Leakage Pathways Research Articles

Detecting and tracking thiram in leakage pathways using bioinspired nanograss with thuja fruit-like nanoparticles

Thiram is a widely employed pesticide that has been instrumental in agricultural practices owing to its efficacy in fungal disease control. However, its extensive and unregulated application has prompted concerns regarding environmental contamination and associated human diseases, including neurological disorders, gastrointestinal disturbances, respiratory problems, dermatological disorders, and endocrine disruption. Thus, detecting thiram residues across diverse environments where these residues are likely to leak has emerged as a critical endeavor to address the aforementioned pressing concerns. This study introduced a hybrid structure by integrating bioinspired nanograss (NG) with thuja fruit-like nanoparticles (TFNPs) to achieve sensitive thiram detection within potential leakage pathways. The hybrid architecture, amalgamating TFNPs and NG, underwent optimization to augment the overall sensing capabilities, thus mitigating the limitations of individual substrates. The resulting hybrid surface-enhanced Raman spectroscopy (SERS) substrate, termed TFNPs@NG, demonstrated outstanding performance by achieving a detection limit of 1 pM for thiram in ethyl alcohol. It proficiently discriminated thiram from various sulfur-containing pesticides. The sensor also demonstrated remarkable reproducibility, signal uniformity, and selectivity, rendering it well-suited for practical applications. This TFNPs@NG-based SERS sensor shows considerable potential for the sensitive and reliable detection of thiram in diverse leakage pathways, encompassing soil, drinking water, and milk. This advancement contributes significantly to the fields of environmental monitoring and public health protection.

Risk-based analysis of squeeze cementing operations

The success rate of a squeeze cementing operation is generally low due to complexities in the range of potential leakage pathways and enormous uncertainty in characterizing these pathways. This work aims first to develop a physically based numerical model of the key part of the squeeze cementing operation, i.e. invasion of cement slurry into a realistic microannuli geometry under operational conditions. Secondly, it aims to account for uncertainties by using statistical tools combined with the deterministic model to deliver probabilistic information regarding outcomes. This paper develops this novel risk-based approach. The first objective of this work is to build upon our previous model, which focused on a single-perforation injection scenario. We reconstruct a multi-perforation injection scenario by collocating radial (single-perforation) invasion flows. We use this model to investigate the invasion of a viscoplastic fluid, representing a cement slurry, into a randomized varying width microannulus channel. The second objective is to investigate the effect of relevant parameters on leakage reduction. This is broken into two parts. First, how effectively the squeeze operation fills the microannulus around a perforation, explored through metrics that quantify penetration/filling. Second, what effect the penetration/filling has on the leakage of the entire well, i.e. given that the squeeze operation is local. We compare the effect of different perforation patterns and rheological parameters on penetration/filling metrics and on the reduction of leakage. We generate a probability distribution of leakage rates before and after the operation. In this way can estimate both the mean reduction in leakage for different perforation patterns, and associated confidence intervals. Such predictions have not been made before. Our results demonstrate the inherent uncertainty of a squeeze operation, which arises from the geometrical complexity. Practically, this suggests that higher perforation density and lower yield stress slurry will lead to higher success rates, as is intuitive. However, the spread of uncertainty in outcomes is not reduced much by such practices, meaning that one can still be unlucky in perforating the wrong part of the annulus.

Reduced-order models for the greenhouse gas leakage prediction from depleted hydrocarbon reservoirs using machine learning methods

