页岩气井超临界二氧化碳压裂起裂压力预测

Fracture initiation pressure prediction for shale gas well fracturing with super-critical carbon dioxide

超临界二氧化碳流体在页岩气井少水及无水压裂方面的应用潜力巨大。为分析超临界二氧化碳压裂的起裂机制及受控因素,首先考虑井筒内温度、压力变化对二氧化碳物性的影响规律,构建了页岩气井憋压过程中井筒内温度场及压力场的数值计算方法;其次以此为基础,建立了超临界二氧化碳压裂条件下套管-水泥环-地层的热流固耦合力学模型,结合岩石破坏准则,提出了起裂压力的计算方法,并以某页岩气井为例进行了实例研究。结果表明:超临界二氧化碳压裂起裂压力显著低于常规滑溜水压裂,但是受增压速率的影响,破裂时间大幅度提高;超临界二氧化碳压裂条件下,起裂压力随着排量和注入温度的增加而增大,随着开井压力的降低而增大;破裂时间随着排量、开井压力的增加而降低。

SC-CO2(super-critical carbon dioxide)has great potential in anhydros or waterless fracturing for shale gas well.In order to analyze the crack initiation mechanism and influence law of supercritical carbon dioxide fracturing,considering the influence law of wellbore temperature and pressure change on carbon dioxide physical characteristics,numerical calculation methods for the wellbore temperature field and pressure field during shale gas well pressure build-up were figured out.On this basis,a thermo-fluid-solid coupling mechanical model of casing-cement sheath-rock under the supercritical carbon dioxide fracturing was built.A calculation method for crack initiation was proposed by combining the failure criterion of rocks.A case study was conducted using a certain shale gas well.The result indicates that the crack initiation pressure for supercritical carbon dioxide fracturing is significantly lower than that of conventional slickwater fracturing.However,the fracture time is greatly increased because of the pressurization rate.The crack initiation pressure increases with the pump rate and the injection temperature under the supercritical carbon dioxide fracturing and increases with the decrease of open pressure.The fracture time decreases with the increase of pump rate and open pressure.