Geomechanics for Energy and the Environment最新文献

The decarbonization of the built environment requires rapid growth in energy storage solutions due to the intermittent nature of most renewable energy sources. This paper focuses on the efficacy of so-called energy tunnels (i.e., tunnels equipped with pipe heat exchangers) used for underground thermal energy storage. By harnessing a 3-D thermo-hydraulic finite element model validated against full-scale experimental data, this work specifically explores seasonal, medium-temperature, thermal energy storage operations of energy tunnels. Numerical simulations are performed to unravel the influence of convection resulting from groundwater flows and airflows on the thermal energy storage performance of energy tunnels. The analyses address the impact of different groundwater flow velocities, air temperatures, and airflow velocities on the thermal losses and storage efficiency of energy tunnels used as thermal batteries. The study discourages underground thermal energy storage in the presence of convection due to significant heat losses. It shows that thermal energy storage operations via energy tunnels are feasible in site conditions characterized by no groundwater flow, limited temperature differentials between the heat carrier fluid circulating in the pipe heat exchangers and the surroundings, and thermal insulation on the tunnel intrados.

In order to characterise a rock formation prior to subsurface operations, it is required to find a microscale rock volume for which the homogenised property does not fluctuate when the size of the sample is increased; the Representative Elementary Volume (REV). Its determination usually comes at the cost of a large number of simulations, making it overall a computationally expensive process. Therefore, many scientific studies have been dedicated to optimising the process of finding REV. Using statistical numerical methods, it is shown that the fluctuation of the effective property corresponds overall to a cone-like shape convergence. We suggest determining the generic evolution law of the cone of convergence, which can be used to predict the size of the REV and the effective physical property. This study is based on simulations of Stokes flow through idealised microstructures from which the permeability is upscaled. By tracing and plotting the convergence of permeability for multiple samples, the full cone of convergence appears. The cone shows exponential growth and decay, converging towards the effective permeability of the microstructure. By fitting a log-normal distribution on the collected data points, we show that the generic evolution law of the cone of convergence can always be described with two parameters, independently of the porosity. We show that the determined law of the cone also applies to real microstructures, despite the presence of natural heterogeneities. The new method allows us to reduce the computational costs of finding all characteristics related to REV by simulating several subsamples rather than the full-sized sample, unlocking thereby high-resolution samples which are often too computationally expensive. The use of a statistical model provides quantification of the precision level we can obtain on the REV determination.

To investigate the use of thermally-activated helical piles in shallow geothermal energy systems, a 1-g modelling study was conducted. Helical piles with either a single- or double- helix were installed in a medium dense, dry sand, and subjected to mechanical, thermal only and thermo-mechanical loading. The results indicate that during the thermal tests (1 – 3 cycles), a small upwards residual displacement was observed and pile head movements ranged between about 90% and 100% of the free expansion of the pile shaft above the shallowest helix, suggesting that the helices fixed the shaft and little restraint was offered by the surrounding soil. In the thermo-mechanical tests (30 thermal cycles), the pile head developed irrecoverable settlement as a function of the number of helices (more helices, less settlement) and initial load (higher load, greater settlement). No significant alteration in pile axial stiffness or resistance was found for piles with zero mechanical load that underwent only a few thermal cycles; however, an increase in stiffness and resistance, beyond that due to inherent variability in the test setup, was observed for piles with an initial load and following a large number of thermal cycles. The testing of thermally-activated helical piles in sand has confirmed that the response is similar to conventional piles and that thermal ratcheting effects can be managed by the application of suitable margins of safety in design and/or the use of multi-helix piles.

The present paper is devoted to multi-scale constitutive modeling of the brittle–ductile transition in rocks. The rocks are considered as heterogeneous media composed of solid phase weakened by microcracks at the microscale and two different populations of pores at the micro and mesoscales. A Drucker–Prager type criterion is first formulated considering microcracking-induced damage in the solid phase. By means of a two-step modified secant variational method, this criterion is then adopted to derive a micro–macro model for double porous medium taking into account the effects of pores. Considering that the operative deformation mechanism in brittle rocks is microcracking, the Drucker–Prager type microcrack damage model is applied to describe the transition of three typical brittle rocks from brittle faulting to dilatant ductile flow by establishing a linear relation between the critical damage caused by microcrack propagation and confining pressure. By introducing an appropriate plastic hardening law and taking into account the influence of confining pressure on plastic hardening parameter and dilatancy coefficient, the micro–macro model for porous rocks is applied to describe the transition from brittle faulting to compactive ductile flow in two typical porous rocks. Comparisons between numerical simulations and experimental data show that the main features of brittle–ductile transition of two types of rocks are well captured by the proposed model.

