Lifetime Of Reservoir Research Articles

Geothermal system lifetime with fracture heterogeneity and thermal nonequilibrium impact

Geothermal energy is a clean and sustainable alternative energy, and enhanced geothermal systems are an essential technology offering an industrial-scale method to tap geothermal energy. This technology stimulates numerous reservoir fractures, creating primary flow channels for fluids to extract heat. The important question of how the permeability of fracture swarm evolves during fluid circulation and how this evolution potentially affects the geothermal system life remains to be further answered. To address this, we develop and validate a fully coupled thermo-hydro-mechanical model based on the discrete fracture network. Results show a positive feedback mechanism between pore pressure/temperature changes and fracture permeability evolution, leading to monotone permeability-increasing in a limited number of specific fractures, indicating uneven permeability evolution of fracture swarm is crucial in shortening reservoir lifetime. Furthermore, we conduct a comparative analysis of multiple factors to determine what most significantly affects reservoir lifetime, including fracture heterogeneity, the local thermal non-equilibrium effect, and layout strategies. Results indicate the strong heterogeneity of the fracture network is dominant, and the inherent heterogeneity of hydraulic fractures is further exacerbated during geothermal production. Neglecting fractures’ mechanical response leads to an overestimated outcome. A simplified thermal-hydraulic analysis indicates a 34 % higher temperature and a 6 % higher heat extraction rate than the coupled thermo-hydro-mechanical modeling. In contrast, the impact of the local thermal non-equilibrium effect is negligible, with comparable results between equilibrium and non-equilibrium models. We further find the multi-well layout can effectively suppress the heterogeneity impact and prevent preferential flow paths. Compared to dual-well systems, the multi-well scheme has a 24 % higher temperature and a 6 % higher heat extraction rate.

Estimating future bathymetric surface of Kainji Reservoir using Markov Chains and Cellular Automata algorithms

The menace of sedimentation to reservoirs has a significant implication for water quality, storage capacity and reservoir lifetime. Rainfall patterns and other anthropogenic and environmental impacts alter the erosion rate and, by extension, directly affect sedimentation rates if left unchecked. This research focused on using the integration of Markov Chains and Cellular Automata (MC – CA) models to estimate and forecast the future bathymetric surface of the Kainji reservoir in Nigeria for the year 2050. The bathymetric datasets used for this research comprise two different epochs (1990 and 2020). The datasets were acquired using a Single Beam Echosounder at Low and High frequencies of 20 kHz and 200 kHz. The preliminary investigation revealed that sedimentation is exacerbating a greater danger to the reservoir functionality. The results show that the maximum observed depth is 71.2 m, indicating a 7.53% loss in depth from the 1990 archived data and a 16.24% depth loss to sedimentation from 1968 to 2020 and 22.35% depth loss in the year 2050 as shown on the projected surface. Consequently, the integrated model (MC and CA) efficiently predicted the future bathymetric surface of the Kainji reservoir for the year 2050 based on the data characteristics. However, the proven techniques for analysing spatial data, such as the Markov Chain and Cellular Automata, best suited for analysing categorical transition data, show some artefacts (black spots) on the projected generated map which is subject to further investigation.

Field monitoring and modelling of sediment transport, hydraulics and hydroabrasion at Sediment Bypass Tunnels

