Field-scale Model Research Articles

Effect of chemical corrosion on propagation of complex fracture networks under different hydraulic pressures in enhanced geothermal systems

Chemical corrosion and hydraulic fracturing are important ways to improve the permeability of the enhanced geothermal systems. In this work, we developed a discrete element code for simulating the hydraulic fracturing of three different fracture network models after chemical corrosion. Mechanisms of the crack extension and coalescence in the field-scale models with fracture network were systematically revealed, in which the effects of corrosion time, inclination of the fracture (γ), pH value and hydraulic pressure were considered. The results show that the simulated cracking behaviors of granites agree well with those obtained through Brazilian tests and triaxial compression tests, verifying the validity of the developed discrete element code. During hydraulic fracturing, the impact of γ on the fracture network connectivity increases with the increment of hydraulic pressure (Pin). With the increase in hydraulic pressure, crack propagation accelerates whereas the connectivity of fracture network models decreases. The reasons are that the excessive hydraulic pressure causes the crack to spread in an unfavorable direction and increases the effect of γ on the connectivity of models of fracture networks. As the pH value increases from 2 to 12, the crack growth rate first decreases and subsequently raises. The numbers of cracks per time step of sparse mixed fracture networks with artificial fractures (SM), dense mixed fracture networks with artificial fractures (DM) and dense mixed fracture networks without artificial fractures (NDM) are in the ranges of 0.06601–0.1259, 0.05176–0.2072 and 0.1727–0.6846, respectively. The connectivity of SM is better than that of NDM under the same hydraulic fracturing conditions, indicating that artificial fractures can greatly improve the fracture network connectivity and the utilization efficiency of EGS.

Reduction of Chlorine Disinfection Dosage through Optimal Jet Design in the Hong Kong Harbor Area Treatment Scheme

Chlorine is commonly used in disinfection processes in wastewater treatment plants. The chlorine solution is typically dosed by turbulent mixing through multiport jet diffusers. The chlorine reacts with the inorganic and organic nitrogenous compounds at a fast rate (in the order of 1 s or less), and a significant portion of the dosed chlorine can be rapidly consumed by the reactions instead of destroying the pathogens. Despite extensive previous research, the chlorine consumption in the complex sewage flow remains an elusive problem. Field-scale experiments have been carried out to study the chlorine consumption in the turbulent mixing of dense chlorine jets with the primary treated sewage effluent of the Stonecutter’s Island Sewage Treatment Works (serving a population of 5.7 million). Based on jet theory and the field tests, it is proposed to discharge the chlorine jets at a location of highest velocity to minimize the contact time of the sewage with high concentration chlorine. Full-scale in-plant experiments demonstrated that significant savings in chlorine can be achieved with optimal jet diffuser design and placement; at nominal applied chlorine dosages of 12 and 20 mg/L in winter and summer, respectively, the chlorine demand can be reduced by up to 30% compared with the conventional dosing design. The relatively simple dosing reconfiguration results in significant savings in chlorine and energy, reduces harmful impact to the environment, and improves plant operation reliability. The chlorine demands from daily operations are well-correlated with the results of the field-scale models. Predictions of the chlorine demand as a function of applied dosage and temperature were obtained from the collective field data.

