A Two-Phase Flow Theory For Thermal-Moisture-Hydro-Mechanical Multi-Physical Coupling In Fractured Shale

Gas-liquid two-phase flow is widely present in nature and industrial processes.Due to the characterastics of non-linear flow,the complexity of coupling process and the non-uniformity,two-phase flow has been a basic theoretical topic that still needs further study.In the application of energy engineering,the flow process in shale gas production and CO₂ geological storage has opposite target,but shares the same scientific problem:two-pase flow coupling with multi-physical interactions in fractured shale.Therefore,a two-pase flow theory coupling with multi-physical model in porous media with structural complexity should be developed for fractured shale.This theory can reveal the flow mechanism of shale gas flowback and CO₂ storag and thus the research is of important scientific background and engineering significance.In this study,experimental tests,theoretical modelling and numerical simulations are comprehensively used to investigate the thermal-moisture-hydro-mechanical-chemical coupling mechanisms.The model is vertified with field fata and compared with other models.It is expected that the research outcomes can provide better theoretical guidance for these two engineering applications.Based on the research of this thesis,the following results have been achieved:(1)The shale immersion tests revealed the changes of shale composition,surface morphology,internal pore structure and tensile strength under acid-base deterioration.First,the tests of dissolution effect on shale with three solutions have been completed.X-ray diffraction(XRD)was used to semi-quantitatively analyze the change of mineral composition.Results show that the dissolution effect of three solutions on shale ranks:alkaline solution>acid solution>distilled water.Then the field emission electron microscopy(FE-SEM)was used to observe the change of shale surface morphology.It is found that the water-rock reaction has a significant impact on the shale surface morphology and mainly concentrates on micron-scale cracks and inter-particle pores.The adsorption experiments with nitrogen and CO₂ were conducted to qualitatively analyze the internal microscopic pore structure of shale,and to quantitatively describe the fractal characteristics of pore size distribution.Finally,the Brazilian splitting experiment further revealed the change in the mechanical properties of shale under acid-base degradation.The average tensile strength of shale dropped significantly after being soaked in distilled water,reaching 27.4%.(2)A novel gas-water relative permeability fractal model was derived.This model considered pore-structure parameters(pore-size distribution fractal dimension and tortuosity fractal dimension),water film,geometric correction factor,and real gas effect.This model was then verified by comparing with two classic relative permeability models and several sets of experimental data.Finally,the effects of pore structure parameters,water film,geometric correction factors and real gas effects on the gas-water relative permeability were explored in details.Results show that the pore size distribution determines the flow pattern and the fractal dimension of the pore size distribution has more significant impacts on the change of gas-water effective permeability.The pore geometry directly affects the gas flow mechanism.When the irregularity of pore geometry increases,the Knudsen number decreases,the collision between gas molecules is strengthened,and the gas flow is gradually transitioned into a continuous medium flow.(3)A three-zone model with multi-scale flow-diffusion was proposed to investigate the effect of water-based fracturing fluid on shale gas production.The effects of fracture parameters(such as fracture spacing,fracture width,fracture uniformity,and fracture geometry)on shale gas production were investigated.The contribution of multi-scale flow-diffusion and the gas exchange rate in different zones to shale gas production were carefully studied.It is found that the cumulative shale gas production of this two-phase flowback model has decreased by58.2%after the initial stage of flowback(230 days in our example).The permeability of the micro-fractures in the matrix gradually increases and approaches the permeability of the fractured zone.Shale gas production results from the flow consistency between fracture-scale flow and matrix-scale diffusion.(4)A moisture-hydro-mechanical multi-physical coupling model was estabilished after considering the water film in shale gas flowback and the migration mechanism of water-based fracturing fluid after the two-phase flow stage was revealed.With the moisture transport,the effects of threshold pressure gradient under the residual water saturation,the water film evaporation on the fracture surface and the gas-liquid-solid mixed adsorption mechanism in the matrix are further investigated.It is found that the structure of the water film in the fracture is the main reason for the non-Darcy flow.The relationship of the gas adsorption decay coefficient,the water coverage factor and the amount of gas adsorption in the matrix is clarified.(5)A thermal-hydro-mechanical multi-physical coupling model was developed to consider the coexistence of CO2 three phases in the CO2 critical-depth caprock.The effects of temperature and pressure on the sealing efficiency of a shallow caprock at the burial depth of800 m were numerically studied.The physical properties of CO₂ in the phase transition zone varying with gas partial pressure and formation temperature were discussed.By defining the CO₂penetration depth in the caprock,the sealing efficiency of the caprock was effectively evaluated.This study found that the CO₂ penetration depth increased by 5.9%after considering the real gas effect in 400 years of storage.(6)The thermal-hydro-mechanical multi-physical coupling and migration mechanism of CO2 in deep saline aquifers.The thermal effects(thermal stress and Joule-Thomson cooling)on CO₂ migration in deep saline aquifers were studied.The variation of CO₂ physical properties,the accumulation of pore pressure,adsorption expansion and thermal contraction were included in our revised porosity model.A thermal-hydro-mechanicial multi-physical coupling model for CO₂ storage in deep saline aquifers was then formulated.The evolutions of temperature and pressure of injected were analyzed through the coupling of two-phase flow,porous media deformation,heat transfer,and Joule-Thomson effect.Finally,the effect of capillary entry pressure on the distribution of CO₂ plume was numerically compared.The effect of injection boundary conditions on the CO₂ accumulation pressure was investigated.These results indicate that an appropriate injection rate is critical to the efficiency of CO₂storage.This dissertation includes 105 figures,16 tables and 202 references.