Geologic storage of carbon dioxide (CO2) is one of the potential technological options to mitigate human-induced climate change. Depleted hydrocarbon reservoirs are promising candidates for storing CO2. However, these reservoirs contain residual hydrocarbons that may migrate beyond primary reservoirs during CO2 storage operations. Therefore, it is imperative to quantify the leakage risks of residual hydrocarbons (primarily methane) and CO2 to shallow aquifers. We focus on developing models that quantify leakage risks from wellbores that provide potential leakage pathways. To offset the computational intensity of high-fidelity reservoir simulations, we develop a Reduced-order model (ROM) enabling fast and accurate CO2 and hydrocarbon leakage predictions through wellbores. The ROM is generated using a dataset of 1000 high-fidelity, compositional reservoir simulations of CO2 injection into generic hydrocarbon reservoirs. The input parameters include reservoir depth, permeability, net to the gross ratio (NTG), reservoir pressure multiplier, wellbore permeability, average water saturation, oil compositions, and time. We analyze the performance of various ROM development techniques, including multivariate adaptive regression splines, gradient boosting, and neural networks. While all ROM development techniques yield excellent agreement with R2 higher than 0.95, the neural network model performs best compared to the other two methods. Furthermore, we explore developing a ROM as a collection of multiple sub-ROMs to improve accuracy across a wide range of predictions. We observe that using a set of sub-ROMs outperforms the prediction accuracy of a single ROM. Our work enables fast and reliable risk-assessment tools for CO2 geologic storage in depleted hydrocarbon fields.

Research on android user privacy permission analysis and protection mechanism under big data environment

With the rapid development of big data technology, the issue of user privacy security on the Android platform is becoming increasingly prominent. This paper aims to conduct an in-depth analysis of the privacy permissions of Android users under the big data environment and explore effective protection mechanisms. Through research on permission management, application behavior, and user privacy leakage pathways in the Android system, this paper proposes a comprehensive privacy protection strategy to enhance the privacy security level of Android users in the big data environment.

Subcritical crack growth and arrest in the presence of a material interface

Reservoir systems often consist of layered rock formations that are naturally fractured. These can include both fractures that are confined within the main reservoir layer, and ones that penetrate adjacent confining layers or cut through into nearby reservoir strata. Layer-bound fractures have the effect of improving the overall permeability of a reservoir layer, however through-cutting fractures act as pathways for both fluid leakage and pressure communication between otherwise isolated formations. In this study, we investigate the subcritical propagation of a single crack from a stiff inner layer into more compliant and ductile outer layers. We model the crack growth numerically, by making use of recently formulated correction factors that account for the effect of mechanical boundaries on the crack tip energy release rate. Furthermore, we utilize a modified arc-length method to handle the stiff behaviour of the differential equations governing crack tip motion in a layered medium. Under different conditions, a fracture may be arrested permanently or temporarily at the layer boundary, propagate a short distance into the outer layer before arrest, or propagate critically in the outer layer forming a through-cutting fracture. We find that fractures are most likely to remain layer-bound in layers less than 5 m thick, a subcritical index below 10, and a low critical energy release rate relative to the confining layers. If the outer layer modulus is less than half that of the inner layer, the fracture is more likely to propagate a short distance into the confining layer before arrest. In the case of tectonic deformation characterized by steadily increasing far-field extensional strain, crack arrest at the layer boundary is likely to be only temporary, and the occurrence of layer-bound versus through-cutting fractures will be determined by the duration of said deformation together with the thickness and mechanical properties of the different layers.

Methane Emissions from Abandoned Oil and Gas Wells in Alberta and Saskatchewan, Canada: The Role of Surface Casing Vent Flows.

Abandoned oil and gas wells can act as leakage pathways for methane, a potent greenhouse gas, and other fluids to migrate through the subsurface and to the atmosphere. National estimates of methane emissions remain highly uncertain, and available measurements do not provide details on whether the emissions are associated with well integrity failure (indicating subsurface leaks) or aboveground well infrastructure leaks. Therefore, we directly measured methane emission rates from 238 unplugged and plugged abandoned wells across Alberta and Saskatchewan, Canada, separately quantified emissions from surface casing vents and other emissions from the wellhead (non-surface casing vent), and developed emission factors to estimate Canada-wide emissions from abandoned wells. Our highest measured emission rate (5.2 × 106 mg CH4/hr) from an unplugged gas well was two to three times higher than the largest previously published emission rate from an abandoned well. We estimated methane emissions from abandoned wells in Canada to be 85-93 kilotonnes of methane per year, of which surface casing vent emissions represented 75-82% (70 kilotonnes of methane per year). We found that subsurface leaks, as evidenced by surface casing vent flows, occurred at 32% of abandoned wells in Alberta, substantially higher than previously estimated using provincial data alone (6 and 11%). Therefore, well integrity failures and groundwater contamination are likely to be more common than previous studies suggest.

