Geomechanics for Energy and the Environment最新文献

During the operation of hydraulic fracturing as used in many geo-energy and geo-environment applications, chemical stimulation is often incorporated for cracking enhancement in low-permeability geological formations for the purpose of an optimization of energy recovery. The mechanism of subcritical crack propagation in a chemically reactive environment is essential for understanding of the involved coupled chemo-mechanical process and a better control of acid-assisted hydraulic fracturing. It has been postulated that the rate of crack propagation under environmental loads is inherited from the chemical processes involved including the reaction and the diffusive transport. However, laboratory explorations focusing on the evolving interplay between the propagation of a fluid-pressurizing individual crack and the environment it is subject to via a variable chemical intensity imposed have been rare. Here we present an experimental investigation on a single tensile crack propagation in alginate hydrogel as an analogue material for brittle rocks, driven by the injection of a chemically reactive fluid kept at constant pressure using a Hele-Shaw cell setup. We show that an intensified chemical environment can accelerate tensile crack propagation in both subcritical crack growth and fracturing regimes, while leading to the Region III fracturing of less brittle characteristics. During the experiment, crack-tip blunting upon injection of reactive solutions was observed, suggesting a competing mechanism between the crack-tip geometry induced toughening and the chemically induced softening within the process zone as the crack advances. Our results provide quantitative insights into how a chemically reactive environment facilitates the growth of a single macroscopic crack of mode I opening in a low-permeability matrix through coupled chemo-mechanical feedback. The imposed chemical environment promotes crack propagation while alleviating the stress concentration at the advancing crack tip, suggesting a more energy-efficient method compared to pure water fracturing. We anticipate our experimental investigation presented here to be a starting point of sound laboratory support for future studies towards a more controllable technique of chemical stimulation in geomaterials as well as complementing the ongoing modeling efforts in reactive chemo-mechanics.

The tensile strength of remolded granite residual soil under different water content conditions, during wetting and drying, was investigated using a self-made horizontal direct tension apparatus. The variations in tensile strength and the microcosmic mechanism of shrinkage crack formation and development were elucidated from the perspectives of suction stress and cementing force. Experimental results indicated that the tensile strength of granite residual soil initially increased and then decreased under different water content conditions. During the wetting process, the tensile strength followed a similar trend, but the peak strength (10 kPa) was lower compared to that under different water content conditions (22 kPa). The drying process exhibited three stages of tensile strength variation: a linear increase stage, a stationary stage, and a slight decrease stage. The peak value of the tensile strength during drying was much higher (reaching approximately 84 kPa) than that under different water content conditions and the wetting process. The tensile strength of remolded granite residual soil was solely controlled by suction stress under different water content conditions and in the wetting process. However, in the drying process, the tensile strength was also influenced by the cementing force, resulting in a peak value four times higher than that under different water content conditions and seven times higher than that in the wetting process. Suction stress served as the source of tensile stress in the soil during the drying process, and the development of cracks caused by suction stress led to a reduction in the overall tensile strength of the soil. Suction stress acted as both a contributor and a destroyer of soil tensile strength. During the drying process, the soil sample exhibited weakly acidic pH and gradually weakened hydrophilic ability. This led to the formation of stronger binding forces within the soil skeleton. Consequently, for soil samples with the same moisture content, the tensile strength during the drying process was much greater than in the other two situations. This study provides an alternative perspective on the source of soil tensile strength and its main controlling factors.

In this study, we describe strategies to reduce uncertainties in the estimation of in-situ stresses. The strategies are based on the commonly used poro-elastic model with strain corrections for in-situ stress estimation. In addition to the calibration of the minimum horizontal stress magnitudes from measurement data, we further constrain the magnitude of maximum horizontal stress quantitatively with (i) a critical stress state model, (ii) (non)-occurrences of breakout and/or drilling-induced fracture observed from image logs, and (iii) qualitatively with elliptical borehole shapes observed from multi-arm caliper logging data. The methodology is demonstrated using wells near Dawson Creek, Northeast British Columbia and Northwest Alberta, in Western Canada Sedimentary Basin (WCSB). The uncertainty of the estimated maximum horizontal stress magnitude was reduced and the range of the maximum horizontal stress was narrowed after the application of these constraint strategies. It is also observed that in the east of the study area, the in-situ stress regime is a normal fault stress regime. The presence of a normal fault stress regime is unexpected since a strike–slip fault stress regime is typically considered for the entire region. Yet, analysis of caliper data in two horizontal wells has confirmed its presence.

