Pore Pressure Evolution Research Articles

Closed-form solution of stress state and stability analysis of wellbore in anisotropic permeable rocks

This work aims to develop a closed-form solution of stress state around wellbore drilled in the anisotropic permeable rocks taking into account the transient effect of fluid flow. For this purpose, a simplified method was adopted to solve the anisotropic hydraulic diffusion equation. Knowing the explicit transient evolution of pore pressure, the hydraulic and hydro-mechanical potentials were proposed through which stress state around wellbore are determined thanks to the use of the well-known complex potential approach. The validity of this closed-form solution was verified by comparing with the result obtained from the numerical simulation. By combining the analytical solution of stresses on well wall with a chosen anisotropic failure criterion of rock mass, the stability analysis of wellbore was carried out. Through the parametric study, we elucidated the influence of different factors on the safe mud pressure window notably the anisotropic poro-elastic properties, the anisotropic strength of the rock masses as well as the anisotropic initial stress state and the inclined bedding plane of formation.

Utility of inclusions for interpreting reservoir thresholds for tight sandstone gas accumulation in the Longfengshan and Dongling sags, Southeast China

To better understand the thresholds for hydrocarbon migration in tight sandstone, we studied the gas migration and accumulation processes in the Longfengshan and Dongling sags in the Songliao Basin by determining the ancient pore pressure evolution and gas fractionation. The trapping pressure of coeval aqueous inclusions can be used to identify systematic overpressure trends, which increase during gas charging. Early formation of overpressure and a maximum surplus pressure greater than 6 MPa in reservoirs adjacent to high-quality source rocks (total organic carbon > 1.2%) indicate that the formation of surplus pressure is related to the gas generation capacity. The gas-bearing inclusion distribution and systematic geochemical trends suggest that dynamic gas migration was accompanied by gas fractionation. The results indicate that dissolved gas mainly accumulated in reservoirs with a maximum surplus pressure greater than 6 MPa and that the free gas diffused toward reservoirs with maximum surplus pressures less than 6 MPa. The gas saturations were generally stable and greater than 50% in the high surplus pressure zone. However, in the low surplus pressure zone, the gas saturation was less than 25%, although this value was relatively higher in reservoirs where the maximum surplus pressure was less than 3 MPa. Our results demonstrate that the resistance thresholds for the migration of dissolved gas and free gas differ in tight sandstone reservoirs. The surplus pressures greater than 6 MPa are advantageous for dissolved gas accumulation. An obvious resistance threshold is not observed for free gas migration, although the migration of this gas was relatively inefficient and occurred mainly via diffusion in the direction of decreasing surplus pressure. The results of this study confirm the importance of considering the driving force for natural gas in different phases as a key factor when determining optimal exploration targets.

A novel hydro-mechanical coupled analysis for the fractured vuggy carbonate reservoirs

A novel hydro-mechanical coupled analysis is proposed for calculating the pore pressure and stress evolution of fractured vuggy carbonate reservoirs under depletion or injection. Different from the conventional dual- and triple-porosity models in which the vugs are considered as continuous porosity, the vugs are treated as virtual volumes in this study. The fluid exchange at the vug-to-matrix interface is dynamically calculated with time evolution and the pore pressure in the vug is updated through considering both the fluid material balance and the volume change due to the mechanical deformation of the vug. The fluid-mechanical interaction in the rock matrix and natural fractures is calculated based on the framework of Biot’s poroelasticity theory. The mechanical and hydraulic interactions between vugs and matrix are maintained and the stress evolution owing to the depletion can be dynamically updated. The results in this study show that the depletion or injection process is greatly affected by the existence of vugs and natural fractures. Natural fractures provide highly conductive channels that lead to the preferential flow paths. The large fluid storage of vugs impedes the pressure diffusion process and brings perturbation to the pressure pattern near the vugs. The response of a vug to the depletion can be divided into three different stages depending on the depletion time. The effects of fluid storage, matrix/fracture permeability, natural fractures and fluid injection are investigated. The proposed hydro-mechanical coupled analysis provides a new modeling method for studying the hydraulic and mechanical behaviors of fractured vuggy carbonate reservoirs.

Pore pressure evolution and mass loss of broken gangue during the seepage.