Refined Analysis of Leakage Current in SiC Power Metal Oxide Semiconductor Field Effect Transistors after Heavy Ion Irradiation

A leakage current is the most critical parameter to characterize heavy ion radiation damage in SiC MOSFETs. An accurate and refined analysis of the source and generation process of a leakage current is the key to revealing the failure mechanism. Therefore, this article finely tests the online and post-irradiation leakage changes and leakage pathways of SiC MOSFETs caused by heavy ion irradiation, analyzes the damaged location of the device in reverse, and discusses the mechanism of leakage generation. The experimental results further confirm that an increase in the leakage current of a device during heavy ion irradiation is positively correlated with the applied voltage of the drain, but the leakage path is not direct from the drain to the source. The experimental analysis of the source of the leakage current of the device after irradiation indicates that there is also a leakage current path between the device gate and source. The research results suggest that the experimental sample is more prone to a single-event gate rupture effect under this heavy ion radiation condition. The gate breakdown mainly occurs in the gate oxide layer at the neck region. This research can provide a theoretical basis for the radiation resistance reinforcement of SiC power devices.

On the Flow of CO2-Saturated Water in a Cement Fracture

Cement fractures represent preferential leakage pathways in abandoned wells upon exposure to a CO2-rich fluid. Understanding fracture alteration resulting from geochemical reactions is critical for assessing well integrity in CO2 storage. This paper describes a mathematical model used to investigate the physical and the chemical changes in cement properties when CO2-saturated water is injected into a wellbore. This study examines the flow of a solution of CO2-saturated water in a two-dimensional fractured cement. In this approach, a micro-continuum equation based on the Darcy–Brinkman–Stokes (DBS) equation is used as the momentum balance equation; in addition, reactive transport equations are used to study the coupled processes of reactant transport and geochemical reactions, and the model for cement porosity alteration and fracture enhancement. This paper focuses on the effects of cement porosity, fracture aperture size, and surface roughness. Mineral dissolution and precipitation mechanisms are also considered. Our simulations show that smaller initial fracture apertures tend to a high mineral precipitation self-sealing. However, a complete sealing of the fracture is not observed due to the continuous flow of CO2-saturated water. The calcite precipitation mechanism of a rough fracture (random zigzag shape) differs from that of a smooth/flat fracture surface.

“Characteristic features of shale barrier materials”

Deep wells in soft formations are completed with a steel tube (“casing”) that isolates the well from the surrounding rock. The annulus between the casing and the rock is usually left open in long sections of the well, thus creating a potential pathway for leakage which must somehow be sealed by a barrier. In many wells, it has been observed that shale formations creep in and close the annulus all by themselves, thus creating shale barriers. This may represent huge cost savings for the operator. However, this does not happen in all shale formations. It is therefore of great interest for the operator to identify at an early stage whether a given shale is a potential shale barrier material.The objective of this study was to identify some characteristic features of shale barrier materials, that may allow for early identification and classification of shales in terms of their potential as shale barrier material. In this study, the performance of shale barriers created in laboratory tests have been compiled with several characteristic properties for a set of shale materials. As a first step in this process, a quantitative classification of the shales' willingness to form barriers was established, based on their resistance against annulus closure, and their resistance against forming a sealing barrier. The definitions of these parameters reflect that formation of shale barriers depends on several factors in addition to rock properties.Porosity emerges as a key parameter from this study. This result is supported by theoretical considerations. High porosity implies low shear stiffness and strength which is a requirement for a shale barrier material. Low porosity on the other hand is often associated with over-consolidation and brittle behaviour which is unfavourable for the process of forming a sealing barrier by deformation of the rock. An interesting implication of this result is that several other parameters that are often available from well logs, such as density, and P- and S-wave velocities, may be used as indicators for shale barrier potential, since they are all closely related to porosity.Contrary to expectations, the test results revealed only weak connection between shale barrier quality and mineralogy related parameters such as total clay content and quartz content. A possible explanation may be the limited selection of materials in our study, which contained only shales with clay content above 45% and quartz content below 30%. This result may indicate that clay content is less important if it is higher than a given threshold, and that quartz content is less important if it is lower than a given threshold.

