Increase In Pore Pressure Research Articles

Projecting seismicity induced by complex alterations of underground stresses with applications to geothermal systems

Seismicity associated with subsurface operations is a major societal concern. It is therefore critical to improve predictions of the induced seismic hazard. Current statistical approaches account for the physics of pore pressure increase only. Here, we present a novel mathematical model that generalises adopted statistics for use in arbitrary injection/production protocols and applies to arbitrary physical processes. In our model, seismicity is driven by a normalised integral over the spatial reservoir volume of induced variations in frictional Coulomb stress, which—combined with the seismogenic index—provides a dimensionless proxy of the induced seismic hazard. Our model incorporates the classical pressure diffusion based and poroelastic seismogenic index models as special cases. Applying our approach to modeling geothermal systems, we find that seismicity rates are sensitive to imposed fluid-pressure rates, temperature variations, and tectonic conditions. We further demonstrate that a controlled injection protocol can decrease the induced seismic risk and that thermo-poroelastic stress transfer results in a larger spatial seismic footprint and in higher-magnitude events than does direct pore pressure impact for the same amount of injected volume and hydraulic energy. Our results, validated against field observations, showcase the relevance of the novel approach to forecast seismic hazards induced by subsurface activities.

Comparisons of distributed and lumped rainfall-runoff model for soil moisture estimation

Soil moisture is an important parameter in landslides because of increased pore pressure and decreased shear strength. This research aims to derive soil moisture indicators from two hydrological models: the physically-based distributed hydrological model and the lumped model. Rainfall-Runoff-Inundation (RRI) Model is used to simulate the hydrological response of catchments to the rainfall-induced landslide in a distributed manner. Tank Model as a lumped hydrological model is also used in this study to simulate the dynamic of soil moisture. The study area is the upper Brantas River Basin, prone to landslides due to heavy rainfall and steep slope. Calibration of the model is conducted by tuning the model according to the river discharge data. The simulation indicates that acceptable performance is confirmed. Tank Model can provide the dynamic of the soil moisture. However, by using this approach, the spatial variation of the soil moisture cannot be presented. Regarding the quantitative amount of soil water content, RRI Model could make a reasonable simulation though the temporal variation is not adequately reproduced. Validation of this method with satellite soil moisture as well as ground measurement is also presented. The challenges of using these approaches to develop landslide hazard assessment are discussed.

Evolution of the Oil Shale Permeability under Real-Time High-Temperature Triaxial Stress in the Jimusar Area, Xinjiang

This paper adopts a real-time high-temperature triaxial seepage test system to study the permeability evolution of oil shale in the Jimusar area, Xinjiang, with the temperature, pore pressure, and volumetric stress. The results indicate that (1) the variation process of the oil shale permeability with the temperature can be divided into three stages: slow growth stage from 20 to 350°C, rapid growth stage from 350 to 500°C with a threshold temperature of 400°C, and growth deceleration stage from 500 to 600°C. (2) With increasing pore pressure, the permeability gradually decreases. Under a volumetric stress of 17 MPa, the permeability decreases the most rapidly from 1 to 2 MPa, and under a volumetric stress of 34 MPa, the permeability decreases the fastest from 1 to 3 MPa. (3) The oil shale permeability decreases with increasing volumetric stress. At room temperature, the decrease magnitude of the permeability is small and increases with increasing temperature. The results can provide a theoretical reference for the analysis of the seepage process of thermal fluids and pyrolysis oil and gas in oil shale.

A Simple Relation to Constrain Groundwater Models Using Surface Deformation.

A simple relation between pore pressure change and one‐dimensional surface deformation is presented. The relation is for pore pressure change in a confined aquifer that causes surface deformation. It can be applied to groundwater models of any discretization and is computationally efficient. The estimated surface deformation from model results can be compared to observed surface deformation through geodetic techniques such as Differential Interferometric Synthetic Aperture Radar. Model parameters then are constrained using the observed surface deformation. The validity of this relation is shown through constraint of model parameters for surface uplift due to pore pressure increase caused by wastewater disposal injection.