A damaged cement sheath in wells can open a leakage pathway to shallow freshwater aquifers and atmosphere. Quantitative assessment of leakage along wells has become an area of interest for both the industry and the regulatory bodies. The well leakage can be of importance in both active and legacy wells. In order to estimate leakage through cement sheaths, the size of the leakage pathway and the damage in the cement sheath must be estimated. In this work, we have developed a hydro-thermo-mechanically coupled near-well model that aims to calculate the evolution of cement’s stress as it cures. This process takes into account the cement’s gradual increase in stiffness, chemical shrinkage, and the heat of hydration. The results are verified using lab measured cement stress and pore pressure data from the literature. A case study was developed based on a low-enthalpy geothermal doublet in the Netherlands. The results show that during the cold water injection, an outer microannulus may open to 60 µm. The presence of an external source of water and formation stiffness are of significant importance in determining the damage to the cement sheath. The heat of hydration in cement increases the temperature of cement during curing. The subsequent drop in temperature due to drilling or completion reduces the cement stress and exacerbates the damage to the cement sheath. The producer well may not form a microannuli, however shear and cyclical failure may be of higher likelihood. The modelling framework presented here allows for estimation of annular cement stress in the well. The analysis provides quantitative estimates of the size of the leakage pathway along a well that can be used to estimate well leakge. Quantitative estimate of well leakage provides crucial information for quantitative risk analysis and provides a framework to optimize well operations to minimize leakage risk.

Offshore wind is the most mature of the offshore renewable energy technologies and has a significant role to play in the energy transition. 2000 GW of offshore wind capacity is anticipated globally by 2050 in order meet the targets of the Paris Agreement; 35 times the current installed capacity. The pace and scale of offshore wind ambitions to support the energy transition present a range of challenges for the offshore geotechnical sector and the broader offshore wind sector. Challenges extend across the life-cycle of projects from marine spatial planning, site investigation, design, manufacturing, installation, operation and decommissioning, and across the supply chain regarding availability of raw materials for foundations, anchors and mooring systems, vessels and equipment for site investigation and installation, and trained geotechnical personnel. This paper identifies five key challenges and sets out the necessary shifts in technology, culture and practice in geotechnical engineering to achieve the ambitious targets to deliver offshore wind at the pace and scale required for the energy transition. The paper closes with a reflection on the consequence of delaying or not meeting net-zero targets, and thus identifying the urgency for these shifts in technology, culture and practice to be developed and adopted.

A field test was conducted to assess the thermo-mechanical behavior of a full-scale energy retaining pile (ERP) adjacent to a utility tunnel during geothermal extraction. Numerical models were also established and calibrated using measured data. The extracted thermal power, temperature, thermally induced strain and stress, and bending moment of the ERP were analyzed. Additionally, a comparative analysis was conducted using validated numerical models to assess the impact of the air temperature (T_air) inside the adjacent utility tunnel on the thermo-mechanical behavior of the ERP. The findings highlight that the extraction thermal power of the tested ERP was 57 W/m, with the short-term geothermal extraction operation yielding even higher values of 200–250 W/m. The operation of the adjacent high-temperature utility tunnel can lead to an average increase of approximately 15 % in the extracted thermal power of the ERP. Additionally, during the geothermal extraction, regardless of the value of T_air, the ERP primarily reflected in changes to the axial thermo-mechanical behavior. However, the higher-temperature utility tunnel can result in a notable bending moment of the ERP prior to the geothermal extraction operation. Thus, considering the influence of T_air on the thermo-mechanical behavior of the ERP becomes crucial during the preliminary design phase.