Exceeding 10 000 h of Lifetime in Blue Fluorescent Organic Light‐Emitting Diodes by Introducing an Electron Leakage Pathway

Electron leakage is one of the most critical factors for the degradation of blue fluorescent organic light‐emitting diodes (FOLEDs), as it occurs when electrons injected from the cathode flow to an electron‐blocking layer (EBL) without being entirely consumed within an emissive layer (EML). To address this issue, this study introduces the concept of an electron leakage‐pathway (ELP). Anthracene core materials, used as hosts for blue FOLEDs were doped in the EBL to prevent its direct damage to the EBL molecules by constructing the ELP. This study shows that using an anthracene core with high bond dissociation energy (BDE) in the anionic state as the ELP is the most effective way to increase the lifetime of the FOLED, demonstrating over 400 h. of LT90 (the time taken for the luminance to decrease to 90% of its initial value), which corresponds to an increase of 33% compared to the lifetime without the ELP. Remarkably, under a commonly used brightness level of 500 cd m−2 in smartphones, an LT50 of over 10 000 h was obtained. Ultimately, this study outlines a strategy for increasing the lifetime of blue FOLEDs through simple modifications to the device structure, without requiring the development of EBL materials or complex device engineering.

The influence of prolonged instrument manipulation on gas leakage through trocars

BackgroundDuring laparoscopic surgery, CO2 insufflation gas could leak from the intra-abdominal cavity into the operating theater. Medical staff could therefore be exposed to hazardous substances present in leaked gas. Although previous studies have shown that leakage through trocars is a contributing factor, trocar performance over longer periods remains unclear. This study investigates the influence of prolonged instrument manipulation on gas leakage through trocars.MethodsTwenty-five trocars with diameters ranging from 10 to 15 mm were included in the study. An experimental model was developed to facilitate instrument manipulation in a trocar under loading. The trocar was mounted to a custom airtight container insufflated with CO2 to a pressure of 15 mmHg, similar to clinical practice. A linear stage was used for prolonged instrument manipulation. At the same time, a fixed load was applied radially to the trocar cannula to mimic the reaction force of the abdominal wall. Gas leakage was measured before, after, and during instrument manipulation.ResultsAfter instrument manipulation, leakage rates per trocar varied between 0.0 and 5.58 L/min. No large differences were found between leakage rates before and after prolonged manipulation in static and dynamic measurements. However, the prolonged instrument manipulation did cause visible damage to two trocars and revealed unintended leakage pathways in others that can be related to production flaws.ConclusionProlonged instrument manipulation did not increase gas leakage rates through trocars, despite damage to some individual trocars. Nevertheless, gas leakage through trocars occurs and is caused by different trocar-specific mechanisms and design issues.

Fluoroscopy-guided multiphase flat panel CT cisternography to diagnose complex skull base CSF leaks

Skull base cerebrospinal fluid (CSF) leaks can be challenging to diagnose. We describe a first technical report on a fluoroscopy-guided multiphase flat panel computed tomography cisternography to diagnose a CSF leak in a complex postoperative anterior cranial fossa. The entry point, pathway of leakage, and exit point were visualized in detail. The feasibility and technical details are described. This technique could be an additional asset in the diagnostic work up of complex CSF leaks, not only of the anterior cranial fossa, but also complex CSF leaks of the middle cranial fossa.