Broken gangue consists of different particles, and it has more complicated seepage characteristics than intact rock sample. Using the self-designed instrument, the permeability, mass loss and pore pressure of crushed gangue during the seepage are tested. The result shows that permeability parameter k of crushed rock has a polynomial relationship with effective stress σ′ in inverse proportion, and permeability parameter β of crushed gangue has power exponent relationship with effective stress σ′ increasing in direct proportion. The particle size of 8.0–10.0 mm has a good support effect. The inner pressure of crushed rock is mostly linear distribution along the tube wall. After the seepage, mass loss of broken gangue mainly increases with large particle size out of proportion.

Hydromechanical Impacts of Pleistocene Glaciations on Pore Fluid Pressure Evolution, Rock Failure, and Brine Migration Within Sedimentary Basins and the Crystalline Basement

AbstractThe effects of Pleistocene glacial loading on rock failure, permeability increases, pore pressure evolution, and brine migration within two linked sedimentary basins were evaluated using a multiphysics control volume finite element model. We applied this model to an idealized cross section that extends across the continent of North America from the Hudson Bay to the Gulf of Mexico. Our analysis considered lithosphere geomechanical stress changes (σyy > 35 MPa) in response to 10 cycles of ice sheet loading. Hydrologic boundary conditions, lithosphere rheological properties, and aquifer/confining unit configuration were varied in a sensitivity study. We used a Coulomb Failure Stress change metric (ΔCFSp > 0.1 MPa) to increase permeability by a factor of 100 in some simulations. Results suggest that a buildup of anomalous pore pressures up to about 3 MPa occurred in confining units during periods of glaciations, but this had only a second‐order effect on triggering rock failure. In regions prone to failure, permeability increases during glaciations help to explain observations of brine flushing in sedimentary basin aquifers. During the Holocene to present day, deglaciation resulted in underpressure formation in confining units primarily along the northern margin of the northern basin. Holocene‐modern geomechanical stress fields were relatively small (<0.6 MPa). However, pore pressure increases associated with postglacial rebound, especially when a basal sedimentary basin aquifer is present, induced rock failure and seismicity up to 150 km beyond the terminus of the ice sheet. Sedimentary basin salinity patterns did not equilibrate after 10 simulated glacial cycles.

Pore pressure and reservoir quality evolution in the deep Taranaki Basin, New Zealand

The Palaeocene fluvial to shallow marine sandstones of the Farewell Formation are an important proven hydrocarbon reservoir in the Taranaki Basin, New Zealand. The Kapuni Deep-1 well was drilled to target Farewell Formation sandstones in the deeper overpressured (>5000 m, >3000 psi/21 MPa) sections of the onshore Manaia Graben of the Taranaki Basin. However, the Farewell Formation sandstones display anomalously low measured helium porosities (1–4.5%) and intergranular volumes (6–11%) even for their present-day, close to maximum, burial depth (c.5000 m). One dimensional burial history modelling demonstrates initial rapid burial leading to significant porosity reduction via mechanical compaction, enhanced by poor sorting, angular grain morphology and the presence of ductile grains, which allowed efficient packing and plastic deformation. Low intergranular volume (IGV), anhedral nature of quartz overgrowths, a general lack of fluid inclusions, and poor crystalline clay mineral content indicate that early burial compactional processes significantly influenced reservoir quality. The lack of mudstone to siltstone grade lithologies within the overlying Eocene section inhibited the early or shallow onset of overpressure in the Farewell Formation. One dimensional basin modelling has shown that rapid Pliocene subsidence related to exceptionally high sedimentation rates generated overpressure through disequilibrium compaction in the overlying Oligocene to early Miocene section during the past 6 Ma. However, the late and deep (>3000 m) onset of overpressure had no effect on arresting porosity loss in the Farewell Formation. Continued compaction of Farewell Formation sandstones after dissolution of early carbonate cements of CO2 rich fluids creates zones of extremely low permeability, which have the potential to act with interbedded shales to form pressure seals as seen in the Kapuni Field.

Stress and Pore Pressure in Mudrocks Bounding Salt Systems

We simulate the evolution of stress and pore pressure in sediments bounding salt systems. Our evolutionary geomechanical models couple deformation with sedimentation and porous fluid flow. We find that high differential stresses develop near rising diapirs and below salt. Salt emplacement induces significant excess pressures that are comparable to the weight of the salt sheet. In addition, we show that the shear-induced component of the excess pressures is significant. We also find that low effective stresses result in low strength, which enables salt growth. We model salt as a solid viscoplastic and sediments as poro-elastoplastic materials, and calibrate the consolidation properties based on experimental testing on smectite-rich mudrocks typical of those in the Gulf of Mexico. Our approach can be applied to design stable well bores and provide insight into macroscale geological processes. Overall, we show that transient evolutionary models can provide estimates of stress and pore pressure in many geologic systems where large strains, pore fluids, and sedimentation interact. We close with a discussion of the need to better understand material behavior at geologic stress and timescales.

