Geomechanics for Energy and the Environment最新文献

To date, hydraulic fracturing for shale gas extraction has been used at three wells in the UK. In each case, the resulting microseismicity exceeded the UK’s red-light threshold of magnitude 0.5. The three wells all targeted the Bowland Shale Formation, and all were located within close proximity of each other on the Fylde Peninsula in west Lancashire. Observations of hydraulic fracturing-induced seismicity (HF-IS) elsewhere shows that the prevalence of induced seismicity is highly spatially variable. Hence, it is by no means clear whether hydraulic fracturing elsewhere in the Bowland Shale would be likely to generate seismicity at similar levels. In this study we examine the geological and geomechanical conditions across the Bowland Shale with respect to their potential controls on induced seismicity. The abundance of pre-existing faults is likely to play an important control on the generation of HF-IS. We use an automated fault detection algorithm to map faults within a selection of 3D reflection seismic datasets across the Bowland Shale play. For the identified faults, we compute the effective stresses acting on these structures in order to identify whether they are likely to be critically stressed. We find that the Bowland Shale within the Fylde Peninsula contains a significant number of critically stressed faults. However, there is significant variation in the density of critically stressed faults across the play, with up to an order of magnitude reduction in fault density from the west (i.e., the Fylde Peninsula) to the east. We use these observations to inform a seismic hazard model for proposed hydraulic fracturing in areas to the east of the Bowland Shale play. We find that the occurrence of felt seismic events cannot be precluded, however their likelihood of occurring is reduced.

In the process of coalbed methane (CBM) extraction, coal seam penetration modification is frequently subjected to several cycle impact due to drilling-blasting method and deflagration fracturing method. Therefore, the split Hopkinson pressure bar (SHPB) was utilized to investigate the impact cycle effect and confining pressure effect on dynamic behavior of coal. Furthermore, the low-field nuclear magnetic resonance (NMR) was utilized to evaluate the modification of multiscale pore before and after 5 cycles impacts. Finally, the 3D profile scanner was utilized to quantify fracture surfaces and assess fracture roughness variation. The results showed that there existed the 6 MPa critical confining pressure that altered the dynamic mechanical properties of coal. Due to the combined effect of the confining pressure and cycle impact, the damage variable based on the energy method showed a log-normal distribution. With increasing strain rate, the micropores evolved into mesopores and macropores. There was a critical strain rate that caused the ratio of effective porosity to total porosity to shift from increasing to decreasing. Furthermore, the fracture roughness was shown to be positively correlated with the ratio and negatively correlated with seepage fractal dimension. The research findings can provide theoretical guidance for the safer and more efficient CBM exploitation.

Significant efforts are made to reduce the carbon dioxide concentrations in the atmosphere as part of a global scheme that aims to mitigate climate change. Carbon geological storage involves the storage of CO₂ permanently in a subsurface reservoir, commonly a brine saturated aquifer, or a depleted reservoir. Carbon dioxide is also injected for enhanced oil or gas recovery (EOR/EGR). This work applies a material balance to CO₂ for injection and storage in a single-phase dry and/or condensate gas reservoirs. The developed framework based on piston-like displacement can be either used for pressurising depleted gas reservoirs with CO₂ or for EGR. Sensitivity studies of carbon dioxide injection in pressure depleted gas reservoirs and piston-like injection under water drive are presented for various production rates and initial reservoir pressures. Monte Carlo simulations are conducted for combinations of porosity and permeability of different formations such as sandstone, shale, and unconsolidated sand. The results show that CO₂ piston-like injection in EGR is more efficient compared to first depleting the reservoir and then injecting CO₂ as it controls the water influx. The recovery factors in CO₂ EGR are almost insensitive to initial pressures and production rates for both single-phase and condensate gas. Higher permeability formations are much more effective, however, a formation with very high permeability may lead to stability problems.

This paper focuses on understanding trends in the swelling potential of MX80 bentonite under temperatures up to 200 °C using a high-pressure cell. The free swelling behavior of expansive clays under high temperature and high fluid pressure conditions that may be encountered in geological repositories for high-level radioactive waste is important as the swelling potential is closely linked with key transition points on the physical and chemical properties of these clays. Free swell tests performed at temperatures ranging from 22 to 200 °C under sufficient pressure to ensure that the pore water remains as a superheated liquid were performed to assess whether the swell index of bentonite follows similar non-monotonic trends with temperature as observed in the literature for the cation exchange capacity. The measured swell indices follow an increasing-decreasing trend with a transition close to 100 °C. The experimental results can be used to guide parameter selection in long-term simulations on the buffer behavior of the buffer material, which requires an understanding of temperature effects on the coupled thermal-hydraulic-mechanical properties governing these processes.

