低温诱发岩石破裂的理论与数值模拟研究

Theoretical and numerical study on the cryogenic fracturing in rock

低温诱发岩石开裂在油气开发、地热开采、液化石油气储存等方面都有所涉及,低温液体引起的岩石破裂值得关注。针对这一问题,首先运用热弹性理论获得平板模型单面遭受冷冲击时,其内部温度和应力演化的理论解。运用理论公式研究温度的传导规律和应力的演化特征,并分析换热系数对降温速率和拉应力增加速率的影响作用。结果表明,平板模型表面遭受冷冲击时,冲击边沿处温度先急剧降低,然后逐渐减缓,并最终达到与环境相同的温度;拉应力也先急剧升高而后逐渐降低,并最终降低至0。此外,为了重现冷冲击作用下岩石的破裂过程,揭示冷冲击参数对岩石低温下的破裂机制和特征,运用数值模拟方法研究岩石承受降温冲击作用下的裂纹扩展过程,分析换热系数对岩石温度和最小主应力分布的影响作用,并对不同换热系数条件下岩石产生的裂纹数量进行定量分析。数值模拟结果表明,增大换热系数不仅提高了换热边界的拉应力,使岩石更易于开裂,同时也增加了产生的裂纹数量和扩大了裂纹区的范围,有利于提高低渗透油气藏的渗透性。

Thermal cracking of rock induced by cryogenic liquid is widely observed in oil and gas exploitation,geothermal development and liquefied petroleum gas storage etc. To study the cryogenic fracturing,a theoretical solution of the temperature and stress distribution in a plate was derived with the thermo-elastic theory. The heat conduction characteristics and associated thermal stress evolution in the plate was theoretically studied with the derived theoretical formula. The effects of heat transfer coefficient on cooling rate and increasing rate of tensile stress were also analyzed. The results show that when the rock surface contacts with cold liquid,the temperature on the contact surface decreases rapidly at first,then slowly and finally the same as the environment temperature. The tensile stress rises sharply firstly and then decreases,and finally decreases to zero. In addition,the crack initiation,propagation process of cryogenic fracture in rock under thermal shock are numerically simulated. The effect of the heat convection coefficient on the evolution of temperature,stress distribution,the fracture mode and the number of fractures were analyzed. The numerical results indicate that increasing the heat convection coefficient not only raises the tensile stress on the contact surface between liquid and solid which results in rock easier to crack,but also raise the number of cracks which are helpful for enhancing the permeability of low permeability reservoirs.