Effects of the Anisotropy of the Fault Zone Permeability on the Timing of Triggered Earthquakes: Insights from 3D-Coupled Fluid Flow and Geomechanical Deformation Modeling

In the last years, numerous seismological evidences have shown a strict correlation between fluid injection and seismicity. An important topic that is currently under discussion in the scientific community concerns the prediction of the earthquake magnitude that may be triggered by fluid injection activities. Coupled fluid flow and geomechanical deformation models can aim at understanding the evolution of pore pressure and rock deformation due to fluid injection in the subsurface. To perform an accurate numerical study of the correlation among fluid injection, seismicity rates and maximum earthquake magnitude, it is necessary to characterize the model with two fundamental features: first, the presence of a system of faults possibly intersecting among each other; second, the variability of the hydro-mechanical properties across the region surrounding each fault plane (fault zone). The novelty of this work is to account for these two aspects combining together two different numerical techniques that have been proposed in literature for the fault’s modelling: for the first feature, interface elements are used to describe the frictional contacts occurring on the fault surfaces; for the second feature, solid elements are adopted to describe the heterogeneous hydro-mechanical behavior across the fault zone. Moreover, we account for a spatial variation of the permeability in the fault zone both along the dip and the normal direction with respect to the fault plane. We compute the numerical solution for six models among which we vary the permeability contrasts between protolith rocks and damage zone and between damage zone and fault core. We demonstrate that the anisotropy of permeability of the fault zone has a strong impact both on the timing and on the magnitude of triggered earthquakes. We suggest that a similar approach, which includes the entire architecture of the fault zone, shall be included in fluid-flow-geomechanical simulations to improve fault stability analysis during fluid injection.

Promoting wellbore stability in active shale formations by water-based muds: A case study in Pabdeh shale, Southwestern Iran

Minimizing water invasion into shales is a critical property for many water-based muds. By improving drilling fluids rheology in HPHT conditions, it is possible to minimize water invasion and maintain true overbalance pressure conditions at near wellbore scale which endorses wellbore stability. In order to propose an inhibitive mud formulation for drilling troublesome swelling Pabdeh formation frequently encountered in southern to southwestern regions of Iran, a comprehensive experimental approach was adopted. After performing rheology and thermal stability tests for 81 different formulations of drilling fluids, 5 high performance salt/polymer solution muds were chosen. They showed good rheology properties and all of them were thermally stable. In order to find the best formulation which accomplishes all requirements pertaining shale stability and wellbore integrity, some complementary analyses were performed. Using a roller oven simulating down hole temperature conditions, physical integrities of shale samples immersed and rolled in these 5 mud formulations were investigated. Pore pressure transmission tests were also carried out using a membrane efficiency screening equipment to evaluate membrane efficiency of the Pabdeh shale and also inhibitive performances of 5 designed mud formulations. Accordingly, pore pressure evolution in shale samples in contact with designed muds were obtained by direct measurements, which resembles the near wellbore pore pressure alterations. According to the obtained results, Pabdeh shale shows negligible membrane efficiency of about 4%. Although all mud formulations show high surface coating capabilities resulting in almost high shale recovery after exposure, pore pressure transmission tests revealed different inhibitive performances. Among them, a combination of polymers and sodium silicate shows promising mud formulation that hinders filtrate invasion into shale samples by either physical or chemical plugging of the shale pore throats. Regarding both environmental and economical aspects of drilling operations, derived mud formulations can also be advantageous for drilling safe and stable wells in Pabdeh formation.

Numerical investigation on permeability evolution behavior of rock by an improved flow-coupling algorithm in particle flow code

