超深、特深碳酸盐岩多场-损伤耦合破裂压力计算

Numerical Calculation of Multi-field Damage Coupling Fracture Initiation Pressure for Ultra-deep and Extra-deep Carbonate Reservoirs

超深、特深碳酸盐岩储层破裂压力高,面临压不开的难题,酸液会与储层基质反应起到扩孔增渗以及劣化岩石力学性质的作用,进而降低破裂压力,但目前缺少碳酸盐岩储层酸损伤下的破裂压力精准计算方法,难以设计针对性的降破措施。针对上述问题,通过测试钻井液浸泡、酸液驱替后岩芯动态杨氏模量建立了不同流体扰动状态下的碳酸盐岩损伤演化方程,进一步建立了酸压过程中流动场、化学场和应力损伤场多场耦合的破裂压力数值计算模型,结果表明,钻井液酸液综合扰动条件下,孔隙度低于4.32%、酸化时间小于4.08 min时损伤因子小于0,无法解除钻井液污染导致的杨氏模量升高;P1井“井筒替酸+静止浸泡+浸泡后酸压”施工第73 min时8833m附近井段达到起裂条件,此时损伤因子为0.301,破裂压力降低了29 MPa,酸损伤降破后成功压开地层;模型计算结果与实测破裂压力误差为1%∼5%,较传统解析模型降低了3∼10个百分点,对于蓬莱气区灯影组或类似碳酸盐岩储层破裂压力计算与酸损伤降破措施设计具有一定指导意义。

Ultra-deep carbonate reservoirs are characterized with high initiation pressure,leading to difficulties in fractures initiating.Acid can react with the reservoir matrix,enhancing porosity,increasing permeability,and deteriorating the mechanical properties of the rock,thereby reducing the initiation pressure.However,there lacks accurate calculation methods of fracture initiation pressure for acid-damaged carbonate reservoirs,making it challenging to design initiation pressure reduction measures.This study tested the dynamic Young′s modulus of cores after drilling fluid immersion and acid displacement,establishing a damage evolution equation for carbonate rocks under different fluid disturbance states.In addition,a numerical calculation model was established to couple flow,chemical,and stress-damage fields during acid fracturing to estimate the fracture initiation pressure.The results indicate that when both drilling fluid and acid fluid are affected,for reservoirs with a porosity below 4.32%and an acidizing time less than 4.08 minutes,the damage factor is below 0,which means that it is unable to mitigate the increase in Young’s modulus caused by drilling fluid.During the operation of“acid displacement of wellbore+acid immersion damage+acid fracturing”,the 8833 m section of Well P1 reached the fracture initiation condition at the 73rd minute with the damage factor of 0.301 and fracture initiation pressure decrease of 29 MPa;as a result,this well was opened successfully.The calculation deviation of the model ranged from 1%to 5%,resulting in an improved accuracy of 3 to 10 percentage points when compared to traditional analytical models,this indicates that the model is particularly valuable for the calculation of fracture initiation pressure and designing acid damage measures in the Dengying Formation,as well as in similar carbonate reservoirs.