Diffusion Effect on Shale Oil Recovery by CO2 Huff-n-Puff

The CO2 diffusion effect is an important mechanism of injected CO2 penetrating the reservoir matrix and diffusing into the oil phase during the huff-n-puff (HNP) process. However, the CO2 diffusion coefficient of shale is still uncertain under the reservoir conditions, and the contribution of diffusion to enhanced oil recovery (EOR) needs to be further explored. In this study, the CO2 diffusion coefficient of shale is determined under the simulated reservoir conditions (9–10 MPa, 90 °C) in the laboratory, and the determined CO2 diffusion coefficient is applied in the reservoir numerical model to evaluate the contribution of the CO2 diffusion effect to EOR in the HNP process. First, a radial constant volume diffusion (RCVD) device was built to measure the CO2 diffusion coefficient by the pressure decay method. After that, both the analytical solution and the numerical solution are used to ensure that the result is correct. Finally, a field-scale model consisting of one stage of hydraulic fracture in a shale oil reservoir is established. The effects of reservoir properties such as fracture spacing, fracture half-length, and operational properties of HNP like injection rate, injection time, and soaking time on oil recovery were discussed when the diffusion was included and excluded. The CO2 diffusion coefficient of a shale core is ranging from 1.44 × 10–7 to 5.50 × 10–7 cm2/s, and it is reasonable to be applied in the field reservoir numerical simulation model. According to simulation results, the lower fracture spacing and longer fracture half-length make more CO2 diffuse into the shale matrix, which are critical parameters, to promote the CO2 diffusion effect. When the injection rate is slow and the injection time is short, the CO2 diffusion effect in the reservoir may contribute more to EOR. A high injection rate means sufficient pressure supplement, and the CO2 diffusion effect may become relatively negligible. The soaking time may not be necessary for the EOR of HNP, but the longer soaking time will promote the contribution of CO2 diffusion to EOR, and the incremental recovery is about 0.15–0.26%.

Field-Scale Modeling of Interwell Tracer Flow Behavior to Characterize Complex Fracture Networks Based on the Embedded Discrete Fracture Model in a Naturally Fractured Reservoir

Summary In a naturally fractured reservoir, natural fractures can not only provide main paths for fluid flow and increase its permeability but also complicate flow behavior and production performance. Interwell tracer tests have been widely applied to estimate the petrophysical properties; however, limited attempts have been made to accurately identify the natural fracture networks. In this study, the newly proposed numerical models have been verified and used to characterize the fracture distributions in a naturally fractured reservoir conditioned to tracer transport behavior. The stochastic fracture modeling approach is implemented to generate the randomly distributed natural fractures which are dealt with the embedded discrete fracture model (EDFM) while ensuring its sufficient accuracy. To be specific, the matrix domain is discretized using the structured grids, within which each embedded fracture is divided into a series of segments. Subsequently, nonneighboring connections (NNCs) allow us to couple the flow of fluid and tracer between the nonneighboring grid cells, while the historical tracer profiles are matched to delineate the geometry and properties of the fractures by taking multiple tracer transport mechanisms into account. Furthermore, the influences of fracture number, fracture length, fracture orientation, and tracer dispersion on the tracer production concentration have been investigated through sensitivity analysis. The response of an interwell tracer model is sensitive to the fracture parameters rather than tracer properties. A fracture network with its orientation parallel to the mainstream direction will cause the earliest tracer breakthrough. The tracer breakthrough time with an average fracture length of 40 m is 110 days earlier than that with a mean fracture length value of 10 m, while the tracer production peak concentration for the former is nearly two times higher than for the latter. A larger fracture number results in an earlier tracer breakthrough, and an intermediate fracture number will lead to the highest tracer production concentration. Additionally, the newly developed model has been validated through its comparison with the commercial ECLIPSE simulator and then extended to field applications to identify the possible fracture distributions by simulating multiwell tracer tests in the Midale field. The flexible and pragmatic EDFM-based method developed in this study can model the interwell tracer flow behavior as well as characterize the properties and geometries of the natural fractures with better accuracy and calculation efficiency in comparison with other fracture simulation methods (e.g., local grid refinement method).

Calibration and Validation of RZWQM2-P Model to Simulate Phosphorus Loss in a Clay Loam Soil in Michigan