Permeability is a vital property of rock mass, which is highly affected by tectonic stress and human engineering activities. A comprehensive monitoring of pore pressure and flow rate distributions inside the rock mass is very important to elucidate the permeability evolution mechanisms, which is difficult to realize in laboratory, but easy to be achieved in numerical simulations. Therefore, the particle flow code (PFC), a discrete element method, is used to simulate permeability behaviors of rock materials in this study. Owe to the limitation of the existed solid-fluid coupling algorithm in PFC, an improved flow-coupling algorithm is presented to better reflect the preferential flow in rock fractures. The comparative analysis is conducted between original and improved algorithm when simulating rock permeability evolution during triaxial compression, showing that the improved algorithm can better describe the experimental phenomenon. Furthermore, the evolution of pore pressure and flow rate distribution during the flow process are analyzed by using the improved algorithm. It is concluded that during the steady flow process in the fractured specimen, the pore pressure and flow rate both prefer transmitting through the fractures rather than rock matrix. Based on the results, fractures are divided into the following three types: I) fractures link to both the inlet and outlet, II) fractures only link to the inlet, and III) fractures only link to the outlet. The type I fracture is always the preferential propagating path for both the pore pressure and flow rate. For type II fractures, the pore pressure increases and then becomes steady. However, the flow rate increases first and begins to decrease after the flow reaches the stop end of the fracture and finally vanishes. There is no obvious pore pressure or flow rate concentration within type III fractures.

Fluid Injection and Time‐Dependent Seismic Hazard in the Barnett Shale, Texas

AbstractThe Barnett Shale in Texas experienced an increase in seismicity since 2008, coinciding with high‐volume deep fluid injection. Despite the spatial proximity, the lack of a first‐order correlation between seismic records and the total volume of injected fluid requires more comprehensive geomechanical analysis, which accounts for local hydrogeology. Using time‐varying injections at 96 wells and employing a coupled linear poroelastic model, we simulate the spatiotemporal evolution of pore pressure and poroelastic stresses during 2007–2015. The overall contribution of poroelastic stresses to Coulomb failure stress change is ~10% of that of pore pressure; however, both can explain the spatiotemporal distribution of earthquakes. We use a seismicity rate model to calculate earthquake magnitude exceedance probability due to stress changes. The obtained time‐dependent seismic hazard is heterogeneous in space and time. Decreasing injection rates does not necessarily reduce probabilities immediately.

Precambrian temperature and pressure system of Gaoshiti-Moxi block in the central paleo-uplift of Sichuan Basin, southwest China

Recently, trillions of cubic meters gas resources have been found in Sinian reservoirs of Gaoshiti-Moxi region in the central uplift of Sichuan Basin, which is the first commercial discovery of primary gas field in Precambrian anywhere in the world. To obtain a great breakthrough in Precambrian hydrocarbon exploration, it is necessary to find out the evolution process of temperature and pressure. Reconstructing the thermal history and pore pressure evolution in carbonate reservoirs are challenging problems, especially in deep-old layers without available samples and methods. To study the temperature and pressure system, we conducted a comprehensive analysis combining the paleo-thermal indicators and inclusions Pressure-Volume-Temperature (PVT) simulation with basin modeling. The results showed that the thermal history of the Gaoshiti-Moxi area could be divided into three stages. From the Late Sinian to Late Paleozoic, the thermal regime was stable with low heat flow value (<65 mW/m2). During the Early Paleozoic, the heat flow increased rapidly to the peak value (75–100 mW/m2). From Mesozoic to present, the thermal subsidence occurred in the basin with the heat flow decreasing to the present value (60–70 mW/m2). With the boundary condition of above thermal history, the simulated pressure of Dengying Formation in Gaoshiti-Moxi area could be divided into four stages. The excess pressure began to form during the Triassic. From the end of Triassic to Middle Jurassic, the excess pressure increased steadily and slowly. From Middle Jurassic to the end of Early Cretaceous, the strata subsided rapidly while the excess pressure increased significantly. Since the Late Cretaceous, the Dengying Formation suffered from rapid uplifting and gas lateral migrating with the excess pressure decreasing, until the Late Neogene, the excess pressure in reservoirs declined to zero.

Quantification of a maximum injection volume of CO2 to avert geomechanical perturbations using a compositional fluid flow reservoir simulator