Slope stability analysis of a landfill subjected to leachate recirculation and aeration considering bio-hydro coupled processes

During the operation of landfills, leachate recirculation and aeration are widely applied to accelerate the waste stabilization process. However, these strategies may induce high pore pressures in waste, thereby affecting the stability of the landfill slope. Therefore, a three-dimensional numerical analysis for landfill slope stability during leachate recirculation and aeration is performed in this study using strength reduction method. The bio-hydro coupled processes of waste are simulated by a previously reported landfill coupled model programmed on the open-source platform OpenFOAM and then incorporated into the slope stability analysis. The results show that both increasing the injection pressure for leachate recirculation and maximum anaerobic biodegradation rate will reduce the factor of safety (FS) of the landfill slope maximally by 0.32 and 0.62, respectively, due to increased pore pressures. The ignorance of both waste biodegradation and gas flow will overestimate the slope stability of an anaerobic bioreactor landfill by about 20–50%, especially when the landfilled waste is easily degradable. The FS value of an aerobic bioreactor landfill slope will show a significant reduction (maximally by 53% in this study) when the aeration pressure exceeds a critical value and this value is termed as the safe aeration pressure. This study then proposes a relationship between the safe aeration pressure and the location of the air injection screen (i.e., the horizontal distance between the top of the injection screen and the slope surface) to avoid landfill slope failure during aeration. The findings of this study can provide insights for engineers to have a better understanding of the slope stability of a bioreactor landfill and to design and control the leachate recirculation and aeration systems in landfills.

Land subsidence and rebound in the Taiyuan basin, northern China, in the context of inter-basin water transfer and groundwater management

The freshwater scarcity and sustainability of overexploited aquifers have been recognized as a big threat to global water security for human development. Consequently, much research has focused in the past on negative consequences of groundwater abstraction, but somewhat less has been documented about the impacts of adequate management practices to address water shortages. Here, using an integrated analysis of InSAR displacement data, groundwater, and geophysical modeling we show how combined management provisions and inter-basin water transfer project has affected the aquifer system in Taiyuan basin in North China. Following groundwater recovery, the alleviation of land subsidence was found with rates being reduced by up to ~70% in the period 2017–2020 with respect to the period 2007–2010. The increase in pore pressure caused by rising groundwater in Taiyuan city, north of the basin, turned four subsidence centers with rates exceeding 110 mm/yr in the 1980 to uplift centers with rates up to +25 mm/yr between 2017 and 2020. A simple linear elastic model for homogenous subsurface properties can explain InSAR-measured surface displacements well. In the central basin, we found a significant seasonal displacement with annual amplitude up to 43 mm (negative peak in autumn and positive peak in spring) related to the groundwater recharge and discharge due to agricultural pumping irrigation. Using cross-wavelet method, we showed a relatively short time lags (less than one month) between surface deformation and water level changes in the central basin, indicating the low-permeability clayey units have a limited influence in delaying the compaction of aquifer system. Quantifying the effects of adequate groundwater management measures and large-scale engineering approaches like inter-basin water transfer to recharge pumped aquifers provide insight for local governments and decision-makers to properly evaluate the impacts of their policy in recovering the sustainability and efficiency of aquifers in water-deficient basins.

Probabilistic-based geomechanical assessment of maximum operating pressure for an underground gas storage reservoir, NW China

One of the most cost-effective ways to increase deliverability and working gas capacity in underground gas storage (UGS) reservoirs is to operate the gas storage at higher pressure (increased delta pressure). Maximum operating pressure (MOP) for a gas storage reservoir depends on several geomechanical factors, including in-situ stresses, stresses induced by local and global pressure changes in the storage reservoir, and the stabilities of the caprock and faults. PetroChina’s Hutubi (HTB) gas storage site is located in the west gate of China’s west–east gas pipeline project. Multiple major faults intersect the gas storage reservoir and caprock, which pose safety concerns for increasing the gas storage pressure. The typical practice in an HTB storage site has been to operate gas storage pressures at levels equal to the original reservoir pressure because of concerns related to caprock integrity and fault stability. To investigate the possibilities of increasing the gas storage pressure, a comprehensive geomechanical study has been conducted. By utilizing the data from rock mechanics laboratory testing, in-situ stress measurements, 3D field-specific geomechanical models, and coupled reservoir-geomechanical simulations, this paper presents probabilistic risk assessment of fault slippage in response to injection-related increases in pore pressure. Uncertainties of relevant geomechanical parameters, such as the frictional strengths of faults, orientation, and magnitudes of in-situ stresses, have been incorporated through Monte Carlo simulations. The authors find that: (1) fault slippage is the controlling factor which determines the limit of MOP at a HTB gas storage site; (2) increasing the gas storage pressure by 12.5% above the initial reservoir pressure (34.5 MPa) leads to approximately 0.6% probability of fault instability that could provide a leakage pathway. Thus, from a geomechanical perspective, operating the storage pressure at initial reservoir pressure at a HTB gas storage field is an overly conservative approach that leads to under-utilization of the existing storage capability. This paper provides the first integrative geomechanical study for increasing MOP for large underground gas storage reservoir in China. The methodology as well as comprehensive data repository are potential sources for future geomechanical studies on similar underground gas storage projects.