Thermally-activated (TA) piles are cost-effective technologies with the dual role of transferring structural loads to the ground while exchanging heat with the surrounding soil as part of shallow geothermal energy systems. As TA piles are subjected to both thermal and mechanical loads, the behaviour on the soil-structure interface is particularly complex and is key for the analysis and design of these structures. The present paper aims to review the current state of knowledge regarding the thermal dependency of soil-structure interface behaviour and provide an overview of experimental results obtained from non-isothermal tests investigating soil and soil-structure interface behaviour. This overview includes comparison of the different experimental equipment and procedures, soil types, initial soil state overconsolidation ratio (degree of consolidation or relative density) and thermal loadings. It was found that it is not straightforward to reach a unique interpretation regarding possible variation of the soil-structure interface behaviour at different temperatures: the framework of the experimental evidence is very complex due to the wide variation in testing conditions. Therefore, it was not possible to compare the studies like-for-like, leading to an apparently ambiguous interpretation of the results. Overall, the consensus across this and other studies is that the potential variation of interface resistance with temperature typically appears to be limited and not very significant.

The economic feasibility of gas production from hydrate deposits is critical for hydrate to become an energy resource. Permeability in hydrate-bearing sediments dictates gas and water flow rates and needs to be accurately evaluated. Published permeability studies of hydrate-bearing sediments mostly quantify vertical permeability; however, the flow is mainly horizontal during gas production in layered reservoirs. Additionally, ASTM standards require a hydraulic gradient of 10–30 to be used during laboratory permeability measurements, but the gradient is much higher in the field, particularly near a production well. To address these issues, this study focuses on the hydraulic properties of a sandy silt subsample of the hydrate reservoir and a clayey silt subsample of the fine-grained, hydrate-free interbed recovered from a GC955 deep-water Gulf of Mexico gas hydrate reservoir. We characterize the sediment pore space with water retention curves for both hydrate-free and hydrate-bearing samples (hydrate saturation, S_h =80 %). Vertical deformation with increasing stress is also quantified while consolidating the samples to the 4 MPa in situ vertical effective stress. The customized permeameter measures both the horizontal and vertical permeability with increasing stress. Results show that high hydraulic gradients lower permeability in the flow direction, possibly due to increased flow tortuosity and local sediment compaction from the high seepage force. Assuming a single permeability value, even though hydraulic gradients decrease with distance from the well, is not realistic for field estimations. The results highlight that permeability anisotropy, hydrate saturation, stress conditions, and hydraulic gradient all substantially impact reservoir permeability during production.

The CNSC, the Canadian regulator for the nuclear industry, participated in DECOVALEX-2023 Task G that focuses on the thermo (T) - hydro (H)- mechanical (M) behaviour of rock joints. Joints are omnipresent in rock masses and are planes of weakness in the host rock. When deep geological repositories (DGRs) for radioactive waste are being considered in areas where rock joints are present, the joints could be preferential pathways for radionuclide migration. Therefore, their THM behaviour must be better understood to assess the safety of the DGR. Under different possible internal and external perturbations, a joint can move by shear and dilation. If the joint crosses the emplacement area of a waste container, the heat generated from the waste can itself induce shearing of the joint. Excessive shear movement can in turn lead to failure of the container, resulting in earlier release of radionuclides. Furthermore, dilation that might accompany shear, results in an increase in the joint aperture creating a faster flow path for radionuclide transport. Mathematical models are important tools that need to be developed and employed, in order to assess joint shear and dilation under different loading conditions, such as the heat generated from the emplaced waste. The authors have developed such a mathematical model based on a macroscopic formulation within the framework of elasto-plasticity. It is verified against analytical solutions and validated against shear under constant normal load tests and thermal shearing tests of joints in granite.