Sediment Bypass Tunnels (SBTs) are proven to be an effective measure to reduce or even stop reservoir sedimentation by bypassing sediment laden flows around reservoir dams to the downstream river reach. They are mostly used in Switzerland, Japan, and Taiwan. However, hydraulic and sedimentological operating conditions and the resistance of the invert materials against hydroabrasive erosion affect their cost-effectiveness. Hydroabrasion is a pressing issue at SBTs, other hydraulic structures and steep bedrock rivers exposed to high sediment transport rates under supercritical flow conditions. The present study was therefore conducted to address this issue by aiming at improving knowledge on abrasion mechanics and calibrating a mechanistic saltation abrasion model enhanced by Demiral-Yüzügüllü (2021). To this end, the abrasion resistance of fourteen different invert materials installed at Solis, Pfaffensprung and Runcahez SBTs in Switzerland was quantified by annual 3D laser scanning and the hydraulic conditions and sediment transport rates were regularly monitored between 2017 and 2021. The analysis of invert scans and hydraulic conditions revealed that Prandtl’s first and second kinds of secondary currents occurring in the bends and straight sections of the SBTs, respectively, and the observed abrasion patterns were strongly interrelated. The tested potassium aluminate cement and steel fibre concretes, granite, cast basalt and steel plates had better abrasion resistance against impact of sediment-laden flows compared to other materials. Sediment mineralogical composition i.e., bulk hardness relative to the invert material properties significantly affected hydroabrasion. The enhanced abrasion prediction model was calibrated with the present data and a quasi-constant abrasion coefficient of kv = (4.8 ± 2.2) × 104 was obtained. The enhanced model is well-suited for both laboratory and field scales. The present findings will contribute to the sustainable utilization and operational safety of hydraulic structures, optimization of SBT and reservoir operations regarding bypassing efficiency and reservoir lifetime and modelling of bedrock river erosion.

Numerical investigation of electricity generation potential by water circulating through three horizontal wells at Fengshun geothermal field in China

Fengshun geothermal field is the first medium and low temperature geothermal area in China, which is used for electricity generation. At present only vertical wells are employed to extract heat energy. After about 40 years of exploitation, the production temperature and rate are all declined. In this work, based on the hydrogeological data of the study area, the deep reservoir temperature and corresponding circulation depth of underground hot water are predicted with a silica thermometer. To fully develop the deep heat reservoir, we proposed a new heat exploitation scheme through three horizontal wells, computed the electricity generation potential and efficiency, and analyzed the main factors affecting heat production. The results indicate that under the reference conditions, the three horizontal well system attaches an electric power of 2.70–1.5 MW, a reservoir impedance of 0.13–0.27 MPa/(kg/s), a pump power of 0.29–1.16 MW, and energy efficiency of 7.34–1.30 during the reservoir lifetime of 43.6 years. Thus, this three-horizontal-well system has significant development potential in Fengshun deep geothermal reservoir. Sensitivity analysis indicates that water production rate, injection temperature and initial reservoir temperature have a significant impact on heat production performance. Reducing the water production rate and injection temperature within a certain range can improve energy efficiency. For the precise design of power generation systems and the accurate evaluation of production performance, field logging data is necessary.

Assessment of Sediments’ Transport Triggering Processes through the Identification of Deposition Shapes in Large Reservoirs

Sediment deposition at the bottom of artificial reservoirs has become a worldwide problem. This comprises a dual issue that is, in the first place, associated with the reduction in storage capacity and lifetime of large reservoirs. The second aspect comprises the threat that the sediment represents for the dam structure. This research is mainly aimed at identifying and inferring the main sediments’ triggering processes through a rigorous analysis of deposition shapes in a large reservoir. For identifying the main deposition shapes, a sequential methodology was designed and developed comprising the following stages. First, an analysis of XYZ cartography from bathymetric development was conducted. Then, a shapes categorization was developed that comprises the identification of six types of shapes based on four parameters: slope continuity, slope break, absolute and relative slope, and arc configuration. The third stage comprised a visualization and spatial calculation of shapes through GIS-based cartography. The fourth stage comprised an interpretation of deposition shapes processes: for that, a dual analysis was developed. First, an analysis based on fluvial sediments transport processes was realized. The second stage implied an analysis of the dam influence on fluvial hydrodynamics and sediments transport. Results comprised a quantitative assessment of each shape as well as physical processes identification and interpretation, generating a robust equivalence between shapes and triggering processes. This research proved successful for the identification and characterization of the main deposition and transport processes that may help to prevent, palliate, and/or correct phenomenon of silting in large reservoirs. This detailed knowledge of deposition forms opens new strategies to release sediments from storage water more effectively.

