We develop a new fully coupled thermo-hydro-mechanical (THM) model to investigate the combined effects of thermal perturbation and in-situ stress on heat transfer in two-dimensional fractured rocks. ...We quantitatively analyze the influence of geomechanical boundary constraints and initial reservoir temperature on the evolutionary behavior of fracture aperture, fluid flow and heat transfer, and further identify the underlying mechanisms dominating the coupled THM processes. The results reveal that, apart from enhancing normal opening of fractures, the transient cooling effect of thermal front may trigger shear dilations under the anisotropic in-situ stress condition. It is found that the applied in-situ stress tends to impose a strong impact on the spatial and temporal variations of fracture apertures and flow rates, and eventually affect heat transfer. The enhancement of reservoir transmissivity during transient cooling tends to be significantly overestimated if the in-situ stress effect is not incorporated, which may lead to unrealistic predictions of heat extraction performance. Our study also provides physical insights into a fundamental thermo-poroelastic behavior of fractured rocks, where fracture aperture evolution during heat extraction tends to be simultaneously governed by two mechanisms: (1) thermal expansion-induced local aperture enlargement and (2) thermal propagation-induced remote aperture variation (can either increase or decrease). The results from our study have important implications for optimizing heat extraction efficiency and managing seismic hazards during fluid injections in geothermal reservoirs.
The identification of underground formation lithology can serve as a basis for petroleum exploration and development. This study integrates Extreme Gradient Boosting (XGBoost) with Bayesian ...Optimization (BO) for formation lithology identification and comprehensively evaluated the performance of the proposed classifier based on the metrics of the confusion matrix, precision, recall, F1-score and the area under the receiver operating characteristic curve (AUC). The data of this study are derived from Daniudui gas field and the Hangjinqi gas field, which includes 2153 samples with known lithology facies class with each sample having seven measured properties (well log curves), and corresponding depth. The results show that BO significantly improves parameter optimization efficiency. The AUC values of the test sets of the two gas fields are 0.968 and 0.987, respectively, indicating that the proposed method has very high generalization performance. Additionally, we compare the proposed algorithm with Gradient Tree Boosting-Differential Evolution (GTB-DE) using the same dataset. The results demonstrated that the average of precision, recall and F1 score of the proposed method are respectively 4.85%, 5.7%, 3.25% greater than GTB-ED. The proposed XGBoost-BO ensemble model can automate the procedure of lithology identification, and it may also be used in the prediction of other reservoir properties.
•An improved thermo-hydro-mechanical coupling model is developed.•Effects of shear dilation and thermal stress on heat extraction are investigated.•Effects of fracture properties and deviatoric ...stress on THM processes are studied.•Practically useful guidelines for developing sustainable EGSs are proposed.
Shear dilation of fractures has been recognized as a main mechanism of permeability enhancement by hydraulic stimulation in Enhanced Geothermal Systems (EGSs); however, the interactive role of fracture shear dilation and thermo-hydro-mechanical (THM) coupling processes in long-term heat extraction performance of EGSs remains unclear. In this study, we develop a novel THM coupling model based on the discrete fracture network approach, which can realistically capture important processes including hybrid normal-shear deformation of fractures, thermal expansion of rocks, fluid flow in both fractures and rocks, and heat convection/conduction as well as local thermal non-equilibrium effect and changes in physical parameters of fluid. We quantitatively investigate the effects of fracture network geometries and geomechanical boundary constraints on fracture shear dilatancy, and the resulting heat transfer characteristics of EGSs. Numerical results reveal that shear dilation of fractures can be triggered by transient pore pressurization and thermal stress under anisotropic in-situ stress condition, and would severely engender flow channeling as well as anisotropic heat transfer, which strongly impact the heat extraction performance. The production temperature tends to be overestimated while the thermal production rate may be underestimated, if the shear dilatational behavior is not incorporated. Increased in-situ stress ratio and injection/production pressure would magnify the effects of shear dilation, and lead to considerable enhancement of fracture permeability, eventually resulting in much earlier and quicker temperature drop. Excessive increase of fracture density and the location of injection-production wells parallel to potential channelized flow paths, formed by intersected fractures preferentially oriented for shear sliding, tend to form short circulating flow paths and reduce the heat extraction performance. Our study demonstrates the importance of considering fracture shear dilation and fully-coupled THM behaviors when evaluating the long-term performance and efficiency of heat extraction in EGSs.