Abstract Subsurface CO2 injection and storage alters formation pressure. Changes of pore pressure may result in fault reactivation and hydraulic fracturing if the pressure exceeds the corresponding thresholds. Most simulation models predict such thresholds utilizing relatively homogeneous reservoir rock models and do not account for CO2 dissolution in the brine phase to calculate pore pressure evolution. This study presents an estimation of reservoir capacity in terms of allowable injection volume and rate utilizing the Frio CO2 injection site in the coast of the Gulf of Mexico as a case study. The work includes laboratory core testing, well-logging data analyses, and reservoir numerical simulation. We built a fine-scale reservoir model of the Frio pilot test in our in-house reservoir simulator IPARS (Integrated Parallel Accurate Reservoir Simulator). We first performed history matching of the pressure transient data of the Frio pilot test, and then used this history-matched reservoir model to investigate the effect of the CO2 dissolution into brine and predict the implications of larger CO2 injection volumes. Our simulation results –including CO2 dissolution– exhibited 33% lower pressure build-up relative to the simulation excluding dissolution. Capillary heterogeneity helps spread the CO2 plume and facilitate early breakthrough. Formation expansivity helps alleviate pore pressure build-up. Simulation results suggest that the injection schedule adopted during the actual pilot test very likely did not affect the mechanical integrity of the storage complex. Fault reactivation requires injection volumes of at least about sixty times larger than the actual injected volume at the same injection rate. Hydraulic fracturing necessitates much larger injection rates than the ones used in the Frio pilot test. Tested rock samples exhibit ductile deformation at in-situ effective stresses. Hence, we do not expect an increase of fault permeability in the Frio sand even in the presence of fault reactivation.

Influence of Permeability in Modeling of Reservoir Triggered Seismicity in Koyna Region, Western India

ABSTRACT The Koyna region located in the west coast of India is a classic example of reservoir triggered seismicity (RTS) that started soon after the impoundment of the Koyna reservoir in 1962. Previous studies have shown that RTS can be explained in terms of stress and pore pressure changes due to poroelastic response of the rock matrix. The permeability of rock matrix is a key parameter for pore pressure diffusion which is mainly responsible for generation of stress perturbation related to seismicity. Based on the poroelastic theory, we employ 2-D finite element models to simulate the evolution of pore pressure up to 5 years after the reservoir impoundment in 1962, using a range in permeability, 10−16-10−14 m2. Constraints on material properties of Deccan basalt and granitic rocks were taken from available studies. The results show the formation of pore pressure front and its propagation with depth and time since the reservoir impoundment as a function of permeability. While a permeability of 10−16 m2 does not produce any significant change in pore pressure, a ten-fold increase in permeability produces significant changes up to a depth of 2 km only beneath the reservoir after 5 years of impoundment. Permeability values between 10−15 m2 and 10−14 m2 are required to induce critical pore pressure changes in the range 0.1-1 MPa up to depth of 10 km, capable of triggering earthquakes in a critically stressed region. Studies on core samples of granitic basement rock down to a depth of 1522 m in the Koyna region provide evidences of fracture zones that may contribute to water channelization. Direct measurements of material properties through the ongoing deep drilling programme would help to develop more realistic models of RTS.

Effect of Crushing and Strain Rate on Mechanical Behavior of Type-F Fly Ash

Fly ash is a by-product of coal combustion from thermal power plant. It comprises of hollow spherical silt size particles with low specific gravity. The increased use of fly ash in highway/railway embankment construction as a structural fill material raises the need to study its mechanical behavior in detail. Although it has been reported that fly ash particles are susceptible to breakage, the effect of crushing on mechanical behavior of fly ash is yet to be explored. The current experimental research was conducted on type-F fly ash to study the following: (a) effect of crushing of fly ash particles on its macroscopic and microscopic behavior and (b) effect of strain rate on pore pressure evolution and stress-strain response of fly ash (uncrushed). Macroscopic (shear strength, compressibility, compaction behavior) and microscopic (particle’s shape and size using SEM images) tests were performed on fly ash to study the crushing effect. Strain rate effect on pore pressure and stress-strain response of uncrushed fly ash was studied by performing CU triaxial tests at different loading rates (0.005–9% per min). Macroscopic response of fly ash exhibited that higher crushing reduced shear strength and compressibility of fly ash and improved its compaction behavior. Microscopic response indicated deformation and breakage of fly ash particles due to crushing. Strain rate study showed large negative pore pressure generation along with brittle stress-strain response of uncrushed fly ash at 9% per min as compared to lower strain rates.

Hydromechanical Modeling of Stress, Pore Pressure, and Porosity Evolution in Fold‐and‐Thrust Belt Systems