Highlights RZWQM2-P was tested and validated for clay loam soil using daily discharge and load data. The model performed satisfactorily in predicting hydrology and TP load, but DRP prediction was unsatisfactory. Inability of the model to simulate P loss in subsurface drainage discharge after fertilization event was one of the reasons for the unsatisfactory model performance. Abstract. Phosphorus (P) loss and transport through subsurface drainage systems is a primary focus for addressing harmful algal blooms in freshwater systems. The recent development of the phosphorus (P) routine of the Root Zone Water Quality Model (RZWQM2-P) has the potential to enhance our understanding of the fate and transport of P from subsurface-drained fields to surface water. However, there is a need to test the model under different fertilization, soil, climate, and cropping conditions. The objective of this study was to test the model's performance with daily drainage discharge, dissolved reactive phosphorus (DRP), and total phosphorus (TP) load collected from a subsurface-drained field with clay loam soil. We calibrated RZWQM2-P using two years of measured data. Subsequently, we validated RZWQM2-P using a year and nine months of measured data. We used the Nash-Sutcliffe model efficiency (NSE) and percentage bias (PBIAS) statistics for the RZWQM2-P model evaluation. The results showed that the model performance was “good” (daily NSE = 0.66 and PBIAS = -7.16) in predicting hydrology for the calibration period. For the validation period, the hydrology prediction of the model was “very good” (daily NSE = 0.76), but it had a “satisfactory” underestimation bias (PBIAS = 23.57). The model’s performance was “unsatisfactory” in simulating DRP for both calibration (daily NSE = 0.31 and PBIAS = -61.50) and validation (daily NSE = 0.32 and PBIAS = 43.68) periods. The P model showed “satisfactory” performance in predicting TP load for both calibration (daily NSE = 0.46 and PBIAS = -32.41) and validation (daily NSE = 0.39 and PBIAS = 42.90) periods, although both periods showed “unsatisfactory” percent bias. The underperformance may have been due to the model’s inability to partition fertilizer P into different P pools under high water tables or ponding conditions when using daily data. In conclusion, the RZWQM2-P model performed well for drainage discharge with daily data, but further investigation is needed to improve the P component of the model. Keywords: Field-scale modeling, Nutrient load, Phosphorus modeling, Subsurface drainage, Tile drainage, Water Quality.

Probabilistic simulation of phosphorus loss using the Vermont P-index: a bottom-up field to watershed approach

Abstract Anthropogenic nutrient loading from land use, especially agriculture, is a major threat to waterbodies worldwide. Efforts to govern nutrient pollution are increasingly based on simulation modeling for research, evaluation, and regulation. This study develops a novel approach to simulate nutrient losses from agriculture applied to the Lake Champlain basin in the US state of Vermont. The Vermont Phosphorus-Index—a farm-based empirical model regularly used for site evaluation—is scaled up to the basin level with high-resolution geographic data and probabilistic estimation of unknown parameters and management practices. Results are comparable with analyses using more data and computationally intensive tools. Important insights into basin-wide management include: (1) nutrient-management planning can significantly reduce P losses in a livestock-agriculture-dominated watershed by re-distributing manure applications from areas of high loss to low loss; (2) hotspot identification from geographic data alone may be deeply complicated by high underlying heterogeneity of soil phosphorus; and (3) probabilistic modeling using simple, field-scale models is a potentially useful complement to complex watershed process models. Findings suggest that currently available best-management practices will likely be insufficient to reach reduction targets in the most impaired sub-watersheds. Reductions of agricultural land use and herd size, particularly in intensive dairy operations, may be necessary.

