Study On Mechanism And Parameters Of Geothermal Exploitation Using Multilateral Wells With CO₂ As Working Fluid

Geothermal energy is one of important renewable and clean resources.Geothermal energy in China is very rich,so it is extremely significant to promote geothermal development for improving energy strategy and air quality.For a double-well enhanced geothermal system(EGS),two wells need to be drilled for injection and production,the contact area between wells and reservoir is limited and the connection degree between wells and fractures is poor.Therefore,we proposed a novel multilateral-well EGS with CO2 as working fluid using one main wellbore to achieve injection and production.The multilateral wells are liable to enhance the connections with fratures and reservoir,therefore increasing swept area and improving heat extraction performance.Based on numerical simulation and machine learning,mechanism and parameters of multilateral-well CO2-EGS were studied and its productivity was intelligently predicted in this paper.Considering multi-physics of fluid flow,heat transfer and rock deformation,the thermal-hydrualic-mechanical coupling model was presented.Considering CO2 pressure work,friction heat,heat transfer between central tubing and annulus,heat transfer between annulus and surrounding formation,the wellbore fluid flow and heat transfer model was established.Taking average temperature and pressure of multilateral wells as coupling data,a mathematical model was developed to couple the fluid flow and heat transfer in reservoir and wellbore of multilateral-well EGS.It provides a simulation tool for the study on mechanism and parameters of a mutlateral-well CO2-EGS.The thermal-hydraulic-mechanical coupling process in the multilateral-well EGS reservoir was analyzed.The characteristics of CO2 wellbore fluid flow and heat transfer were studied.We compared heat extraction performances between CO2 and water as working fluids,also heat extraction performances between multilateral-well EGS and double-well EGS.It demonstrated that the induced thermal stress could increase fracture permeability,reduce injection pressure and promote thermal breakthrough.The CO2 pressure work could lead to dramatic temperature reduction.A larger diameter central tubing and smaller wellbore depth were beneficial for alleviating the temperature reduction caused by CO2 pressure work.The CO2-EGS had obvious advantages against water-EGS in shallow and low-temeprature reservoir,while water-EGS had better heat extraction performance than CO2-EGS in deep and high-temperature reservoir.The multilateral-well EGS had better heat extraction performance than double-well EGS.The effects of reservoir properties,multilateral-well geometries,injection and production parameters and fracture parameters on heat extraction performance of the multilateral-well CO2-EGS were studied.It revealed that the major parameters affecting heat extraction performance were rock elastic modulus,thermal expansion coefficient,lateral-well length,lateral-well quantity,lateral-well spacing,injection temperature,mass flow rate and production temperature.A complex fracture network with longer fractures and more fracture orientations was beneficial for heat extraction.The non-planar fracture network could reduce the injection pressure and promote the thermal breakthrough.The long short-term memory(LSTM)and multi-layer perceptron(MLP)combining neural network was presented,which could accurately and steadily predict the production temperature and thermal power of the multilateral-well CO2-EGS.The generalization ability of the LSTM and MLP combining neural network was proved to be excellent.The prediction accuracy and stability of LSTM and MLP combining neural network were better than those of conventional LSTM and MLP.The study findings could provide theoretical foundations for the field application and technological development of the multilateral-well CO2-EGS.