Experimental study of the change of pore structure and strength of granite after fluid-rock interaction in CO2-EGS

Deep geothermal energy plays an essential role in the future renewable energy supply. To investigate the mechanism of fluid-rock interaction on the pore structure and the strength of granite in various zones of CO2-EGS, this paper carried out experiments on the fluid-rock interaction at 240 °C and 37 MPa for the three zones of CO2-EGS. Experiments were conducted to test and analyze the elemental composition, surface micromorphology, pore structure, P-wave velocity, and mechanical strength of the granite before and after the fluid-rock interaction. The results show that the water/ScCO2 dissolves the albite and potassium feldspar in the granite, and the carbon in ScCO2 may be sequestered in rock as carbonate mineral precipitates. The injection of ScCO2 fluids increases the pore volume of the rock by 43%, 65%, and 45% in the presence of no water, a small amount of water, and a large amount of water, respectively. There is no chemical reaction, but only physical interaction, between ScCO2 and granite when no water is involved in the core zone of CO2-EGS. The dissolution and precipitation are slight when there is limited water in the rock of surrounding zone, and the P-wave velocity and uniaxial compressive strength also only decrease by ignorable amounts. Only with sufficient water in the outer zone, the ScCO2 fluid can significantly change the uniaxial compressive strength and elastic modulus of rock sample. The findings indicate that the CO2-EGS won't suffer from the blockage of mineral precipitation, decrease of permeability and short service lifetime of geothermal reservoir, which are big problems in conventional water based EGS.

The Risk-Level Change of Dam Break Due to the Population Growth in the Dam Downstream

This research intends to evaluate the risk-level change of dam break due to the number of population growth rates in the dam downstream. The number of population growth rates in the dam downstream area causes the number of populations that is impacted by disaster risk. In this research, there is conducted the danger class classification of dam failure based on the increasing number of population growth rates. In this research, there is conducted a class classification about the dam failure based on the increasing of population growth rate. The methodology is started from data collecting such as PMF design flood that is used for analyzing the dam break to be obtained the flood boundary of the dam downstream area, Map of the Digital Elevation Model that is for knowing the topography of the dam downstream area, population data which is obtained from BPS that is for knowing the population growth rate so it can predict the population number until the life time of the reservoir, and the land use data that is for knowing the population growth rate in the downstream dam area so it can be known the development of residence until the life time of reservoir. Then there is carried out the evaluation and assessment, which is based on the number of risk populations. The result show that there are 6 dams with very high danger level of the downstream damage level and the classification assessment of dams is extreme such as the dams of Bintang Bano, Rotiklot, Napungette, Lolak, Kuwil, and Pandanduri. However, the other four dams also show a very high danger level but in the assessment classification of dam is high (not extreme) such as the dams of Batu Nampar, Kengkang, Sepit, and Jangkih Jawa. The result is hoped can predict the danger classification of dam failure during the life time of the reservoir. Additionally, it can give input to the policymaker in determining the risk classification of dam failure. Moreover, it can be as one of the inputs in developing the risk classification model of dam failure.

Assessment of Reservoir Sedimentation and Mitigation Measures using 2D Hydrodynamic Modeling: Case Study of Pandanduri Reservoir, Indonesia

In this paper, the accumulated sediment in the Pandanduri Reservoir, Indonesia with a storage capacity of 27 million m3 was evaluated and estimated by means of a 2D numerical model (MIKE 21 FM). This model was employed to simulate the sediment transport along the river into the reservoir with a total modeled area of 3.15 km2 and 5 km of the tributary river. The field measurement and sediment sampling in the tributary river were conducted in order to analyze the sediment parameters and later used for the calibration of the numerical model. According to the computational results, the current sedimentation rate was 53,075 m3/year, which came from the main tributary river (Suradadi River). One of the proposed alternatives was to design a check dam structure downstream of the Suradadi River so that the sediment transport continuity could be interrupted, thus reducing the river sedimentation rate. Based on the numerical results, adding a check dam upstream of the reservoir could reduce the sedimentation rate of up to 30%. Therefore, the reservoir sedimentation rate reached 34,690 m3/year due to the sediment trapping at the upstream river. The 2D model also showed the sedimentation pattern in the reservoir, where most of the sediment accumulated in the middle of the reservoir near the spillway. In conclusion, it is recommended to combine the check dam structure with mechanical excavation and flushing to eliminate the deposited sediment and increase the reservoir lifetime.