Practical workflows for monitoring saturation and pressure changes from 4D seismic data: A case study of Malay Basin

Discrimination between fluid saturation and pore pressure-related changes using 4D seismic response is a challenging issue in reservoir studies. The main objectives of this study are the estimation of pore pressure and water saturation for baseline and monitor seismic surveys as well as determination of bypassed hydrocarbons location. To achieve this goal, we propose two practical workflows. The workflows originate from the fact that changes in pressure and saturation result in different trends on a Vp/Vs ratio versus P-Impedance cross-plot. The fact that an increase in hydrocarbon saturation results in a decrease in the Vp/Vs ratio and that an increase in pore pressure causes an increase in the Vp/Vs ratio plays a key role in this study. In this approach, after rock physics modeling and seismic data conditioning, elastic seismic inversion is performed for baseline and monitor surveys. The Bayesian probability classification technique is used to differentiate pay zones from non-pay zones which is then incorporated for prediction of fluid saturation changes. In the pore pressure prediction workflow, the Bowers equation is replaced with an empirical relationship in which a consistent match is attained between the calculated effective pressure and Vp/Vs ratio at injected water zones. Pore pressure prediction reveals noticeable anomalies around the injector wells but pressure depletion around the producers remains uncertain due to the insignificant pore pressure drop and the poor quality of seismic data near the platform area. However, at most of the producers' locations, the predicted pore pressure shows a depletion trend confirmed by the field-measured pressure. In this case study, the unswept oil and injected water areas are clearly outlined, which helps to improve sweep efficiency and update EOR scenarios.

A New Method to Estimate Formation Pore Pressure in Fractured Areas of Shale Gas Reservoir

The matrix of shale gas reservoir has the characteristics of low porosity and ultra-low permeability, but also develop natural and hydraulic fractures. Many field operations observed that the formation pore pressure after hydraulic fracturing is higher than the original formation pore pressure. Drilling in the fractured areas of shale gas reservoir, including natural fractures and artificial fractures, always faces greater overflow risk. The pore pressure test after fracturing also shows that the test value is higher than the formation initial pore pressure. However the conventional formation pore pressure predicting methods are difficult to obtain the accurate pore pressure in the fractured areas of shale gas reservoir.In this paper, the causes of formation pore pressure changing and fracturing pressurization effect in the fractured zones of shale gas reservoir are analyzed. Based on the assumption of closed and constant volume material balance and the gas equation of state, the internal mechanism of the increase in pore pressure caused by fracturing operation in low-permeability shale gas formation is theoretically analyzed. A new method to predict pore pressure changing of shale gas reservoir during drilling and hydrofracturing operations is proposed. The pore connectivity of shale gas reservoir is poor, and the permeability is tens of nano-darcy. The permeability is usually between 0.01-0.20 millidarcy when natural fractures are developed. After fracturing, the permeability can be increased by 2-3 times. Based on the principle of effective stress, a pore pressure calculation model with the influence of permeability change is established by using logging data. Three influence terms are considered in the model: the first one is the overburden pressure term, the second one is the skeleton stress term, and the last one is the influence term of permeability change rate on local formation pore pressure. The proposed method shows simple principle, clear mechanism and easy to use in the field. Field applications show that this method could quickly and accurately predict dynamic formation pore pressure changing of shale gas reservoir. It offsets the shortcomings of traditional formation pore pressure calculation methods.