Study Evaluates Surfactant EOR From Tight Carbonates

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 209418, “Surfactant Enhanced Oil Recovery From Tight Carbonates: Core-Scale Experiments to Reservoir-Scale Modeling,” by Yue Shi, SPE, and Kishore Mohanty, SPE, The University of Texas at Austin. The paper has not been peer reviewed. _ Most carbonate reservoirs are oil-wet/mixed-wet and heterogeneous at multiple scales. The majority of the injected water flows through the high-permeability regions and fractures and bypasses the oil in the matrix because of high negative capillary pressure (Pc). To enhance oil recovery from such reservoirs, the sign of the Pc should be changed by wettability alteration (WA) or the Pc should be reduced by lowering interfacial tension (IFT). In the complete paper, surfactants that can either alter wettability or develop ultralow IFT were identified through laboratory measurements for the target carbonate reservoir. Introduction Contact-angle, IFT, phase-behavior, and imbibition tests were first performed, and surfactants that can either alter wettability or develop ultralow IFT were identified. Then, a laboratory-scale imbibition model was built with commercial reservoir simulation software. A sensitivity study was performed to evaluate the effects of residual oil saturation (Sor), alteration of Pc and relative permeability (Kr), IFT, and matrix scale on oil recovery by surfactant. Next, the laboratory-scale imbibition model was used to history match the experimental data and parameters were tuned for reservoir-scale simulation. Finally, a 3D layered field-scale model was developed to study the performance of the selected surfactants in injection/soak/production (ISP) tests. Materials Eight anionic surfactants (An 1 through An 8) and two quaternary ammonium cationic surfactants (Cat 1 and 2) were used. Silurian dolomite (SD) outcrop cores were used in this study. Fluid samples were obtained from a West Texas dolomite-rich reservoir. The oil has a density of 0.84 g/mL at 25°C and a viscosity of 7.46 cp at reservoir temperature (35°C). It has an acid number of 0.25 mg KOH/g and a base number of 1.0 mg KOH/g. The reservoir has been waterflooded for years, and total dissolved solids of brine has decreased from 112,668 to 40,393 ppm for recent produced water (PW). The authors detail the methodology for aqueous-stability, contact-angle-measurement, IFT-measurement, phase-behavior, and spontaneous-imbibition tests in the complete paper.

Kinetic Model and Numerical Simulation of Microbial Growth, Migration, and Oil Displacement in Reservoir Porous Media.

Microbial enhanced oil recovery (MEOR) is a potential tertiary oil recovery method. However, past research has failed to describe microbial growth and metabolism reasonably, especially quantification of reaction equations and operating parameters is still not clear. The present study investigated the ability of bacteria extracted from Ansai Oilfield for MEOR. Through core flooding experiments, bacteria-treated experiments produced approximately 6.28–9.81% higher oil recovery than control experiments. Then, the microbial reaction kinetic model was established based on laboratory experimental data and mass conservation. Furthermore, the proposed model was validated by matching core flooding experiment results. Lastly, the effects of different injection parameters on bacteria growth, bacteria migration, metabolite migration, residual oil distribution, and oil recovery were studied by establishing a field-scale model. The results indicate that the injected bacteria concentration and nutrient concentration have a great influence on bacteria growth in a reservoir and the low nutrient concentration seriously restricts bacteria growth. Compared with the injected bacteria concentration, nutrient concentration has a decisive effect on bacteria and metabolite migration. The injected bacteria concentration has little effect on oil recovery, while nutrient concentration and slug volume have a significant effect on oil recovery.

Modeling water and nitrogen dynamics from processing tomatoes under different management scenarios in the San Joaquin Valley of California

Study regionThe Southern San Joaquin Valley (SSJV), California. Four representative soil types were evaluated. Study focusEvaluation of regional scale hydrology and nitrogen (N) fate and transport requires a robust modeling framework that considers critical local scale environmental conditions and crop root zone physical processes. Two widely used yet conceptually and mechanistically different models, HYDRUS(2D/3D) and SWAT (Soil and Water Assessment Tool), were used to investigate water and N dynamics in processing tomato fields across a range of soil types and management scenarios over a 30-year period in the SSJV. New hydrological insights for the regionThis study implements a framework to evaluate a regional scale model against a field scale model and provides a framework for assessing percolation and N transport results from regional scale simulations in the absence of extensive field observations. Field scale models developed in HYDRUS (2D/3D) produced similar results to a regional-scale SWAT model in four specific soils in terms of responses to percolation and N losses as a function of varying N fertilizer and irrigation management practices. This model-to-model agreement at the local scale provides confidence in region-scale hydrological and N fate and transport findings, particularly in the absence of extensive field observations. Both models suggest that promoting conservation practices targeting the optimization of irrigation and fertilizer-N management would have a large impact in protecting regional groundwater quality.