Development of a new approach using an artificial neural network for estimating oil formation volume factor at bubble point pressure of Egyptian crude oil

Understanding the phase behavior and volumetric changes of reservoir fluids throughout the extent of reservoir lifetime are crucially required for effective-commercial oil recoveries. Ideally, reservoir fluid properties are experimentally measured by laboratory experiments known as PVT tests nonetheless, these tests are prohibitive, time-consuming, and required to restrict sampling and transporting procedures. For these discernible reasons, several modeling approaches have been developed. By reviewing the literature, one crucial obstacle that encounters field applicability of most extant models is the selection of input variables. Moreover, a great percentage of extant models employ the results of lengthy experimental tests such as differential gas solubility or even the sample’s chemical composition. Replicability of models’ results using different datasets is also one of the main challenges when employing AI models. Frequently, inadequate descriptions for AI models have been provided in many studies which limits their utility. The inadequate description includes the analysis of ANN model weights and biases besides, the final mathematical model. In this study, a rigorous ANN model with its mathematical model has been implemented to predict oil formation volume factors based on 600 compiled datasets from Egyptian oilfields.A detailed comparison between widely used empirical correlations and the proposed new ANN model is provided in this study. Statistical and graphical analysis depicted the outstanding performance of the new model with R2 = 0.974, ARE = −0.0017%, and AARE = 2.13%. The ANN model provides remarkable sustainable performance compared to local Egyptian empirical correlations and all the other global models.

Assessment of current reservoir sedimentation rate and storage capacity loss: An Italian overview

Sedimentation has a prominent impact on the functionality and lifetime of reservoirs and is a growing concern for stakeholders. Various parameters influence sedimentation caused by soil erosion. Here we have examined fifty Italian reservoirs to determine sedimentation rates and storage capacity loss. The reservoirs studied have an average age of 78 years as of 2021, with the highest loss of capacity observed, equal to 100%, for Ceppo Morelli. For the fifty Italian catchments covering north, south, central and islands of Italy, we found the mean annual sediment yield varying between 17–4000 m3/km2. year. Six of fifty reservoirs studied (Quarto, Colombara, Ceppo Morelli, Fusino, Vodo and Valle di Cadore) are already in a very critical situation in terms of storage capacity loss. Out of the fifty reservoirs, half of them will reach their half-life year by 2050. For example, for the Fusino reservoir located in northern Italy, we observed a loss of 90% of the storage volume as of 2020 with respect to its operation year 1974, compared to 6% in 2015 as available in literature. Modelling the sediment delivery ratio (SDR) is an open question, due to the lack of adequate data and uncertainties about the variability in hydrological, geomorphological, climate and landcover parameters. Here, we addressed the issue with a simplified multiple regression approach based on sediment delivery ratio values retrieved by the RUSLE model. We found different multi regressions for reservoirs belonging to the Alpine and Apennine regions. This analysis offers a starting point for the management and prioritization of adaptation and remediation policies necessary to address reservoir sedimentation.

Influential factors on the development of a low-enthalpy geothermal reservoir: A sensitivity study of a realistic field

A realistic deep low-enthalpy geothermal reservoir based on real data with high detail and complicated sedimentary structure is utilized to perform sensitivity analyses of the geological features influencing reservoir properties. We perform simulations using the Delft Advanced Research Terra Simulator (DARTS). Compelling numerical performance of DARTS makes it suitable for handling a large ensemble of models including efficient sensitivity and uncertainty analyses. The major finding is that shale facies, generally ignored in hydrocarbon reservoir simulations, can significantly extend the predictive lifetime of geothermal reservoirs exploited by deep well doublets. It is important to accurately account for the shale facies in the simulation, though with an additional computational overhead. The overburden layers can improve doublet performance, but the impact depends on reservoir heterogeneity. In addition, heterogeneity will also divert the flow path with even a minor shift in the well placement. The discharge rate, an essential parameter of geothermal operation strategy, inversely corresponds to the doublet lifetime but positively correlates with the energy production for studied parameter ranges. Low sensitivity of doublet lifetime to vertical-horizontal permeability ratio and permeability-porosity correlation is observed. All these systematic findings for a realistic geothermal field with characterization at unprecedented level of detail can help to provide a general guideline for forward simulation and farther improve the profitability of geothermal energy production in realistic deep geothermal reservoirs through computer-assisted modeling and optimization.