Sensitivity Analysis of Geomechanical Constraints in CO2 Storage to Screen Potential Sites in Deep Saline Aquifers

In order to tackle the exponential rise in global CO2 emissions, the Intergovernmental Panel on Climate Change (IPCC) proposed a carbon budget of 2,900 Gt to limit the rise in global temperature levels to 2°C above the pre-industrial level. Apart from curbing our emissions, carbon sequestration can play a significant role in meeting these ambitious goals. More than 500 Gt of CO2 will need to be stored underground by the end of this century to make a meaningful impact. Global capacity for CO2 storage far exceeds this requirement, the majority of which resides in unexplored deep aquifers. To identify potential storage sites and quantify their storage capacities, prospective aquifers or reservoirs need to be screened based on properties that affect the retention of CO2 in porous rocks. Apart from the total volume of a reservoir, the storage potential is largely constrained by an increase in pore pressure during the early years of injection and by migration of the CO2 plume in the long term. The reservoir properties affect both the pressure buildup and the plume front below the caprock. However, not many studies have quantified these effects. The current analysis computes the effect of rock properties (porosity, permeability, permeability anisotropy, pore compressibility, and formation water salinity) and injection rate on both these parameters by simulating CO2 injection at the bottom of a 2D mesh grid with hydrostatic boundary conditions. The study found that the most significant property in the sensitivity analysis was permeability. Porosity too affected the CO2 plume migration substantially, with higher porosities considerably delaying horizontal and vertical migration. Injection rate impacted both the pressure rise and plume migration consistently. Thus, in screening potential storage sites, we can infer that permeability is the dominant criterion when the pore pressure is closer to the minimum principal stress in the rocks, due to which injection rate needs to be managed with greater caution. Porosity is more significant when the lateral extents of the reservoir limit the storage potential.

High-pressure water and gas alternating sequestration technology for low permeability coal seams with high adsorption capacity

Traditional ECBM techniques, which cannot extract methane adsorbed in coal pores effectively, is still difficult to greatly improve coalbed methane recovery rate until now. Waterflooding or CO2-ECBM can displace methane adsorbed in nanopores but increases the risk of coal and gas outbursts. We proposed a novel coalbed methane recovery technique, e.g. high-pressure water and gas alternating sequestration technique (H–P-WGAS), which can displace adsorbed methane in coal pores without introducing other gases with high adsorption capacity. Additionally, the injected H–P gas can eliminate the water lock and be easily extracted during methane drainage. Numerical simulation and field test were conducted to verify effectiveness of the H–P-WGAS technique. The numerical results show that the H–P-WGAS technique can enhance methane migration in coal seam by changing pore pressure distribution and increase influence radius to 70 m at 20 MPa in 48 h. For water and air co-injection, methane content can decrease by up to 37.6%, which is much more effective comparing with that under single water injection or gas injection. Meanwhile, field tests show that the reduction of methane content by percentage lies between 25.14% and 80.62% with an average of 53.17%. Meanwhile, the influence radius of field test reached ∼48 m. The average methane drainage flow rate of single borehole increased by 2.65 times. Average methane drainage concentration increased from 31.3% to 47.6%. The methane drainage flow rate and concentration of the whole coal mine increased by 50.2% and 32.4%, respectively. The above results indicate that the proposed H–P-WGAS has best effectiveness comparing with single water injection or air injection.

The clays involved in the 1963 Vajont landslide: Genesis and geomechanical implications

