干热岩柔性压裂裂缝起裂与扩展规律

Fracture initiation and propagation in soft hydraulic fracturing of hot dry rock

考虑岩石热孔隙弹性效应、循环载荷下岩石强度疲劳劣化本构关系、弹脆性破坏准则和井筒应力叠加效应,建立并验证了热流固-疲劳损伤耦合的干热岩柔性水力压裂裂缝扩展数值模型,在此基础上开展数值模拟研究,探究不同温度和循环载荷共同作用下干热岩裂缝起裂及扩展特征。研究表明:①周期注入、流体渗透、孔隙压力累积与岩石强度劣化共同引发柔性压裂岩石疲劳破坏;②干热岩柔性压裂裂缝扩展模式由温差和循环载荷共同控制,温差越大,热应力越强,利于形成复杂缝网;循环载荷降低,热应力波及范围增大,循环载荷为90%pb和80%pb(pb为常规水力压裂时岩石破裂压力)时,改造面积相较于常规水力压裂分别提升88.33%和120%(注入温度为25℃);③随着循环载荷的进一步降低,储层改造效果减弱,循环载荷降低至70%pb时,距离井筒较远处流体压力无法达到岩石最低破裂压力,不会产生宏观水力裂缝。

By considering the thermo poroelastic effects of rock,the constitutive relationship of fatigue deterioration of rock under cyclic loading,elastic-brittle failure criteria and wellbore stress superposition effects,a thermal-hydraulic-mechanical-fatigue damage coupled model for fracture propagation during soft hydraulic fracturing in hot dry rock(HDR)was established and validated.Based on this model,numerical simulations were conducted to investigate the fracture initiation and propagation characteristics in HDR under the combined effects of different temperatures and cyclic loading.The results are obtained in three aspects.First,cyclic injection,fluid infiltration,pore pressure accumulation,and rock strength deterioration collectively induce fatigue damage of rocks during soft hydraulic fracturing.Second,the fracture propagation pattern of soft hydraulic fracturing in HDR is jointly controlled by temperature difference and cyclic loading.A larger temperature difference generates stronger thermal stress,facilitating the formation of complex fracture networks.As cyclic loading decreases,the influence range of thermal stress expands.When the cyclic loading is 90%pb and 80%pb(where pb is the breakdown pressure during conventional hydraulic fracturing),the stimulated reservoir area increases by 88.33%and 120%,respectively,compared to conventional hydraulic fracturing(with an injection temperature of 25℃).Third,as cyclic loading is further reduced,the reservoir stimulation efficiency diminishes.When the cyclic loading decreases to 70%pb,the fluid pressure cannot reach the minimum breakdown pressure of the rock,resulting in no macroscopic hydraulic fractures.