Study in Sangasanga Field, Indonesia

Paraffin deposition is a problem in producing hydrocarbons that can result in losses. Paraffin can form near a wellbore, tubing, or other production facilities. Temperature differences are one of the causes of paraffin depositions. When the temperature of a fluid starts to fall below its pour point, oil flows more slowly. The temperature at which paraffin begins to form is known as the "wax appearance temperature." The development of paraffin can reduce production rates, damage production equipment, and lead to economic losses. In many cases, paraffin can also develop near the wellbore, which can cause formation damage. As a result, when paraffin begins to form, the problem must be handled as soon as possible to minimize more severe damage. The chemical method has been proven to be effective in dealing with paraffin problems. A solvent dissolves the paraffin and reduces the oil's viscosity, allowing it to flow more easily. This study aims to discuss laboratory studies of two different solvents and analyze the best scenarios that can be applied in the field to overcome paraffin problems in the Sangasanga Field and its effect on permeability improvement. The method used in this study is laboratory testing of fluid properties, solubility tests, and permeability tests. This study shows that the Parasol solvent can dissolve paraffin up to 98.2% and improve permeability up to 94.9%.

NMR log response prediction from conventional petrophysical logs with XGBoost-PSO framework

A combined approach that exploits both the eXtreme Gradient Boosting (XGBoost) method and the Particle Swarm Optimization (PSO) method was used here to predict the nuclear magnetic resonance (NMR) log parameters such as porosity and permeability from field measurements. Data obtained by NMR are extremely critical for modeling the reservoir behavior and are among the most vital inputs for reservoir simulations. Therefore, developing an intelligent framework that can accurately predict NMR logging response by using conventional, low-cost logging data can be a valuable tool for field development. A number of conventional well logs, such as neutron, density, sonic, caliper, gamma-ray, and resistivity logs, recorded from sixteen wells in an offshore oilfield in Persian Gulf were used in the XGBoost-PSO framework to predict NMR log response. The outputs are the free fluid porosity (CMFF), bound fluid porosity (BFV), permeability (KTIM), and total porosity (TCMR). Based on the results, in terms of prediction accuracy, comparing correlation coefficients, the XGBoost-PSO model performs in the range of 88.5–91.4%. Moreover, the application of PSO in hyperparameter optimization of predicted parameters confirmed at least five percent improvement. Overall, we were able to demonstrate that ordinary conventional logging data can be utilized to obtain more advanced NMR data without performing expensive, time-consuming, and high-risk logging operations to decrease operational costs without compromising the benefits.

An integrated modeling approach to predict critical flow rate for fines migration initiation in sandstone reservoirs and water-bearing formations

Fines migration causes formation damage in sandstone reservoirs and water-bearing subsurface formations. Several factors affect fines migration such as composition and salinity of permeating brine, flow rate, and system pH. Electrostatic forces of van der Waals attraction and electric double layer repulsion, gravitational force and hydrodynamic force all contribute to fines detachment and migration under certain conditions. The current research presents a new integrated model of electrostatic, gravitational, and hydrodynamic forces to estimate the critical injection rate of NaCl brine for fines migration initiation in subsurface formations.The DLVO modeling approach was modified by considering electrostatic, hydrodynamic, and gravitational forces. Parameters such as injection brine salinity, average fine particle size, and zeta potential of sand-brine systems were used, and effective forces were quantified. A microscopic force and torque balance approach was applied to model hydrodynamic forces to modify the DLVO model. The developed models were validated with experimental results.The profiles of injection velocity versus fine particle radius were generated and the models predicted critical flow rates of 0.9, 2.6, and 3.8 cc/min for 0.15 M, 0.2 M, and 0.25 M NaCl, respectively. For model validation, the coreflood experiments were performed on Berea sandstone core samples under given salinities and critical flow rates were determined. Comparable experimental results were obtained with critical flow rates of 1, 3, and 4 cc/min for 0.15 M, 0.2 M, and 0.25 M NaCl, respectively. In addition, sensitivity analysis assesses the impact of different forces on the fines migration phenomenon.Formation damage caused by fines migration is a well-documented problem in sandstone reservoirs and water-bearing formations. Regulating the injection/ permeating fluid rate and salinity can avoid fines migration and associated permeability impairment. Our study presents a novel approach incorporating electrostatic, gravitational, and hydrodynamic forces to accurately predict fines mobilization in fine-containing subsurface formations.

Relative permeability hysteresis analysis in a reservoir with characteristics of the Brazilian pre-salt

The hysteresis of relative permeability and capillary pressure need to be more widespread in academic studies, in order to understand how they can influence reservoirs with light oil and high pressure. These phenomena become extremely important to have a good prediction of oil production, considering that in many cases, the use of hysteresis in calculations can lead to a better prediction of oil recovery, allowing the exploration of certain fields. Thus, this study had as main objective the analysis of two hysteresis models (Killough and Larsen and Skauge) widely used in commercial software, in order to investigate the behavior of a light oil reservoir using a miscible WAG-CO2 process. Thus, to achieve this goal, a semi-synthetic reservoir, with characteristics similar to those found in the Brazilian pre-salt, was considered and was modeled using commercial software from CMG. Hysteresis reduces fluid permeability, which can generate two effects: increased local sweep efficiency of the oil and loss of injectivity. The former effect contributes to increased oil recovery, while injectivity loss can decrease oil sweep, reducing oil recovery. Furthermore, this work found that hysteresis can cause loss of gas and water injectivity, however this did not prevent hysteresis from increasing oil recovery compared to the case without hysteresis.