Progress Toward Pilot-Scale Simulation of In-Situ Combustion Incorporating Geomechanics

Summary In-situ combustion (ISC) is a promising thermal enhanced oil recovery method with benefits for deep reservoirs, potentially lesser energy requirements as compared to steam injection, and low opportunity cost. Although successful ISC projects have been developed all over the world, challenges still exist including difficulties in monitoring combustion-front progress in the field, describing multiscale physical processes, characterizing crude oil kinetics fully, and simulating ISC at field scale. This work predicts combustion front propagation and the effect of thermally induced stress at the scale of an ISC pilot project. Reservoir deformation was characterized by a geomechanical model to investigate the correlation of combustion front progress with reservoir and surface deformation. We upscaled the reaction kinetics directly from combustion tube experiments and calibrated the laboratory-scale model compared with experimental measurements. We then upscaled numerical simulation to a 3D geometry incorporating a geomechanical model. The change in scale is significant as the combustion tube is 6.56 ft (2 m) in length, whereas the dimensions of the 3D model are 1,440 ft by 1,440 ft (439 m) by 1,400 ft (427 m). The elastic properties were defined by Young’s modulus and Poisson’s ratio, whereas the plastic properties were defined by a Mohr-Coulomb model. A sensitivity study examined the reliability of the model, showing the reaction progress and geomechanical responses were not significantly impacted by gridblock dimensions and reservoir heterogeneity. Finally, a field-scale model was developed covering an area of 5,960 ft (1817 m) by 4,200 ft (1280 m). We observed successful ISC simulation including ignition as air injection started. The temperature increased immediately to more than 800°C (1,400°F) based on the chemical kinetics implemented. The temperature history indicated that the combustion front propagated from the injection well into the reservoir with an average velocity of 0.16 ft/D (0.049 m/d). A surface deformation map correlated with the progress of ISC in the subsurface. Land surface uplift because of ISC ranges from 0.1 ft (0.03 m) to several feet, depending on the rock properties and subsurface events. This proof-of-concept model indicated strong potential to detect the surface movement using interferometric synthetic aperture radar (InSAR) and/or tiltmeters to monitor dynamically combustion front positions in subsurface.

Mineral surface area accessibility and sensitivity constraints on carbon mineralization in basaltic aquifers

Estimating mineral reactive surface areas in geologic media remains one of the key challenges limiting the accuracy of reactive transport modeling (RTM) predictions of subsurface processes, particularly those controlling the fate of carbon dioxide (CO2) during geologic storage. Although there have been numerous attempts to combine imaging and experimental techniques to estimate mineral reactive surface area for use in RTM predictions of geologic CO2 storage, these techniques have yet to be adapted to basaltic reservoirs, which have pore structure, mineralogy, and chemical composition that is unique compared to their more often-studied sedimentary counterparts. Here, we address this issue by quantifying fluid-accessible mineral surface areas through image analysis of scanning electron microscope (SEM) backscatter electron images (high-resolution 500 nm/pixel) and Raman spectroscopic mapping of a basaltic rock sample from the Eastern Snake River Plain, Idaho. To evaluate whether the determined pore fluid-accessible mineral surface area accurately reflects reactive surface area, a micro-continuum scale RTM was developed and compared with a high-temperature, high-pressure flow-through CO2 mineralization experiment conducted on the characterized basalt. Importantly, simulations employing the image-derived pore fluid-accessible mineral surface areas match the experimental effluent chemistry well within uncertainties. These mineral surface areas were then used to parametrize a field-scale model representative of the Cascadia basin, Northeastern Pacific, to evaluate impacts of surface area variations on mineral carbonation. Simulations were carried out using variations in image-derived surface areas that cover one to two orders of magnitude increase and decrease in surface area, analogous to previously reported magnitudes of difference between total and reactive surface areas. Carbonation efficiency in terms of CO2 volume mineralized over the simulated period was tracked and compared. Simulations with surface area increased and decreased by two orders of magnitude show basalt carbonation efficiency that is three times faster and six times slower, respectively, than predictions with image-derived mineral surface area. These sensitivity analyses demonstrate that accurate quantification of mineral surface area is crucial for efforts to predict CO2 mineralization, and that efforts such as those employed here can dramatically reduce the uncertainty of field-scale predictions of basalt carbonation.