Analysis of the effect of formation dip angle and injection pressure on the injectivity and migration of CO2 during storage

Carbon dioxide (CO2) geological storage (CGS) can help realize the plan of “Carbon Neutrality”. Injection capacity and storage safety are crucial factors in evaluating CO2 storage efficiency. Deep saline aquifers are major sites for CGS. This study focused on evaluating the CO2 injection amount, migration safety and leakage risk. A three-dimensional (3D) model of the Shenhua CO2 capture and storage (CCS) project in the Ordos Basin, China, was established to discuss the impact of injection pressure (1.3P (equal to 1.3 times the top reservoir formation hydrostatic pressure), 1.4P, and 1.5P) and formation dip angle (0°, 5°, 10°, 15°) with fault on the total CO2 injection and migration safety. Twelve schemes were designed for simulation. Based on the results, faulting provided a pathway for CO2 leakage, posing a threat to CO2 storage safety. Injection pressure and formation dip angle had significant effects on injectivity and migration during CO2 storage. The influence of the injection pressure on the CO2 injection amount was more evident than that of the dip angle, though formation dip angle had a significant impact on the CO2 migration during storage. Increasing the injection pressure and formation dip angle increased the gas and dissolved-phase CO2 migration and decreased the CO2 leakage time and storage safety. The CO2 leakage through the fault occurred after 160, 110 and 80 years in a 15° sloping formation with 1.3P, 1.4P, and 1.5P, respectively. The CO2 injection amount was the largest for the 0° formation with 1.5P. These results provide fundamental information for CO2 storage safety and capacity, suggesting that un-faulted reservoirs with smaller dip angles should be selected for CGS projects; however, larger injection pressures can result in greater injection amounts, decreasing the migration safety of CO2 during storage.

Diffusive Leakage Through Existing and/or Induced Microfractures in the Caprock of CO2 Storage System

Summary Safe sequestration of CO2 in subsurface formations requires a caprock that inhibits the unwanted flow of the stored fluids. However, the preexisting fractures and the induced fractures initiated by overpressure and thermal stresses may act as potential leakage pathways. In this study, we describe a physical problem where the caprock with tiny embedded microfractures is subjected to overpressurization at the storage layer. The average effect of the existing and induced fractures is accounted for by employing a spatially variant permeability field. Also, the alteration of effective local stress is considered by applying Pedrosa’s stress-sensitive model. The coupled model for the pressure diffusion in the caprock and monitoring aquifer is solved using a general Bessel function solution and Laplace transform. The nonlinearity due to stress sensitivity was attenuated by Pedrosa’s transform, and a perturbation solution was obtained. The obtained solution was verified analytically and compared against classical solutions. The spatial variability of the caprock permeability field is effectively represented by the caprock’s two endpoint permeability values, storage/caprock interface permeability, and caprock/monitoring aquifer interface permeability. The ratio between the caprock’s two endpoint permeability values, intactness ratio, is observed to decrease with the increase in the permeability modulus, indicating permeability enhancement due to pressure buildup in the caprock. We identified that the temporal change in the spatially variant permeability field due to stress sensitivity is negligible. The results revealed that all averaging methods underestimate the pressure inside the caprock compared to a spatially variant case. This underestimation is minimum at the interface of caprock and monitoring formation for the harmonic average. The pressure evolution in the monitoring aquifer shows an overestimation of pressure when arithmetic and geometric average permeabilities are considered, while the results obtained using a harmonic average are similar to those of the spatially variant case. The reported work is unique as it accounts for pressure diffusion through preexisting and induced fractures and provides a coupled solution for pressure evolution in monitoring aquifers. This analytical model can be extended for double porosity formations.

