深部地热能系统主要挑战与耦合储能的增强型创新开发模式

Major challenges of deep geothermal systems and an innovative development mode of REGS integrated with energy storage

中国在中低温地热能直接利用方面早已领先全球,但在深部地热能发电方面却发展缓慢。深部高温高压环境下岩石渗透性降低,深部地热能开采需要建立工程型地热系统(Engineered Geothermal System,EGS),通过水力压裂对储层进行改造,以获得具有较高渗透性的人工地热储层。由于目前常用的深部地热能储层改造技术主要借鉴油气增产领域的水力压裂工艺,在热储改造效果、地震风险控制、高效取热等方面受到限制。首先总结深部地热能水力压裂的特点为:裂缝破坏主要以剪切机理为主;冷水回灌引起的温差效应产生拉应力会促使裂缝向更远处扩展;持续的注水使注入井井筒压力高于地层压力,有助于保持裂缝处于张开状态。因此,EGS水力压裂不需要使用支撑剂,与依靠支撑剂的油气井增产压裂完全不同。同时,系统剖析EGS面临的发电产能低、注采连通差、诱发破坏性地震以及无补贴难盈利4大难题与挑战。从创新压裂和循环利用层面提出耦合储能的增强型创新开发模式(Regenerative Engineered Geothermal System,REGS),通过数值模拟研究REGS的优点。结果表明,采用水平井非等距、非等面积、非等注水量分段压裂,可以提高注水井与生产井的连通能力。通过优化压裂工艺,采用多段压裂模式,每次压裂初始期间快速提高注水速率,而后期缓慢降低注水速率,避免井筒压力的突然波动,可以达到控制诱发地震震级的目的,避免产生实质性伤害地震。结合可再生能源大规模地下存储,既能实现多能互补,又能提高REGS项目生产寿命和盈利能力。研究成果有助于为我国深部地热能热电联产和储能一体化技术的试点和标准化推广奠定基础。

China has long been a global leader in the direct utilization of moderate-to low-temperature geothermal energy,in contrast to its sluggish progress in power generation using deep geothermal energy.Rocks in deep reservoirs exhibit decreased permeability under high-temperature and high-pressure conditions,necessitating the establishment of engineered geothermal system(EGS)for the exploitation of deep geothermal energy.In an EGS,hydraulic fracturing is employed for reservoir stimulation to create artificially enhanced geothermal reservoirs with higher permeability.The current techniques for deep geothermal reservoir stimulation are predominantly borrowed from hydraulic fracturing processes employed in the oil and gas sector,placing limitations on the stimulation performance,earthquake risk control,and efficient heat extraction of geothermal reservoirs.This study summarized the features of hydraulic fracturing for deep geothermal energy:(1)fracturing-induced damage is dominated by the shear mechanism.(2)The tensile stress generated by cold water injection-induced differential temperature encourages fractures to propagate further.(3)Continuous water injection keeps the wellbore pressure higher than the formation pressure,creating favorable conditions for fractures to maintain open.Therefore,no proppant is required for hydraulic fracturing in an EGS.This is totally different from the hydraulic fracturing of oil and gas wells for production growth,which relies heavily on proppants.Furthermore,this study systematically analyzed four major challenges to EGS:low power generation capacity,poor connectivity between injection and production wells,risks inducing damaging earthquakes,and difficult profit making with no subsidy.From the aspects of innovative fracturing and energy recycling,this study proposed an innovative enhanced development mode integrated with energy storage,termed regenerative engineered geothermal system(REGS).This study investigated the advantages of the REGS through numerical simulation.The results indicate that multistage fracturing using horizontal wells and unequal spacings,areas,and volumes of injected water can enhance the connectivity between injection and production wells.The fracturing process is optimized in the REGS.Specifically,multistage fracturing is adopted.In each fracturing,the water injection rate is rapidly increased in the early stage and gradually decreased in the late stage.This can avoid the abrupt fluctuations in the wellbore pressure,thus governing the magnitude of induced earthquakes and preventing damaging earthquakes.The REGS integrates large-scale underground storage of renewable energy,achieving multi-energy complementation and enhancing REGS projects’production lifespan and profitability.The results of this study will lay the foundation for the pilot projects and standardization promotion of the technology for combined heat and power generation integrated with energy storage of deep geothermal energy in China.