Experimental and numerical investigation of seepage and heat transfer in rough single fracture for thermal reservoir

Seepage characteristics and heat transfer efficiency in rough fracture are indispensable to evaluate thermal reservoir lifetime and production performance. This study presents the seepage experiment, experiment and simulation on convective heat transfer in rough fracture for two artificial specimens. The repeatable artificial specimens are prepared by cement mortar. The roughness on the fracture surface is manufactured based on JRC profiles and 3D printing technology. The effects of fracture surface property (including proppant) and specimen temperature on seepage characteristics under confining pressure of 30 MPa are obtained. The convective heat transfer performance is evaluated considering roughness, multiple flow rates and specimen temperatures. Moreover, the effects of roughness on temperature distribution, local heat transfer and flow velocity in two specimens are analyzed and discussed. The main results indicate that the proppant and higher specimen temperature can improve hydraulic aperture and be conducive to seepage in rough fracture. The higher specimen temperature can strengthen thermal convection and conduction. The higher flow rate can extract more heat to develop thermal breakthrough. Furthermore, the anisotropy of roughness can affect heat transfer efficiency by reducing effective heat transfer area. The bulge and groove on rough fracture can interfere with temperature distribution and local heat transfer by controlling retention time of fluid.

Influence of oil field production life on optimal CO2 flooding strategies: Insight from the microscopic displacement efficiency

We investigate the influence of the microscopic displacement processes on optimal gas flooding strategies. We couple a 1-D compositional reservoir model with an economic model of the flooding to assess profitability of the strategies. In general, we aim at the net present value maximization, although the oil recovery and CO2 storage efficiencies are also estimated. Under certain assumptions, we reduce the number of parameters controlling selection of optimal strategy to just a few dimensionless quantities characterizing both physical and economic processes. We show that the production life of oil fields should not be fixed in optimization studies, especially at low oil prices. A significantly larger net present value can be achieved by varying the reservoir lifetime in addition to the injection rates and volumes and other well controls. Herewith, the optimal strategy can differ from that in the case of a presumed production time. We conclude that waterflooding is the optimal recovery method if the injection rate is low, whereas gas (WAG) flooding applied as a primary method and followed by waterflooding is most optimal for large injection rates. Gas flooding applied as the tertiary recovery method is most optimal for an intermediate range of the rates. In the latter case, gas injection should begin much earlier than water breaks through to producing wells. Finally, we investigate how oil price influences the range of parameters suitable for gas injection.

Implementation of comparative detection approaches for the accurate assessment of sediment thickness and sediment volume in the Passaúna Reservoir

Siltation has significant economic and social impacts as it directly reduces the useable amount of water in reservoirs. Giving a solution to the issue of sedimentation is a complicated task and maybe one of the most important engineering and environmental challenges of the 21st century. The deposited volume and the distribution pattern of the sediment are often unknown and not easy to assess. The sedimentation process is highly dynamic, initially due to the hydrological conditions of the incoming rivers, but also due to common internal phenomena like resuspension or density currents. Sediment remediation measures such as mechanical sediment removal or flushing are planned based on the sediment thickness distribution and the overall sediment volume/mass. Often, the sediment thickness is calculated through topographic differencing between the pre-impoundment reservoir lake bottom and the actual lake bottom. However, it is common that the previous depth distribution map is not available or in insufficient quality. In this regard, alternative measurement techniques have to be taken into consideration. In this study, we assessed the best possible approach depending on the characteristics of the sediment and of the reservoir. We combined three different acoustic systems (a multibeam, a sub-bottom profiler, and a single beam dual frequency system) with sediment coring and dynamic free fall penetrometer measurements for an improved assessment of sediment stock and sediment distribution in the Passaúna Reservoir. Our results showed that topographic differencing could not be applied, as the data for the pre-impoundment lake bottom was insufficiently accurate. The parametric sub-bottom profiler could detect the sediment thickness in high accuracy, but significant limitations were recorded in areas with high gas contents. The dual-frequency echosounder derived the sediment thickness with a normalized mean absolute error of 56% due to the high volumetric gas content in the sediment. The dynamic free-fall penetrometer showed satisfying results compared to the other systems. The normalized mean absolute error was 22%, and sediment thickness could be detected in areas with up to 1.8 m of sediment. Sediment coring is also a reliable technique for sediment thickness determination. However, the results showed that if only traditional coring devices are used (gravity corer), the limited penetration depth of the equipment combined with sampling disturbances often prevent a correct assessment of the sediment thickness. The overall results of this study can help for an improved decision-making regarding reservoir management. The accurate assessment of sediment volume and distribution can reduce costs for sediment removal and assist in having a precise overview of the reservoir lifetime.