Integrating underground gas storage experience for CCS optimisation

Carbon capture and storage (CCS) sites require diligent assessments to account for potential leakage over time. Leakage pathways include both natural and engineered (e.g. wells) invoking the requirement for a structured technical analysis as part of a risk inventory. This paper focuses on lessons learnt from underground gas storage (UGS) sites to guide CCS technical workflows to handle natural geological risk elements. UGS facilities have been operating for over a 100 years. Their injection and withdrawal histories, by their very nature, test the complex changes that occur to the reservoir and cap-rock over time and provide valuable insight to demonstrate the feasibility of CCS and its potential for long-term storage. Geomechanical assessment of the reservoir and cap-rock requires an inventory of core material, log and seismic data with the aim of selecting the appropriate data to model the ability of that formation to contain a column of CO2. As supercritical CO2 is injected into the formation and pore pressure builds, the geomechanics of the reservoir needs diligent review to assess the potential to trigger failure on the cap-rock and pre-existing faults and fractures. UGS injection/withdrawal histories assist in the reservoir and overburden static and dynamic modelling along with designing a measurement, monitoring and verification process for CCS sites. Technical workflows, from seismic and static modelling to geomechanics and dynamic simulation and back to seismic for containment monitoring, are discussed that have been refined by UGS experience to benefit the selection of potential CCS sites.

Enabling site-specific well leakage risk estimation during geologic carbon sequestration using a modular deep-learning-based wellbore leakage model

Amid growing climate concerns, geologic carbon sequestration (GCS) is a promising technology for mitigating net carbon emissions by storing CO2 in reservoirs. Oil and gas brownfields are an attractive option for CO2 storage, but these sites have many historical wellbores from petroleum production that can provide potential leakage pathways for CO2 or formation brine. Therefore, risk management of GCS operations requires an assessment of potential well leakage. Due to the high uncertainty of subsurface systems, stochastic approaches are ideal for quantifying the range of risk behaviors, but they must be computationally efficient in the face of complex physics. Here, we develop a new deep learning wellbore model to predict the leakage of CO2 and brine through wellbores. Full physics numerical simulations were used to generate data sets. A complex regression problem was divided into sub-problems of classification and regression to improve model performance. Feature analysis quantifies the impact of each feature on model prediction and enables us to select only effective features for model training. The model shows high predictive performance across a wide range of geologic and injection conditions and well attributes. A case study illustrates how the model is applied to assess well leakage in GCS operations.

A semi-analytical model for multi-well leakage in a depleted gas reservoir with irregular boundaries

Depleted gas reservoirs are potential sites for CO2 geo-sequestration (CGS). Improperly sealed wells may provide leakage pathways and increase the risk of environmental contamination. A fast and efficient method for injection performance monitoring and leakage risk assessment is essential for implementing CGS projects. This paper presents a new semi-analytical model for predicting the transient behavior of the injection pressure and leakage rate during CO2 injection. The proposed model considers the leakage through multiple abandoned wells and the effect of boundary irregularities on the injection performance. First, the CO2 seepage model for the depleted gas reservoir and the overlying aquifer is derived. Then, the boundaries are discretized into segments, given the boundary irregularities. Using the boundary element method (BEM), Laplace transform, and Duhamel’s principle, the mathematical model is solved, and the solutions are compared with those from numerical simulations for verification. Based on a case study of the Dongfang 1-1 gas field, the impacts of the parameters of the reservoir and leaky well on the injection performance and leakage rate are analyzed.The proposed model has a good fit with the results of the numerical simulations. The sensitivity analysis demonstrates that the cumulative leakage ratio increases linearly with the upper limit of injection pressure and reservoir permeability. The leakage risk induced by the leaky wells is relatively low in the reservoir with low permeability because of high seepage resistance and limited pressure propagation. However, the storage potential is restricted due to the poor injectability. The rate of CO2 inter-layer flow is determined by the leakage permeability of all of the abandoned wells, not just those with a high leakage permeability. Reservoirs with multiple abandoned wells and a large overlying aquifer are still at the risk of gas leakage even if the leakage permeability of the abandoned wells is low.