The enriched degree of freedom method for the absorbing boundary and its application to XFEM in elastodynamic problems

For elastodynamic problems, significant effort has been exerted by numerical researchers to eliminate the reflection of outgoing elastic waves at the boundary of the computational domain. Eliminating these reflection is very important for correctly analyzing the dynamic response of various structures. In this paper, the enriched degree of freedom method is first proposed to absorb outgoing elastic waves; degrees of freedom are added to the nodes in the absorbing domain by using a user-defined function. Next, the gradually increasing mass matrix is derived to further dampen the elastic waves based on the theory of the perfectly matched layer. Then, a novel damping method is proposed based on the enriched degree of freedom method, and it is appropriately coupled with the XFEM and the modified Newmark algorithm to solve elastodynamic problems with infinite boundaries. Finally, the numerical results show that the proposed method is feasible and applicable for absorbing the outgoing elastic waves. Importantly, the proposed method is independent of the frequency; it can be used to simulate the field-scale model and the laboratory-scale model, and it is effective in solving problems with multiple cracks.

Assessment of chemo-mechanical impacts of CO2 sequestration on the caprock formation in Farnsworth oil field, Texas

This study evaluates the chemo-mechanical influence of injected CO2 on the Morrow B sandstone reservoir and the upper Morrow shale caprock utilizing data from the inverted 5-spot pattern centered on Well 13-10A within the Farnsworth unit (FWU). This study also seeks to evaluate the integrity of the caprock and the long-term CO2 storage capability of the FWU. The inverted 5-spot pattern was extracted from the field-scale model and tuned with the available field observed data before the modeling work. Two coupled numerical simulation models were utilized to continue the study. First, a coupled hydro-geochemical model was constructed to simulate the dissolution and precipitation of formation minerals by modeling three intra-aqueous and six mineral reactions. In addition, a coupled hydro-geomechanical model was constructed and employed to study the effects of stress changes on the caprock’s porosity, permeability, and ground displacement. The Mohr–Coulomb circle and failure envelope were used to determine caprock failure. In this work, the CO2-WAG injection is followed by the historical field-observed strategy. During the forecasting period, a Water Alternating Gas (WAG) injection ratio of 1:3 was utilized with a baseline bottom-hole pressure constraint of 5500 psi for 20 years. A post-injection period of 1000 years was simulated to monitor the CO2 plume and its effects on the CO2 storage reservoir and caprock integrity. The simulation results indicated that the impacts of the geochemical reactions on the porosity of the caprock were insignificant as it experienced a decrease of about 0.0003% at the end of the 1000-year post-injection monitoring. On the other hand, the maximum stress-induced porosity change was about a 1.4% increase, resulting in about 4% in permeability change. It was estimated that about 3.3% of the sequestered CO2 in the formation interacted with the caprock. Despite these petrophysical property alterations and CO2 interactions in the caprock, the caprock still maintained its elastic properties and was determined to be far from its failure.

Modeling and assessment of CO2 geological storage in the Eastern Deccan Basalt of India.

In this study, the CO2 carbonatization potential of the Deccan basalt formation in Eastern India is evaluated by establishing a hydro-chemical field-scale model based on the geological, hydrological, and geochemical parameter of the basalt in the Mandla lobe. The reliable initial mineral thermodynamic parameters are obtained by validating the laboratory scale experiment of CO2-water-basalt reaction with a numerical method. Over 50% of injected carbon mineralized within 140days for the Deccan basalt in the Mandla lobe, and the majority of CO2 is sequestered as ankerite, siderite, and calcite, which occupy a percent of 65%, 28%, and 7%, respectively. Clay minerals, including smectite and chlorite, are important secondary minerals contributing to the process of CO2 storage in the basaltic reservoir. Clay precipitation can promote the dissolution of silica- and aluminum-rich plagioclase and release Ca2+ to enhance the carbonatization of CO2 to Ca carbonates but competes for Fe2+ and Mg2+ from siderite and magnesite. Clay precipitation also impacts the CO2 carbonatization efficiency by changing the basalt conductivity. CO2 carbonatization efficiency was found to increase with the reduction of injection rate. However, slow flow rate can increase the pore clogging risk and induce large pressure build-up. This is the first field-scale assessment of CO2 mineralization potential of the Deccan basalt, which is one of the largest terrestrial flood basalt formations in the world. The results can provide valuable information and scientific support for India and global carbon mitigation.