Reservoir compaction and surface subsidence are widely studied consequences of hydrocarbon (HC) production. This is an important topic in the Brazilian Pre-salt, where wells cross thick salt layers to reach carbonate reservoirs. Adding knowledge on the behavior of salt as caprock is strategic, considering the upcoming demands for decommissioning and the trends for energy transition, such as carbon capture and storage (CCS). This work presents a literature review on subsidence resulting from HC production and the associated mechanical behavior of the salt caprock. A numerical study of a conceptual Pre-salt reservoir is performed to assess the combined behavior of reservoir compaction/expansion, salt caprock creep, and subsidence. The production, injection (such as CCS), and abandonment periods are considered. From the assumptions and findings of the study, two conclusions are drawn: i) the salt caprock creep contribution to the stress paths and changes in permeability is of small magnitude, while it is relevant to subsidence if the reservoir is abandoned in the depleted condition; and ii) creep prolongs the evolution or reversal of the mechanical behavior and permeability when the external loads acting in the model are reverted. This study is expected to serve as a reference for more advanced analyses in Pre-salt reservoir geomechanics, specially considering the trends for decommissioning and energy transition.

Salt is an attractive disposal medium for radioactive waste because intact salt is essentially impermeable and non-porous. However, upon drift or borehole excavation a damaged region develops surrounding the excavation which causes increased permeability and porosity creating potential flow paths for brine. Brine leads to corrosion of waste forms and waste packages and is a possible transport vector for radionuclides, so it is important to better understand the early-time behavior and evolution of brine flow in a salt. As a result, this study is part of Task E of DECOVALEX-2023 which focuses on understanding the evolution of thermal, two-phase hydrological, and mechanical processes in the excavation damaged zone in salt. Field measurements from The Brine Availability Test in Salt (BATS) 1a heater experiment are analyzed by implementing a high-resolution three-dimensional numerical model. This salt heater experiment consists of 28 days of heating and 13 days of cooling in a central borehole within bedded salt at the Waste Isolation Pilot Plant (WIPP). Here, the flow simulator PFLOTRAN is utilized; simulations are run on a Voronoi mesh, with temperature-dependent thermal conductivity, permeability and porosity decay away from excavations. The temperature-dependency of permeability is done to match field measurements. Results from the simulation match temperature measured in the field within + /- 0.1 °C and the total brine inflow over the 41-day experiment. This study illustrates that the accuracy of the temperature evolution within salt is critically important when analyzing and modeling experimental data by simulating three heating scenarios of the BATS 1a experiment showing that temperature has a direct effect on total brine inflow.

In the process of integrating supercritical CO₂ (ScCO₂)-enhanced shale gas recovery and geological sequestration, the mechanical properties of shale can be impacted by ScCO₂ under high-temperature and high-pressure conditions. This can affect wellbore stability, production efficiency, and the safety of sequestration. To address this issue, this study investigated the interactions between shale and three types of fluids: ScCO₂, water, and a combination of ScCO₂ and water. Experiments were conducted at high pressure (15 MPa and 45 MPa) and high temperature (100 °C). Changes in shale's mechanical properties before and after immersion were analyzed using uniaxial compression tests and acoustic emission monitoring. The main cation content, microstructure, and element minerals of shale's solution after immersion were also studied. The results show that immersion in ScCO₂ and related fluids deteriorates shale's mechanical properties. Immersion in ScCO₂ has the least effect on shale strength, followed by the change in shale strength caused by immersion in water, and shale strength is the lowest after immersion in a combination of water and ScCO₂. ScCO₂ imbibition promotes the occurrence of micro-cracks, while immersion in water makes shale's matrix loose, forming a pore network structure that is most significantly affected by a combination of water and ScCO₂. For unsoaked and water-immersed shale samples, the acoustic emission events mainly occur during the unstable crack propagation stage, while the acoustic emission events in shale samples treated with ScCO₂ are more dispersed. Compared with previous dynamic pressure immersion experiments, the strength of shale after static pressure immersion increases by 10–30 MPa. This study aims to provide a more comprehensive understanding of the alterations in the mechanical properties of shale when subjected to high temperature and high-pressure immersion conditions. The findings provide valuable data for shale gas extraction and carbon sequestration.