In an enhanced geothermal system (EGS), fractures and fracture networks are the predominant elements for fluid flow and heat transfer through the artificial reservoir. In this work, different ...conceptual discrete fracture networks were generated by characterizing the fracture number, fracture bifurcation and fracture connectivity of the fracturing area. A thermal–hydraulic (TH) coupled mathematical model was applied to evaluate the EGS thermal recovery process. Heat extraction capacity was appraised in terms of the temperature production, net power generation and thermal recovery rate. The results show that an interconnected fracture network with considerable bifurcation results in high heat production and power generation, however the energy efficiency is not optimized due to water loss. The effect of fracture connectivity is more significant than that of fracture spacing. The more fractures (bifurcations) the higher the overall and local heat recovery rates near the production well. For the case with connected fractures without bifurcation, the less characteristic length in the direction of flow results in a lower heat production and power generation.
Enhanced geothermal system (EGS) technologies have been developed to improve geothermal energy production from hot dry rock (HDR). In this study, discrete fracture network models for geometric ...topological networks that consider different parameters (the fracture density and the fracture length index) were built on the basis of fractal geometry theory. The heat extraction processes of CO2 and water as the working fluid through different discrete fracture networks were simulated with the application of the thermal–hydraulic–mechanical (THM) coupled method. A series of sensitivity analyses were carried out to reveal the influences of fracture parameters on heat transfer processes. Based on the simulation results, heat extraction efficiencies and temperature distributions in the reservoir of CO2 and water as the working fluid were compared, which showed that CO2 as the working fluid can bring a faster thermal breakthrough. It was found that the fracture length index a = 2.5 and the fracture density I = 5.0 can provide the highest heat extraction rate compared with other cases. This study provides a detailed analysis of fracture parameters and working fluids, which will contribute to the optimized management of geothermal energy production.
Electromanipulating Water Flow in Nanochannels Kou, Jianlong; Yao, Jun; Lu, Hangjun ...
Angewandte Chemie (International ed.),
February 16, 2015, Letnik:
54, Številka:
8
Journal Article
Recenzirano
In sharp contrast to the prevailing view that a stationary charge outside a nanochannel impedes water permeation across the nanochannel, molecular dynamics simulations show that a vibrational charge ...outside the nanochannel can promote water flux. In the vibrational charge system, a decrease in the distance between the charge and the nanochannel leads to an increase in the water net flux, which is contrary to that of the fixed‐charge system. The increase in net water flux is the result of the vibrational charge‐induced disruption of hydrogen bonds when the net water flux is strongly affected by the vibrational frequency of the charge. In particular, the net flux is reaches a maximum when the vibrational frequency matches the inherent frequency of hydrogen bond inside the nanochannel. This electromanipulating transport phenomenon provides an important new mechanism of water transport confined in nanochannels.
A vibrational charge outside a nanochannel can promote water flux within the channel. A decrease in the distance between the charge and the nanochannel causes an increase in the water net flux, which is contrary to that of the fixed‐charge system. This electromanipulating transport phenomenon provides an important new mechanism of water transport confined in nanochannels.