Assessment of Sewage Sludge and Coir Pith as Alternative Cover Materials to Red Soil in Bioreactor Landfills

This study assessed the performance of sewage sludge and coir pith as a potential alternative to virgin red soil as cover material in bioreactor landfills. Three field scale models of anaerobic bioreactor landfills were set up so as to study the performance of the cover materials on leachate generation time, quality and quantity. In one reactor (R1), red soil was used as the cover and was kept as control. In the other two reactors (R2 and R3), modified sewage sludge and coir pith were used as the cover materials, respectively. The leachate produced from each were analyzed based on its quality (Total Solids, pH, BOD, COD, Nitrate and Iron), quantity and generation time. Red soil provided respectively 41.08% and 25.5% reduction in BOD than modified sewage sludge and coir pith. However, coir pith showed better performance in reduction of leachate quantity, total solids, nitrate and iron, thus making it suitable for use as an alternate cover material.

Gas diffusion through variably-water-saturated zeolitic tuff: Implications for transport following a subsurface nuclear event

Noble gas transport through geologic media has important applications in the characterization of underground nuclear explosions (UNEs). Without accurate transport models, it is nearly impossible to distinguish between xenon signatures originating from civilian nuclear facilities and UNEs. Understanding xenon transport time through the earth is a key parameter for interpreting measured xenon isotopic ratios. One of the most challenging aspects of modeling gas transport time is accounting for the effect of variable water saturation of geological media. In this study, we utilize bench-scale laboratory experiments to characterize the diffusion of krypton, xenon, and sulfur hexafluoride (SF6) through intact zeolitic tuff under different saturations. We demonstrate that the water in rock cores with low partial saturation dramatically affects xenon transport time compared to that of krypton and SF6 by blocking sites in zeolitic tuff that preferentially adsorb xenon. This leads to breakthrough trends that are strongly influenced by the degree of the rock saturation. Xenon is especially susceptible to this phenomenon, a finding that is crucial to incorporate in subsurface gas transport models used for nuclear event identification. We also find that the breakthrough of SF6 diverges significantly from that of noble gases within our system. When developing field scale models, it is important to understand how the behavior of xenon deviates from chemical tracers used in the field, such as SF6 (Carrigan et al., 1996). These new insights demonstrate the critical need to consider the interplay between rock saturation and fission product sorption during transport modeling, and the importance of evaluating specific interactions between geomedia and gases of interest, which may differ from geomedia interactions with chemical tracers.

Streamline Tracing and Applications in Dual-Porosity Dual-Permeability Models

Summary Streamline-based methods have proven to be effective for various subsurface flow and transport modeling problems. However, the applications are limited in dual-porosity and dual-permeability (DPDK) system due to the difficulty in describing interactions between matrix and fracture during streamline tracing. In this work, we present a robust streamline tracing algorithm for DPDK models and apply the new algorithm to rate allocation optimization in a waterflood reservoir. In the proposed method, streamlines are traced in both fracture and matrix domains. The interfluxes between fracture and matrix are described by switching streamlines from one domain to another using a probability computed based on the ratio of interfluxes to total fluxes within the domain. The approach is fundamentally similar to the existing streamline tracing technique and can be utilized in streamline-assisted applications, such as flow diagnostics, history matching, and production optimization. The proposed method is benchmarked with a finite volume (FV)-based approach where grid-based time-of-flight (TOF) distributions were obtained by solving the stationary transport equation. We first validated our method using simple examples. Visual TOF comparisons as well as tracer concentration and allocation factors at wells show good agreement. Next, we applied the proposed method to field-scale models to demonstrate the robustness. The results show that our method offers reduced numerical artifacts and better represents reservoir heterogeneity and well connectivity with subgrid resolutions. The proposed method is then used for rate allocation optimization in DPDK models. A streamline-based gradient-free algorithm is used to optimize net present value (NPV) by adjusting both injection and production well rates under operational constraints. The results show that the optimized schedule offers significant improvement in recovery factor, NPV, and sweep efficiency compared to the base scenario using equal rate injection and production. The optimization algorithm is computationally efficient, as it requires only a few forward reservoir simulations. A novelty of this work is the streamline tracing algorithm in DPDK reservoir models. The proposed method is simple and easy to implement to existing streamline-based frameworks and is applicable to field-scale reservoir models for flow diagnostics and rate allocation optimization.