Oilfield development system optimization under reservoir production uncertainty

An optimization approach is proposed to support the design decisions of floating production systems and their subsea layout, regarding the production and investment planning. The optimization methodology extends a previous approach by including the dynamics and uncertainty of the oil production during the reservoir lifetime, and the associated dynamics of the gas and water production. The new model integrates production forecast with facilities requirements and economic optimization. An uncertainty assessment methodology is provided and applied to the reservoir production including its decline curve. A case study from Brazil waters is presented and discussed to demonstrate the performance of the method applied to a specific production system.

Sand production by hydraulic erosion during multicycle steam stimulation: An analytical study

The production of sand particles associated with the reservoir hydrocarbons becomes one of the most common problems a well of multi cycle steam stimulation may experience during reservoir lifetime. Therefore, it is essential to the investigation of sand particles migration and sand production mechanisms coupled with fluid flow for optimization design of injection and profile control parameters. This paper presents a model based on hydraulic erosion with heat transfer that can analysis release of sand fines under high temperature. The model describes the continuous hydraulic erosion of rock core by the mass conservation equation and particle transport equation, moreover, the sand production and heat transfer are connected through it is coupled with steam injection model. Then, a coupled nonlinear governing equation is derived by using porosity and liquefied sand concentration to evaluate pore enlargement behavior of flow channel and predict sand production behavior under different steam breakthrough channel. Finally, in terms of numerical processing for model, an effectively difference solution method is adopted based on PDE module in COMSOL Multiphysics. The simulation results show that the sand production model can effectively couple heat and sand production, and the relationship between sand production and channeling flow can be studied through seepage process. The degree of steam channeling affects sand production, that is, sand production increases with the increase of the number of steam breakout channels. In addition, it reveals why the sand production can be effectively slowed down by adjusting reasonable injection parameters for multicycle steam stimulation.

Sedimentation and its response to management strategies of the Three Gorges Reservoir, Yangtze River, China

The Three Gorges Dam (TGD), on the upper valley of the Yangtze River, China, is the largest water conservancy and power generation project in the world. Problematic sedimentation in the Three Gorges Reservoir (TGR) is related to the reservoir lifetime, hydropower generation, riverbed erosion, and sustainable development of ecosystems. In this study, measures to holistically and systematically address sedimentation in the TGR are -discussed and demonstrated, including upstream sediment trapping, regulatory optimisation with respect to sediment peaks and reservoir tail sedimentation reduction, and sediment dredging. Owing to upstream reservoir sediment trapping, the incoming sediment load of the TGR has decreased markedly. Results showed that the annual incoming sediment was only 164 million tons, nearly 70% lesser than the preliminary design value. By intercepting the sediment in the upstream reservoirs and conducting sediment peak regulation, the volume and rates of sedimentation in the TGR decreased significantly. Meanwhile, through sediment peak regulation during the flood season, increased volumes of sediment could be discharged downstream, thereby relieving sediment starvation downstream of the TGD. Overall, through adoption of the related sedimentation management strategies, sediment deposition decreased considerably, to approximately 33% of the preliminary design value, thereby successfully addressing the sediment problems in the TGR. Useful sedimentation management strategies learned from the TGR experience can be applied elsewhere to ensure the sustainable use and management of reservoirs.

