Research On Flow-Heat Transfer And Rock Damage During CO₂ Fracturing Based On Multiphysics Coupling

Carbon dioxide?CO₂?fracturing is considered to be a new non-aqueous fracturing technique to efficiently develop unconventional oil and gas including shale gas.In this study,the theoretical analysis and numerical simulation are carried out to investigate the flow and heat transfer behavior and rock damage characteristics during CO₂ fracturing.The main research work and results are as follows:1.A numerical model is developed to study the characteristics of CO₂ friction loss in circular pipes and the effect of pressure and temperature on CO₂ friction.The results indicate that the friction loss of CO₂ follows the phenomenological Darcy-Weisbach equation despite the sensitivity of CO₂ properties to pressure and temperature.The commonly used correlations can also give good predictions of Darcy friction factor of CO₂ with an acceptable tolerance.The fluid pressure and temperature have a slight effect on the Darcy friction factor of CO₂ but have a major effect on the friction loss of CO₂.The friction loss increases with the increasing pressure and decreases with the increasing temperature at the constant volume flow rate.2.An analytical model considering heat source/sink effects is developed to investigate fluid flow and heat transfer in the welbore during CO₂ fracturing.The model is solved by a coupling method in both depth and radial directions and is used to study welbore heat transfer mechanism and flow behavior.The results show that annular fluid is the most important thermal resistance in the welbore.The natural convection of liquid in annulus is much stronger than that of gas,which can greatly promote heat transfer.The heat resulted from gas expansion and friction should be considered at relatively large flow rates.The influence of Joule-Thompson effect can be ignored for most occasions.3.Numerical simulation is carried out to assess the impact of thermal effect on fracture initiation and the build-up pressure is calculated to evaluate the energized effect of CO₂ fracturing.The results indicate that the temperature of CO₂ increases in the new fracture while decreases in the initial fracture during fracture initiation as a result of fluid compression and expansion.The temperature change results in thermal stress which might induce microfractures.Pumping CO₂ at low injection temperature in a lower permeability formation such as shale play can increase the build-up pressure,which is favorable for enhancing the energized effect of CO₂.4.A fuly coupled hydro-mechanical-thermo model is developed to analyze the coupling effect of CO₂ fracturing.The damage parameters are also included to ilustrate the effectiveness and mechanism of CO₂ fracturing.The results indicate that the injection of CO₂ into formation is a multiphysics coupling process.The unique characteristics of parameters present under the interaction of fluid flow,heat transfer,and geomechanics behaviors.Rock damage occurs first in the direction of maximum in-situ stress.As time goes on,rock damage occurs in the direction of minimum in-situ stress under the effects of pore pressure and thermal stress.However,large rock damage area is still concentrated in the direction perpendicular to the minimum in-situ stress.This study would provide theoretical foundations and guidelines for parameter design and optimization and reservoir adaptability evaluation for liquid-supercritical CO₂fracturing.