注水井增能压裂升压速率对岩石力学响应模式的影响

The Influence of Pressure Rise Rate on Rock Mechanics Response Mode under High Pressure Water Injection

  • 摘要: 水井增能压裂成为低渗透油藏高效开发的新途径,聚焦该过程中岩石损伤破裂力学响应机制这一关键问题,开展了室内岩心增能压裂物理模拟实验,分析了井周应力场变化规律,建立了储层增能压裂起裂及破裂压力计算准则,明确了低升压速率下岩石力学响应模式的主控因素及影响规律。研究发现:相对传统压裂过程,增能压裂可大幅度提高井周孔隙压力,使孔隙压力衍生的第三应力场发生剧烈变化,进而使岩石力学响应模式由传统压裂的张型破裂转变为渐进性损伤演化模式—控制注入排量和黏度,随着井底升压速率提升,岩石依次呈现孔隙性弹塑性增渗、通道性塑性增透和裂缝性破裂导流三种力学响应模式,实验条件下三种响应模式的升压速率界限分别为0.1和1.0 MPa/s;较低升压速率下,井周岩石受孔隙内流体压力细观胀裂与有效应力宏观破坏的协同作用,起裂、破裂压力随井底升压速率降低而减小;起裂点位置偏离井壁,与井壁的距离随升压速率减小而增大;基于断裂力学的判别准则可较好地表征这种起裂压力、破裂压力、起裂点位置与升压速率相关性。研究所得井底升压速率对岩石力学响应模式的影响规律,可为以大范围增能压裂改造为目标的工艺设计提供支撑。

     

    Abstract: Abastrat: Water well enhanced fracturing has become a new approach for efficient development of low-permeability reservoirs. Focusing on the key issue of rock damage and fracture mechanics response mechanism during this process, core enhanced fracturing physical simulation experiments were conducted, the variation law of stress field around the well was analyzed, calculation criteria for initiation and fracture pressure of reservoir enhanced fracturing were established, and the main controlling factors and influencing laws of rock mechanics response mode under low pressure rise rate were clarified. The results show that compared with the traditional fracturing process, enhanced fracturing can significantly increase the pore pressure around the well, causing a drastic change in the third stress field derived from pore pressure, which in turn changes the rock mechanical response mode from the tensile fracture mode of traditional fracturing to a progressive damage evolution mode: By controlling the injection flow rate and viscosity, as the bottom hole pressure rise rate increases, the rock sequentially exhibits three mechanical response modes: pore elastic-plastic permeability enhancement, channel plastic permeability enhancement, and fracture induced flow diversion. Under the experimental conditions, the pressure rise rate limits of the three response modes are 0.1 MPa/s and 1.0 MPa/s, respectively; Under a lower pressure rise rate, the rock around the wellbore is subjected to the synergistic effect of microscopic expansion and fracturing caused by fluid pressure in the pores and macroscopic failure caused by effective stress, and the initiation and fracture pressures decrease with the reduction of bottom hole pressure rise rate; The location of the initiation point deviates from the wellbore, and its distance from the wellbore increases with the decrease of pressure rise rate; The discrimination criteria based on fracture mechanics can better characterize this correlation between initiation pressure, fracture pressure, initiation point position and pressure rise rate. The influence law of bottom hole pressure rise rate on rock mechanics response mode presented in this study can provide support for process design aimed at large-scale enhanced fracturing transformation.

     

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