Microbially induced calcium carbonate precipitation (MICP) technology has garnered significant attention for enhancing soil engineering properties, presenting a potential alternative to traditional cementitious materials for soil seepage control. This study investigates the application of MICP to enhance the hydraulic characteristics, specifically reducing porosity and hydraulic conductivity, of loose sandy soils. Three types of sand-river sand, sea sand, and quartz sand-underwent MICP treatment in cylindrical molds using multiple treatment schemes. Laboratory experiments, including permeability tests, porosity tests, scouring and soaking resistance tests, microstructural testing and analysis, and microfluidic chip tests, were conducted to evaluate the hydraulic characteristics and microstructure contributing to sealing. The results revealed that the structural integrity of the MICP-treated sand declined with an increase in cementation solution (CS) concentration, which were then categorized into intact, discontinuous, and loose blocks. The average decreases in porosity and hydraulic conductivity were 5.5% and 97.2%, respectively, from 0.382 and 4.33 × 10^-4 m/s (before treatment) to 0.361 and 1.2 × 10^-5 m/s (after treatment). Three cementation patterns, G-C-G, G-G, and G-C, were identified in the MICP-treated sand, with corresponding pore-filling rates decreasing successively. Furthermore, the study explores the feasibility of individually distinguishing and characterizing the contributions of biofilms and calcium carbonate precipitation to the reduction in porosity and permeability in biocemented sand through simulation.

The brittleness index of rocks is able to provide crucial guidance to the operation of the drilling and hydraulic fracturing. The definition and evaluation method of the rock brittleness has not yet been unified and standardized due to their diversity. We therefore develop an evaluation method of the rock brittleness based on statistical damage relation. A set of uniaxial compression tests on different rock samples are conducted, and corresponding experimental data is collected and analyzed. Then we establish piecewise damage constitutive models based on different combinations of statistical distribution functions, including power function distribution, Weibull distribution, lognormal distribution and logistic distribution. A new evaluation method of rock brittleness based on energy method and the piecewise statistical damage constitutive model is proposed, and the evaluation results show that the increase of damage variable of peak strain will undermine rock brittleness, and the mineral composition contents have influence on the brittleness of rocks. Comparison work between this proposed method and previous brittleness criteria shows that the brittleness index $B_{43}$ exhibits enhanced stability and consistency in rock brittleness. This study presents a novel method to studying rock brittleness, enhancing current evaluation methods and deepening our understanding of the rock index. Regarding practical application, some field operations involved with the process of drilling or fracturing lead the continuous damage evolution of rocks under loading conditions. The brittleness index from the proposed evaluation method is comparatively reliable and practical for on-site drilling and fracturing.

In China, municipal solid waste containing large amounts of kitchen waste possesses the characteristics of lower cellulose content (15% on a dry basis) and a faster CH₄ generation rate. This promotes the emission of hazardous gases and the increase in gas pressure in landfill cover systems. The higher temperature further aggravates the aforesaid phenomena. The present work investigated the temperature effect on the gas breakthrough pressure (GBP) and permeability of compacted loess. The water permeability k_w increases with increasing temperature. The intrinsic water permeability K_W independent of pore fluid properties behaves just in an opposite manner. K_W would have been increased with increasing temperature if the rigid wall of the permeameter had not been intervened in the permeability tests. The intrinsic permeability K_G also decreases with the increase in temperature. Although the higher K_G neglects the gas slippage effect, the combination of the thermal expansion of minerals, the transformation of bound water to free water, and the thermal expansion of free water causes pore water to migrate into macropores. Such a pore water migration is accompanied by the water-gas boundary moving to the vadose zone. On the other hand, the capillary pressure shows a correspondence with the GBP value. The higher temperature reduces the difficulty for gas molecules to overcome the surface tension at the water-gas boundary, corresponding to the lower GBP value. The findings provide critical guideposts concerning the design of the gas breakthrough and permeability of compacted loess in landfill covers under the temperature effect.