Microbial-induced calcium carbonate precipitation (MICP) is a new biotechnology that can be used to improve the strength of soils. Unsaturated soils are common in nature and saturation is a significant factor affecting the efficiency of bio-cementation. This study investigated the properties of MICP under different grouting saturation conditions. Unconfined compressive strength (UCS) tests confirmed that biocemented sand could get higher strength under unsaturated grouting conditions with the same calcium carbonate content which helps reduce the material cost. Scanning electron microscopy (SEM) test results show that at lower saturation, the size and amount of calcium carbonate crystals were insufficient but calcium carbonate mainly gathered between the particles. At higher saturation, larger calcium carbonate crystals were produced and exited in pores and on the particle surface, increasing the filling effect. Energy-dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD) test results show that the dominant calcium carbonate morphology detected in samples was calcite, which was the most stable one. X-ray computed tomography (CT) test results show that after cementation, the measured contact surface area became uniform and the coordination number was higher. The flow direction of bacteria and the cementing solution did not induce significant anisotropy in the cementation process. The effective cementation and content of calcium carbonate jointly influenced the improvement of soil mechanical properties.

In this study, we numerically analyze the effect of capillary pressure on gas hydrate deposits through coupled flow and geomechanics simulation, with a focus on the scaled capillary pressure. The scaled effect is predicated on sediment pore-size variations resulting from hydrate dissociation or formation, leading to non-monotonic capillary pressure curves influenced by two primary factors: alterations in pore space and gas saturation. Specifically, hydrate dissociation may increase pore space, thereby reducing capillary pressure. Conversely, enhanced gas saturation owing to dissociation can elevate capillary pressure. We employ a scaled capillary pressure model, accounting for porosity fluctuations caused by hydrate formation or dissociation. Additionally, equivalent pore pressure is utilized to ensure the numerical stability and accuracy in scenarios of strong capillarity. The numerical experiments incorporate two distinct methodologies for hydrate dissociation: heat injection and depressurization. In the heat injection scenario, sensitivity analyses are conducted using a range of model parameters, exhibiting characteristic non-monotonic capillary pressure behaviors attributable to the aforementioned competing factors. Regarding the depressurization approach, the UBGH2-6 site in the Ulleung Basin, East Sea, South Korea, is selected as a real-world field case. Over a 30-day gas production simulation, we observe notable enhancements in hydrate dissociation, signifying improved productivity, and distinctive geomechanical responses, under the influence of the scaled model. This investigation demonstrates that the scaled capillary pressure model, upon the hydrate or ice (i.e., solid) phase change, with coupled flow and geomechanics is crucial for accurate modeling of gas hydrate deposits.

Understanding the impact of repository gas, generated from degradation of waste and its interaction with the host rock, is essential when assessing the performance and safety function of long-term disposal systems for radioactive waste. Numerical models based on conventional multi-phase flow theory have historically been applied to predict the outcome and impact of gas flow on different repository components. However, they remain unable to describe the full complexity of the physical processes observed in water-saturated experiments (e.g., creation of dilatant pathways) and thus, the development of novel representations for their description is required when assessing fully saturated clay-based systems. This was the primary focus of Task A within the international cooperative project DECOVALEX-2019 (D-2019) and refinement of these approaches is the primary focus of this study (Task B in the current phase of DECOVALEX-2023).

This paper summarises development of enhanced numerical representations of key processes and compares the performance of each model against high-quality laboratory test data. Experimental data reveals that gas percolation in water-saturated compacted bentonite is characterised by four key features: (i) a quiescence phase, followed by (ii) the gas breakthrough, which leads to a (iii) peak value, which is then followed by (iv) a negative decay. Three models based on the multiphase flow theory have been developed. These models can provide good initial values and reasonable responses for gas breakthrough (although some of them still predict a too-smooth response). Peak gas pressure values are in general reasonably well captured, although maximum radial stress differences are observed at 48 mm from the base of the sample. Here, numerical peak values of 12.8 MPa are predicted, whereas experimental values are about 11 MPa. These models are also capable of providing a reasonable representation of the negative pressure decay following peak pressure. However, other key specific features (such as the timing of gas breakthrough) still require a better representation. The model simulations and their comparison with experimental data show that these models need to be further improved with respect to model parameter calibration, the numerical representation of spatial heterogeneities in material properties and flow localisation, and the upscaling of the related physical processes and parameters. To further understand gas flow localisation, a new conceptual model has been developed, which shows that discrete channels can possibly be induced through the instability of gas-bentonite interface during gas injection, thus providing a new perspective for modeling gas percolation in low-permeability deformable media.