Heat depletion in sedimentary basins and its effect on the design and electric power output of CO2 Plume Geothermal (CPG) systems

CO2 Plume Geothermal (CPG) energy systems circulate geologically stored CO2 to extract geothermal heat from naturally permeable sedimentary basins. CPG systems can generate more electricity than brine systems in geologic reservoirs with moderate temperature and permeability. Here, we numerically simulate the temperature depletion of a sedimentary basin and find the corresponding CPG electricity generation variation over time. We find that for a given reservoir depth, temperature, thickness, permeability, and well configuration, an optimal well spacing provides the largest average electric generation over the reservoir lifetime. If wells are spaced closer than optimal, higher peak electricity is generated, but the reservoir heat depletes more quickly. If wells are spaced greater than optimal, reservoirs maintain heat longer but have higher resistance to flow and thus lower peak electricity is generated. Additionally, spacing the wells 10% greater than optimal affects electricity generation less than spacing wells 10% closer than optimal. Our simulations also show that for a 300 m thick reservoir, a 707 m well spacing provides consistent electricity over 50 years, whereas a 300 m well spacing yields large heat and electricity reductions over time. Finally, increasing injection or production well pipe diameters does not necessarily increase average electric generation.

The role of fluid chemistry on permeability evolution in granite: Applications to natural and anthropogenic systems

Efforts to maintain and enhance reservoir permeability in geothermal systems can contribute to sourcing more sustainable energy, and hence to lowering CO2 emissions. The evolution of permeability in geothermal reservoirs is strongly affected by interactions between the host rock and the fluids flowing through the rock's permeable pathways. Precipitation of secondary mineral phases, the products of fluid-rock interactions, within the fracture network can significantly reduce the permeability of the overall system, whereas mineral dissolution can enhance reservoir permeability. The coupling between these two competing processes dictates the long-term productivity and lifetime of geothermal reservoirs. In this study, we simulate the conditions within a geothermal system from induced fracturing to the final precipitation stage. We performed batch and flow-through experiments on cores of the Carnmenellis granite, a target unit for geothermal energy recovery in Cornwall (UK), to understand the role of mineral dissolution and precipitation in controlling the permeability evolution of the system. The physico-chemical properties of the cores were monitored after each reaction-phase using ICP-OES, SEM, hydrostatic permeability measurements, and X-ray Computed Tomography. Results show that permeability evolution is strongly dependent on fluid chemistry. Undersaturated alkaline fluids dissolve the most abundant mineral phases in granite (quartz and feldspars), creating cavities along the main fractures and generating pressure-independent permeability in the core. Conversely, supersaturated alkaline fluids, resulting from extended periods of fluid-rock interactions, promote the precipitation of clay minerals, and decrease the permeability of the system. These results suggest that chemical dissolution during geothermal operations could generate permeable pathways that are less sensitive to effective stress and will remain open at higher pressures. Similarly, maintaining the circulation of undersaturated fluids through these granitic reservoirs can prevent the precipitation of pore-clogging mineral phases and preserve reservoir permeability in granite-hosted geothermal systems.

Detection of fossil-fuel CO2\xa0plummet in China due to COVID-19 by observation at Hateruma

The COVID-19 pandemic caused drastic reductions in carbon dioxide (CO2) emissions, but due to its large atmospheric reservoir and long lifetime, no detectable signal has been observed in the atmospheric CO2 growth rate. Using the variabilities in CO2 (ΔCO2) and methane (ΔCH4) observed at Hateruma Island, Japan during 1997–2020, we show a traceable CO2 emission reduction in China during February–March 2020. The monitoring station at Hateruma Island observes the outflow of Chinese emissions during winter and spring. A systematic increase in the ΔCO2/ΔCH4 ratio, governed by synoptic wind variability, well corroborated the increase in China’s fossil-fuel CO2 (FFCO2) emissions during 1997–2019. However, the ΔCO2/ΔCH4 ratios showed significant decreases of 29 ± 11 and 16 ± 11 mol mol−1 in February and March 2020, respectively, relative to the 2011–2019 average of 131 ± 11 mol mol−1. By projecting these observed ΔCO2/ΔCH4 ratios on transport model simulations, we estimated reductions of 32 ± 12% and 19 ± 15% in the FFCO2 emissions in China for February and March 2020, respectively, compared to the expected emissions. Our data are consistent with the abrupt decrease in the economic activity in February, a slight recovery in March, and return to normal in April, which was calculated based on the COVID-19 lockdowns and mobility restriction datasets.