Influence of backfill type on the load-bearing performance of GRS bridge abutments

The influence of backfill type and material properties on the performance of field-scale geosynthetic reinforced soil (GRS) abutment models is investigated. Two alternative types of backfill as recommended in the Federal Highway Administration (FHWA) guidelines (called open-graded and well-graded) were used to build two field-scale model abutments and compare their load-bearing performance under a loading beam. Results are presented and discussed relative to the loading beam settlement, facing deformation and reinforcement strains. The well-graded backfill was found to result in smaller beam settlements and facing lateral deformations, especially at smaller loads that were comparable to service load levels. However, it was significantly faster and easier to compact the open-graded aggregate to the unit weight recommended in the guidelines. Nevertheless, performances of both abutment models were found to be satisfactory relative to the limiting requirements on the beam settlement and facing deformations at service load levels.

Automatic History Matching of Parent-Child Wells in Shale Gas Reservoirs with Complex Fracture Hits

Abstract It is excessively crucial for performing production match of parent-child wells automatically by using assisted history matching approach, especially considering the interwell interference scenario. We explored a new EDFM-AI (Embedded Discrete Facture Model-Artificial Intelligence) workflow by taking into account the multiple uncertainty parameters that were composed of fracture height, fracture half-length, fracture conductivity, fracture water saturation, fracture cluster efficiency, and natural fracture conductivity. The history matching algorithm implemented in this study is advanced machine learning model called XGBoost. It could overcome the issue that the proxy model is always imprecise due to the limited training sample data. Specifically, we established a field-scale reservoir model that includes parent-child shale gas wells with fracture hits in the Sichuan basin. By using this sophisticated workflow, two wells’ history matching results are excellent in terms of the inversed solution of the hydraulic fracture properties and natural fracture properties. And then, we perform the production forecasting for parent and child wells, respectively. Consequently, the results show that the best match value for estimated ultimate recovery (EUR) of parent well is 287.4 million cubic meters whereas the best match value for EUR of child well is 187.6 million cubic meters. The reliable reason is that effective fractured area of parent well is larger than that of child well. Besides, the natural fractures have a significant impact on the performance of shale gas wells with fracture hits observed, especially in the short-term production period.

Improving salt leaching efficiency of subsurface drainage systems using low-permeability surface mulch

Surface ponding is often used in combination with subsurface drainage systems to leach excessive salts from the saline soils of reclaimed coastal areas. However, soils that are not immediately proximal to subsurface drains are typically subjected to reduced leaching efficiency (LE). This study evaluates an approach for increasing the LE through the use of a low-permeability surface mulch (LPSM) above the drain. Sand tank experiments and 2D numerical simulations were conducted to examine the effectiveness of LPSM configurations under continuous surface flooding. The results show that the addition of a LPSM leads to more spatially uniform, accelerated salt leaching towards subsurface drains. As a result, base case leaching periods and freshwater application rates were reduced by up to ∼60% and ∼80%, respectively. The sensitivity analysis based on field-scale models revealed optimal values (i.e., leading to maximum LE) of the LPSM hydraulic conductivity and length. Moreover, an approximate method is applied to estimate the leaching period, and to thereby assist in the design of drainage-LPSM schemes. We conclude that the addition of a LPSM is an effective strategy to enhance salt leaching under surface ponding, adding to existing methods for ameliorating saline soils.