Exposure of geomaterials to an acidic environment is frequently encountered in modern-day geo-energy and geo-environmental engineering activities, in e.g. incorporation of chemical stimulation for unconventional shale gas exploitation, enhanced geothermal systems, geological carbon sequestration, and the long-term regional stability in carbonate-rich coastal areas. The Multiphysics-involved process for each application is complex and an optimised control calls for a better understanding on the coupling mechanism of the chemical, hydraulic and mechanical fields. This laboratory-based study aims to provide a quantitative calibration and derivation of the key coupling parameters accommodating our recently proposed framework of reactive chemo-mechanics, using a bio-cemented rock-like material as a representative for dissolvable rocks. The advantage of bio-cemented specimens (here by microbially induced carbonate precipitation) over natural rocks lies in their more uniform grain-bond structure and laboratory tunable calcite content. An experimental setup is introduced for investigating the role of calcite content on the mechanical and hydraulic properties of bio-cemented silica sands, followed by uniaxial tests on the bio-cemented specimens immersed in acidic environment to allow a reactive chemo-mechanical setting. Our results show that bio-cemented samples appear to be more “resilient” to an acidified aqueous environment in terms of less strength degradation compared to natural carbonate-rich rocks. Ductile failure mode is observed in the bio-cemented specimens within a certain range of the calcium carbonate content and a brittle-to-ductile transition in the failure mode occurs when the calcite content in the specimen decreases. With the calibrated model and the derived coupling parameters, we further illustrate an example of numerical prediction on the mechanical response of bio-cemented specimens under varying acidic environments and loading rates.

Organic matter which is scattered uniformly in shale can respond to the applied stress and result in the variation of stress field. However, the effects of organic matter content in organic-rich shale on stress interference have not been well considered during developing infill-wells. A fully coupled numerical model is proposed in this paper to consider the whole flow spectrum of shale gas and investigate the effect of organic matter content on stress variation and fracture propagation in infill-well. Through simulating the production and fracturing process with only one set of code, some conclusions can be drawn that the alteration angle of the maximum horizontal principal stress increases and then decreases with the production time. Furthermore, the shrinkage of organic matter enlarges the alteration angle and the magnitude of the maximum horizontal principal stress. Certainly, the optimal fracturing effect in the infill-wells vary due to the different mass fraction of organic matter. This study not only helps to understand the effect of mass fraction of organic matter on stress variation and fracture propagation, but also provides theoretical support for increasing production from shale gas reservoirs.

Brittleness Index (BI) is a critical parameter characterising the deformation regime of geo-materials, covering the range from purely brittle (fractures) to ductile (plastic flow). A variety of BI models have been developed based on rock properties such as mineralogy, elastic parameters, or constitutive law. However, very few of them are based on the hydro-mechanical interactions emerging in underground engineering applications. In this study, we propose a BI model based on the partitioning of the injection energy E_I into non-seismic deformation energy E_d associated with hydraulic fracture propagation. To calculate the E_d, we apply a model for temporal fracturing area (A_d) within the penny-shaped fracture; and we also correlate the wellbore pressure and the three-dimensional strain induced by hydraulic fracturing of the different types of rock samples subjected to true triaxial stress conditions (TTSC), either σ_v = 6.5 MPa, σ_H = 3 MPa, σ_h = 1.5 MPa or σ_v = 15 MPa, σ_H = 10 MPa, σ_h = 5 MPa. As a comparison, the BI is also quantified based on the existing models: (i) acoustic measurement from Rickman et al. (2008), and (ii) the Mohr-Coulomb’s criteria from Papanastasiou et al. (2016). The E_d ranges between 32.4% and 90.6% of the total injection energy E_I, which is slightly higher than the value reported from field-scale data (15% to 80%), but comparable to laboratory-derived data (18% to 94%) from literature. The results show that the predictions based on our proposed energy-based BI model are qualitatively consistent with Papanastasiou et al.’s, but less so with Rickman et al.’s. Our BI model is shown to be stress-dependent and capable of capturing the brittle-to-ductile behaviour of geomaterials subjected to hydraulic fracturing. This study demonstrates that our BI model opens a new way for quantifying the brittleness index regarding to realistic fracture propagation scenarios in field.