Comparing air sealing techniques for suite compartmentalization in a multi-unit residential building: Aerosolized sealant vs. conventional air sealing approaches

Recently, the use of aerosolized sealant has been proposed as an alternative to conventional (manual) air sealing approaches for suite compartmentalization in multi-unit residential buildings (MURBs). With this approach, aerosolized sealant is released into a positively pressurized zone; the resulting pressure differential across the zone partition draws the aerosolized product to the leakage paths where the particles accumulate until the leakage path is fully sealed. In theory, aerosolized sealants are more effective than conventional air sealing, as their installation does not require manual identification of leakage pathways. However, prior studies assessing the efficacy of aerosolized sealants have typically not included control groups, making it difficult to quantitatively compare aerosolized sealants against conventional techniques. This paper compares the performance of conventional air sealing approaches against aerosolized sealant (for suite compartmentalization) in a newly constructed MURB, in Toronto, Canada. Compartmentalization was assessed using unguarded, suite-level blower door tests. On average, suites sealed using aerosolized sealant were 27% tighter than those sealed using conventional approaches (q50 = 0.22L/s/m2 of suite surface area (0.04cfm50/ft2) vs. 0.30L/s/m2 (0.06cfm50/ft2), N = 5 suites per test group) – a statistically significant difference (t(8) = -2.01, p = 0.04). Guarded floor-level air leakage testing revealed that aerosolized sealant was most effective for leakage paths on vertical surfaces and had no impact on floor/ceiling airtightness. Apart from quantitative system performance, this paper discusses advantages and disadvantages of aerosolized sealants, based on lessons learned from the case study building. These results can help building designers and contractors weigh the relative benefits of using aerosolized sealants for suite compartmentalization.

Flow regime controls on resin repair material invading a microannulus

In wellbores, the microannulus between the steel and the cement has been shown to be a critical leakage pathway. A wellbore microannulus offers unique roughness, aperture distribution, and wetting conditions, which are critical in understanding and explaining the potential sealing efficiency of a repair material. A polymer-based repair material was injected in a horizontal microannulus to displace water and air at different injection velocities. We studied the flow regimes generated in terms of the capillary number (ratio of viscous to capillary forces) and the mobility ratio (ratio of viscosities), and observed capillary fingering, compact displacement, and a transition between these two regimes. The experiments revealed significant differences in the repair material saturation, and thus the repair efficiency, as a function of the capillary number and the mobility ratio in a small microannulus (∼50 μm), and qualitatively and quantitatively suggests the importance of considering these variables when repairing a wellbore microannulus. Compact displacement was obtained when the repair material was displacing air at a high capillary number, with apparent saturation at breakthrough of ∼90%. For resin displacing water in a microannulus, a low injection velocity facilitated preferential flow through the larger apertures of the microannulus, yielding low apparent resin saturation at breakthrough. Compact displacement was not obtained for resin displacing water, but relatively large Sapp values were obtained at breakthrough (∼80%), highlighting the importance of the mobility ratio.

Pore-scale spatiotemporal dynamics of microbial-induced calcium carbonate growth and distribution in porous media

The naturally occurring bio-geochemical microbial-induced calcium carbonate precipitation (MICP) process is an eco-friendly technology for rehabilitating construction materials, reinforcement of soils and sand, heavy metals immobilization and sealing subsurface leakage pathways. We report pore-scale spatiotemporal dynamics of the MICP process in porous media, relevant for reduced environmental risk by leakage during CO2 geological storage. Effects of hydrodynamics and supersaturation on the MICP with Sporosarcina pasteurii stains were studied using a high-pressure, rock-on-a-chip microfluidic device. Bacterial cell numbers and variation in cementation concentration controlled the crystal size and pore-scale distribution by influencing the local supersaturation. Local pore structure determined crystal nucleation, where low velocity regions tended to nucleate more crystals. CaCO3 crystallization was observed at subsurface pressure (100 barg) with a reduced sealing performance due to the low microbial activity from elevated pressure. We identify that hydrodynamics and supersaturation determine crystal nucleation and growth in porous systems, providing important experimental evidence for subsurface environmental applications and validation of upscaled MICP models.