The catastrophic Vajont landslide that occurred on 9 October 1963 caused about 2000 deaths when 270–300 Mm3 of rock and debris slid from the northern slope of Mt. Toc into the newly created artificial reservoir, displacing some huge water waves that flooded the nearby villages. The 1963 slide was an en-block remobilisation of a prehistoric rockslide lying on the southern reservoir bank. Basal sliding took place within a complex lithostratigraphic sequence of cherty–marly limestones and clay interbeds belonging to the Fonzaso Formation of Upper Jurassic age. In this work, we investigate the geological origin and provenance of the Vajont clays according to the results of mineralogical and geochemical analyses performed on several clay samples which were collected from the landslide failure surface and the same limestone sequence in the surrounding area. The sampled clayey materials contained variable amounts of clay minerals (36–96%), calcite (4–64%) and quartz (0–6%). The dominant clay minerals were illite/smectite (I/S) mixed layers with a high illite content (85–50%), which was consistent with the high percentage of K2O (1.17–5.77%). The Vajont clays included in the Fonzaso Fm. can be referred to as K-bentonites and can be interpreted as distinct deposits of volcanoclastic materials (tephra), representing the sedimentary inputs of volcanic ashes that interrupted the “normal” calcareous sedimentation during the late Oxfordian–upper Tithonian (158–145 Ma). The soft clay interbeds played a different mechanical role in the occurrence of the prehistoric rockslide and the 1963 en-block remobilisation. When referring to the ancient slope failures, distinct clay-rich layers that were concentrated within some specific lithostratigraphic intervals of the Jurassic cherty limestone sequence caused a localised decrease in the available shear strength, allowing for relative shear displacements along the dip-slope bedding planes. This caused additional shear/tensile fracturing of limestone beds and the formation of a stepped basal failure surface. During the 1963 remobilisation, the clay layers contributed to the average shear strength decrease of the debris material forming the basal shear zone of the slide, but the main triggering factor was the increase in pore pressures caused by the reservoir-induced groundwater inflow.

Stress-dependent fluid dynamics of shale gas reservoirs: A pore network modeling approach

In this study, the pore network modeling approach is implemented to study the flow behavior of the methane gas in shale reservoirs in the presence of slip flow and gas desorption under different effective stress conditions through the effect on apparent permeability. A single pore and a pore network are constructed using a down-scaled Berea sandstone image. The compressible Stokes is applied with proper boundary conditions to capture slip flow and gas desorption effects. To include the effect of stress, poroelasticity principles are applied to describe the interaction between fluid flow and solid deformation. The model performance is verified against an experimental data taken from the literature. Both models show a negative effect of stress on gas flow when slip flow and desorption mechanisms are activated in the models. In the single pore model, slip has a positive and complementary effect on desorption at a specific stress value when their coupled contributions are summative. However, the results of pore network model show that slip has a destructive effect on desorption contribution specially at low effective stress values. That is, the changes in pore pressure induced by the slip mechanism act to limit the desorption contribution. This is clearly noted with the consideration of stress effects. At high effective stress values, the slip effect is limited due to the increase in pore pressure by compression, hence a closer matching between the permeability enhancement produced by the desorption mechanism alone and the coupled effect of desorption and slip can be noticed. Stress acts to limit the slip flow and also the desorption mechanism as they both are a function of pore pressure, however, as stress increases the contribution of desorption no longer would be reduced by slip flow. This study provides insights into stress role in multiphysics gas flow in shale reservoirs.

Analysis of unsaturated soil columns with application to bulk cargo liquefaction in ships

Over the last 40 years, there have been several ship losses carrying granular mineral ores that are believed to have been caused by the sudden shifting of the cargo as a result of liquefaction. It is known that the initial moisture content during loading is important for cargo types which are considered susceptible to liquefaction. To limit the moisture content, the degree of saturation must be below 80% in compaction tests designed to reproduce the energy associated with loading cargo into a ship’s hold. During transport, ships can experience storms causing cyclic loading which leads to densification of the cargo and increases in the degree of saturation. There can also be transient increases in pore pressure and associated reductions in the cargo material’s resistance, which when the ship heels increase the likelihood of the cargo shifting and in the extreme, the ship capsizing. In this paper, a recently developed fully coupled dynamic finite element analysis and constitutive model for unsaturated soil is used to perform a parametric study to explore the capabilities of the numerical code and the mechanics of the problem. The numerical analyses are shown to be capable of running for up to 2500 cycles and simulating both cyclic loading and simultaneous consolidation and drainage, a computationally very demanding task. Outcomes of the analyses are used to discuss the factors influencing cargo stability and vessel instability.