AbstractWe present coupled, critical state, geomechanical‐fluid flow simulations of the evolution of a fold‐and‐thrust belt in NW Borneo. Our modeling is the first to include the effects of both syntectonic sedimentation and transient pore pressure on the development of a fold‐and‐thrust belt. The present‐day structure predicted by the model contains the key first‐order structural features observed in the field in terms of thrust fault and anticline architectures. Stress predictions in the sediments show two compressive zones aligned with the shortening direction located at the thrust front and back limb. Between the compressive zones, near to the axial plane of the anticline, the modeled stress field represents an extensional regime. The predicted overpressure distribution is strongly influenced by tectonic compaction, with the maximum values located in the two laterally compressive regions. We compared the results at three notional well locations with their corresponding uniaxial strain models: the 2‐D thrust model predicted porosities which are as much as 7.5% lower at 2.5 km depth and overpressures which are up to 6.4 MPa higher at 3 km depth. These results show that one‐dimensional methods are not sufficient to model the evolution of pore pressure and porosity in contractional settings. Finally, we performed a drained simulation during which pore pressures were kept hydrostatic. The predicted geological structures differ substantially from those of the coupled simulation, with no thrust faulting. These results demonstrate that pore pressure is a key control on structural development.

Modeling debris flow: On the influence of pore pressure evolution and hypoplasticity

Modeling debris flow: On the influence of pore pressure evolution and hypoplasticity

Liquefaction response of loose gassy marine sand sediments under cyclic loading

Gassy sediments have often been encountered in the marine seabed, and they have different features from common saturated and unsaturated soil. By developing and improving an effective methodology, triaxial gassy sand specimens with different initial gas content (saturation ≥85%) were prepared in the laboratory. Their state parameters can be controlled in real time. A series of undrained dynamic triaxial tests by a Global Digital System (GDS) dynamic testing apparatus were conducted to investigate the liquefaction characteristics of gassy sand sediments. The results show that the gassy sand can liquefy the same as the fully saturated sand, but gas existence monotonically increases the sand liquefaction resistance. The occluded gas bubbles have significant influences on sand liquefaction properties. The dynamic pore pressure of gassy sand shows obvious features of slower accumulation, greater amplitude fluctuation, and deeper groove shape in time history curves of pore water, resulting from the effects of gas compression/expansion, migration, and dissolution/exsolution. By introducing a parameter of saturation, a modified model was proposed to describe the evolution of dynamic pore pressure of gassy sands. It was found that the model parameter θ is linearly dependent on the initial gas content (or initial saturation degree Sr).

Evaluating model complexity in simulating supercritical CO2 dissolution, leakage, footprint, and reservoir pressure for three-dimensional hierarchical aquifer

A hierarchical fully heterogeneous aquifer model (FHM) provides a reference for developing and testing 3 facies-based hydrostratigraphic models (HSMs) each representing a CO2 storage aquifer with reduced permeability (k) heterogeneity resolution: 8-unit, 3-unit, and 1-unit homogeneous models. Under increasing aquifer lnk variances (0.1, 1.0, 4.5), flow upscaling was conducted to calculate equivalent permeabilities for the HSMs. Within a Design of Experiment uncertainty analysis framework varying geothermal gradient, salinity of formation water, caprock permeability, and injection rate, CO2 injection coupled to convective mixing was simulated by all models. In addition to the injection phase, all simulations were carried out for 2000 years using PFLOTRAN, a massively parallel, multiphase, multicomponent numerical simulator that ran on the NCAR-Wyoming Supercomputing Center's Yellowstone supercomputer. Simulation outcomes of the HSMs were compared to those of the FHM within their full parameter space, and four performance metrics were evaluated: dissolved CO2, CO2 leakage, plume footprint, and pore pressure evolution in response to injection and migration. Results suggest that aquifer variance, heterogeneity resolution, and salinity can all affect the development of fingering and convective mixing, and therefore the amount of dissolution storage. For the modeling choices and assumptions made in this study, the 3-unit HSM was found to be an all-around optimal model by capturing both the sensitivity of the FHM and the performance metrics under different reservoir storage or operational conditions. Implications for modeling long-term CO2 storage in data-poor systems are discussed and future research indicated.

Cyclic degradation and non-coaxiality of soft clay subjected to pure rotation of principal stress directions

Foundation soils are often under non-proportional cyclic loadings. The deformation behaviour and the mechanism of non-coaxiality under continuous pure principal stress rotation for clays are not clearly investigated up to now. In order to study the effect of pure principal stress rotation, a series of cyclic undrained tests on Shanghai soft clay subjected to cyclic rotation of principal stress directions keeping the deviatoric stress constant under the pure rotation condition were conducted using hollow cylinder apparatus. Based on this, the evolutions of excess pore pressure and strains during cyclic loading were investigated, together with the effects of the intermediate principal stress parameter and the deviatoric stress level on stress–strain stiffness and non-coaxiality. The result can provide an experimental basis for constitutive modelling of clays describing the behaviour under non-proportional loadings.