We investigated the effect of in situ stresses on fluid flow in a natural fracture network. The fracture network model is based on an actual critically connected (i.e., close to the percolation ...threshold) fracture pattern mapped from a field outcrop. We derive stress-dependent fracture aperture fields using a hybrid finite-discrete element method. We analyze the changes of aperture distribution and fluid flow field with variations of in situ stress orientation and magnitude. Our simulations show that an isotropic stress loading tends to reduce fracture apertures and suppress fluid flow, resulting in a decrease of equivalent permeability of the fractured rock. Anisotropic stresses may cause a significant amount of sliding of fracture walls accompanied with shear-induced dilation along some preferentially oriented fractures, resulting in enhanced flow heterogeneity and channelization. When the differential stress is further elevated, fracture propagation becomes prevailing and creates some new flow paths via linking preexisting natural fractures, which attempts to increase the bulk permeability but attenuates the flow channelization. Comparing to the shear-induced dilation effect, it appears that the propagation of new cracks leads to a more prominent permeability enhancement for the natural fracture system. The results have particularly important implications for predicting the hydraulic responses of fractured rocks to in situ stress fields and may provide useful guidance for the strategy design of geofluid production from naturally fractured reservoirs.
Two-phase flow in two digital cores is simulated by the color-gradient lattice Boltzmann method. This model can be applied to two-phase flow with high-density ratio （on order of 1000）. The first ...digital core is an artificial sandstone core, and its three-dimensional gray model is obtained by Micro-CT scanning. The gray scale images are segmented into discrete phases （solid particles and pore space） by the Otsu algorithm. The second one is a digital core of shale, which is reconstructed using Markov Chain Monte Carlo method with segmented SEM scanning image as input. The wettability of solid wall and relative permeability of a cylindrical tube are simulated to verify the model. In the simulations of liquid and gas two phase flow in dig- ital cores, density ratios of 100, 200, 500 and 1000 between liquid and gas are chosen. Based on the gas distribution in the dig- ital core at different times, it is found that the fingering phenomenon is more salient at high density ratio. With the density ratio increasing, the displacement efficiency decreases. Besides, due to numerous small pores in the shale, the displacement effi- ciency is over 20% less than that in the artificial sandstone and the difference is even about 30% when density ratio is greater than 500. As the density ratio increases, the gas saturation decreases in big pores, and even reaches zero in some small pores or big pores with small throats. Residual liquid mainly distributes in the small pores and the edge of big pores due to the wettabil- ity of liquid. Liquid recovery can be enhanced effectively by decreasing its viscosity.
Fracture aperture change under stress has long been considered as one of primary causes of stress sensitivity of fractured gas reservoirs. However, little is known about the evolution of the ...morphology of fracture apertures on flow property in loading and unloading cycles. This paper reports a stress sensitivity experiment on carbonate core plugs in which Computed Tomography (CT) technology is applied to visualize and quantitatively evaluate morphological changes to the fracture aperture with respect to confining pressure. Fracture models were obtained at selected confining pressures on which pore-scale flow simulations were performed to estimate the equivalent absolute permeability. The results showed that with the increase of confining pressure from 0 to 0.6 MPa, the fracture aperture and equivalent permeability decreased at a greater gradient than their counterparts after 0.6 MPa. This meant that the rock sample is more stress-sensitive at low effective stress than at high effective stress. On the loading path, an exponential fitting was found to fit well between the effective confining pressure and the calculated permeability. On the unloading path, the relationship is found partially reversible, which can evidently be attributed to plastic deformation of the fracture as observed in CT images.
Optimization of the depressurization pathways plays a crucial role in avoiding potential geohazards while increasing hydrate production efficiency. In this study, methane hydrate was formed in a ...flexible plastic vessel and then gas production processes were conducted at constant confining pressure and constant confining temperature. The CMG-STARS simulator was applied to match the experimental gas production behavior and to derive the hydrate intrinsic dissociation constant. Secondly, fluid production behavior, pressure-temperature (P‐T) responses, and hydrate saturation evolution behaviors under different depressurization pathways were analyzed. The results show that integrated gas-water ratio (IGWR) decreases linearly with the increase in depressurizing magnitude in each step, while it rises logarithmically with the increase in the number of steps. Under the same initial average hydrate saturation and the same total pressure-drop magnitude, a slow and multistage depressurization strategy would help to increase the IGWR and avoid severe temperature drop. The pore pressure rebounds logarithmically once the gas production is suspended, and would decrease to the regular level instantaneously once the shut-in operation is ended. We speculate that the shut-in operation could barely affect the IGWR and formation P‐T response in the long-term level.
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