Impact of Sorption-Induced Strain and Effective Stress on the Evolution of Coal Permeability under Different Boundary Conditions

Coalbed methane (CBM) is a relatively common unconventional natural gas, which has great exploitation value. Coal permeability is an important parameter that affects the production and production efficiency of CBM, which is mainly controlled by the sorption expansion/contraction strain and effective stress. To study the seepage characteristics of coal in the process of CBM production, we have used CH4 and CO2 as test gases separately and conducted comparative seepage tests of different gases under constant pore pressure conditions. At the same time, the elastic modulus reduction coefficient Rm has been introduced to characterize the sorption strain of coal, following which the permeability models suitable for different boundary conditions were derived according to the stress–strain relationship. Under the two gases, the new model could not only better reflect the law of coal sorption strain but also better reflect the relationship among effective stress, pore pressure, and coal permeability. Under the conditions of constant pore pressure, coal permeability was mainly controlled by effective stress; with the increase of effective stress, permeability decreased sharply initially and then gradually. Under the conditions of uniaxial strain and constant external stress, with an increase of pore pressure and Rm, the matrix sorption expansion strain increased, resulting in a narrowing of the seepage channel, and Rm indirectly inhibited permeability. At this point, coal permeability was mainly controlled by sorption expansion/contraction strain and effective stress. In addition, compared with other permeability models, the new permeability model possesses higher applicability both in theoretical mechanism and in data matching. The general change trend concerning coal permeability, determined by rebound pressure prb, was consistent with the test results, which further verified the applicability of the model. It is believed that the results of this study could provide a basis for subsequent research on the stress–strain–permeability relationship and for the study of efficient development of CBM.

Experimental Study on Silt Liquefaction by Shaking Table Test

Based on the shaking table test, the liquefaction of silt under earthquake load and the earthquake damage after liquefaction are studied. The test results show that the lower silt is relatively dense, the pore pressure increases slowly, and the pore pressure ratio can not reach the initial liquefaction state. However, under the stronger earthquake action, the dense silt may also produce liquefaction, which indicates that the increase of pore water pressure has a great relationship with the compactness and earthquake intensity under the earthquake load. Because the permeability coefficient of silt is relatively low, the increase and dissipation of pore pressure are relatively slow. After the earthquake, he pore pressure of silt continues to rise, and the liquefaction will be further aggravated.

Coulomb stress changes triggering surface pop-up during the 2016 Mw 6.4 Meinong earthquake with implications for earthquake-induced mud diapiring in SW Taiwan

• Coseismic surface pop-up of the 2016 Meinong earthquake is due to crust volumetric strain change. • The coseismic strain change is characterized by shallow dilatation and underlying contraction. • Mud diapiring under pop-up is likely triggered by ‘squeezing” contract strain at base of reservoir. We investigated the 2016 Meinong earthquake (Mw 6.4) in southwestern Taiwan, which caused surface pop-up in an area of 10x15 km 2 with maximum uplift of 12 cm, where lies an array of mud volcanoes and possible underlying mud diapir. We calculated 3D strain tensor in a 3D mesh with 5x5x2 km grids in the epicentral area induced by the Coulomb stress change due to coseismic fault slip. We obtained substantial contraction strain (10 −5 –10 −6 ) that occurred in a lobe showing “squeezing” at the depth of 5–14 km below the surface pop-up area. Dilatation strain (10 −5 –10 −6 ) occurred at shallow level (0–3 km) with a radial pattern around the surface pop-up area. Combining with local geology, which is composed of Mio-Pliocene ~5-km-thick mudstone in a fold-thrust belt, we interpret that the 2016 Meinong coseismic surface pop-up was closely related to mud diapirs/volcanoes, which were likely reactivated by sudden increase of fluid pore-pressure in the basal reservoir (at 5–6 km depth) and dilatation in the shallow level. We also explored the potential effects of the Coulomb stress transfer on nearby receiver faults – including three arrays of mud diapir, the regional decollement, a suspected backthrust and one thrust close to the pop-up area. Our results show that the Coulomb stress transfer a) favors NNE-trending mud diapirs in the coseismic pop-up area, with a combination of clamping stress changes at 5–6 km depth and unclamping stress changes at 0–4 km depth, and b) it does not favor triggered thrust slip on the regional thrusts.