Pore-scale investigation on the effect of icing on relative permeability of two-phase immiscible fluids

In this study, the flow behavior and relative permeability (kr) curve of the immiscible two-phase flow accompanied by icing in a horizontal channel were studied using the lattice Boltzmann method-phase field method (LBM-PFM) coupling model at the pore-scale. A novel LBM-PFM coupled model is proposed and tested by comparison with analytical and experimental results, and good agreement is achieved in general. Then, the model is used for simulating two-phase fluids (brine water–oil) flow during icing. In addition, the effects of initial phase distribution, capillary number, and heat flux are numerically discussed. The results show that, the distribution characteristics of the relative permeability curves of the immiscible fluids during the crystallization process are determined by the initial phase distribution. The Ca number is a positive correlation function of kr, while the kr curve is not sensitive to the change of heat flux.

Effect of closed and open system freeze–thaw cycles on GMB–GCL interface transmissivity

An experimental study of the effect of closed system and a limited study of open system freeze–thaw cycles on the interface transmissivity at applied stresses between 10 and 150 kPa is described. The effect of closed system freeze–thaw is most apparent after 100 freeze–thaw cycles and permeation at a stress ≤25 kPa. The effect of permeating fluid chemistry is also evident but decreased with increasing stress. One notable effect of permeant is the potential for internal erosion along weaknesses in the bentonite created by freeze–thaw cycles when permeated by distilled or reverse osmosis water. This is not observed when permeated with simulated pore water containing cations. In terms of practical application, closed system freeze–thaw cycles appear to have little effect on interface transmissivity, and to the extent that there is an effect, it is beneficial. In contrast, cryogenic suction arising with open system freeze–thaw cycles is shown to result in the formation of ice lenses both within the geosynthetic clay liner (GCL) and at the GCL geomembrane interface. Although limited, the data suggest that the effect of ice lens formation on interface transmissivity after open system freeze–thaw cycles is largely eliminated at applied stress above 20–25 kPa, although more research is warranted.

Numerical Study on the Migration of Leaked Oil in Submarine Sediment Caused by Pipeline Leakage Using Two-phase Flow Model

Oil leakage from submarine pipelines can cause severe environmental problems and economic losses. In this study, a two-phase immiscible flow model in porous media is proposed to simulate the migration of leaked oil caused by the leakage of a buried submarine pipeline in sandy sediment. The relationship between saturation, pore pressure, and relative permeability of different fluids in porous media is considered in present model, which is distinguished from previous studies. The spilled oil spreads radially from the leakage point into surrounding sediment driven by the leakage velocity. Moreover, it continues to migrate in sediment under the capillary pressure and gravity. Interestingly, the effect of wave-induced transient response on the ultimate migration of spilled oil in a sandy sediment is negligible. However, increasing sediment porosity can greatly enhance the upward migration of leaked oil based on systematically parametric analysis. Meanwhile, the migration rate of leaked oil decreases with increasing oil density or viscosity. The oil-contaminated area significantly expands with the increase of leakage rate. Fitting formulas between dimensionless time required for the leaked oil to reach the overlying seawater and non-dimensional influencing parameters are well established, which can provide useful guidelines for similar disasters.

Numerical investigation of poroelastic effects during hydraulic fracturing using XFEM combined with cohesive zone model

Hydraulic fracturing is an effective technology used to stimulate hydrocarbon production from unconventional reservoirs. Numerical simulation of the hydraulic fracturing process plays a key role in the design of hydraulic fracturing. However, most of existing models are overly simplified by neglecting the poroelasticity of rock matrix, which may have a substantial effect on the growth of hydraulic fractures, especially in oil and/or water saturated formations. In this study, we developed a fully coupled finite element model for hydraulic fracture propagation in permeable formations by combining XFEM technique with cohesive zone model, and used it to investigate the effects of poroelasticity on the geometry evolution of single fracture which initiated from an injection point. Fluid flow within the fractures, Darcy flow within the rock matrix, hydraulic fractures propagation, and fluid leak-off into the formation are simultaneously taking into account in the model. Then, the model is validated by comparing the results with available analytical solutions. To understand and quantify the poroelastic effects on the propagation of hydraulic fracture, several cases with different matrix permeability, leak-off coefficients, and bulk modulus of pore fluid are performed. The simulation results show that the total volume of leakage is controlled by the combined action of matrix permeability and leak-off coefficient. The fracture aperture and length decrease with the increase of matrix permeability or leak-off coefficient, while as the bulk moduli of pore fluid increases, the fracture aperture and length tend to increase.