Effects of inherent spatial variability of rock properties on the thermo-hydro-mechanical responses of a high-level radioactive waste repository

In France, a deep geological disposal for high level radioactive waste (HLW) and intermediate-level long-lived radioactive waste (ILW-LL) called Cigéo is planned to be constructed in a deep Callovo-Oxfordian claystone (COx) formation. The heat released from the HLW packages leads to a temperature increase in the host rock, which causes a pore pressure increase essentially due to the difference between the thermal expansion of the pore water and the solid skeleton. The low permeability of the COx and its relative rigidity inhibit the discharge of the induced pressure build-up. Moreover, thermal loading also induces thermo-mechanical stresses within the formation. The French National Radioactive Waste Management Agency (Andra) participated in the DECOVALEX-2019 project (Task E) to model the Thermo-Hydro-Mechanical (THM) responses of a benchmark exercise inspired of a HLW repository based on the French concept. This paper aims at studying the THM behavior of the HLW repository by taking into account the inherent spatial variability of the properties of the host rock through thermo-poro-elastic numerical simulations. Only the most influential parameters were selected to perform the spatial variability analysis by using Monte Carlo simulations coupled with the Random Finite Element Method. The selection of the most influential parameters was based on two parametric sensitivity analyses consisting in computing the Sobol indices, which determine the contribution of each input parameter to the overall model output variance. The temporal evolution of the Terzaghi effective stress in the far field is defined as indicator of the influence of each parameter on the THM response.

Cyclic Brittle‐Ductile Oscillations Recorded in Exhumed High‐Pressure Continental Units: A Record of Deep Episodic Tremor and Slow Slip Events in the Northern Apennines

AbstractThe geological record of deep fossil seismogenesis in subduction zones is limited due to common rock overprinting during exhumation and only a few regions expose well‐preserved deeply exhumed structures. We investigated a mesoscopic contractional duplex formed at blueschist facies conditions in continental metasediments in the Northern Apennines (Italy). Field observations reveal strain partitioning within the duplex between metapelite bands, corresponding to high‐strain zones, and metaquartzarenite bands, which form low‐strain, imbricated metric horses. Dilational shear veins occur in both lithotypes and are composed of quartz and carpholite fibers defining a stretching lineation parallel to the regional SW‐NE transport direction. Geometrical, cross cutting and petrographic relationships suggest that veins formed broadly syn‐mylonitization. Thermodynamic modeling constrains the formation of the mylonitic foliation to >0.7 GPa and ∼400°C and the dilational shear veins vein to ∼1.1 GPa and 350°C. Therefore, we document a top‐to‐the‐E‐NE thrust that formed at the deepest conditions recorded by this unit of the Northern Apennines. In the thrusted continental metasediments, aqueous fluid locally released by metamorphic dehydration reactions transiently increased pore pressure, in turn triggering brittle‐ductile cyclicity. We propose that blueschist facies dilational shear veins and mylonitic foliation represent a geological record of deep episodic tremor and slow slip events. To the best of our knowledge, this is the first time that these events are genetically associated with brittle and ductile structures of a mesoscopic duplex. We suggest that these structures could be common features of the high‐pressure metamorphic units of the Apennine orogenic belt.

Experimental study on effect of effect time of supercritical CO2 fracturing fluid on mechanical properties of sandstone

Supercritical CO2 fracturing is a novel hydro-free fracturing technology, which has the characteristics of minor damage to reservoirs and higher efficiency. During the process of fracturing, the CO2 fracturing fluid will infiltrate into the pore of rock, resulting in the change of rock mechanics properties. The rock samples were drilled from the same layer, then the supercritical CO2 fracturing fluid was used to soak the rock samples to measure the changes of rock mechanical properties under different soaking time. When the pore pressure is 0 MPa, the fracture initiation stress of rock sample is about 40% of its peak strength, and with the increase of pore pressure, the elastic stage decreases and the plastic stage increases, while the pore pressure reaches 31 MPa, the core fracture initiation stress is about 20% of its peak strength. The results show that with the longer CO2 soaking time, the shorter elastic stage with the longer plastic stage and the weaker compressive strength, the smaller elastic modulus, and the higher Poisson's ratio. This indicates that the strength of rock decreases gradually with the increase of injection time in supercritical CO2 fracturing process, which is conducive to the formation of longer fractures.