考虑多重滤失效应的前置液酸压有效缝长模拟

为了提高天然裂缝较发育的碳酸盐岩储层酸化压裂设计的准确性和有效性,在采用经典裂缝拟三维延伸数学模型模拟前置液造缝过程中裂缝几何尺寸动态变化的基础上,结合液相反应平衡原理和局部反应平衡原理,建立了考虑天然裂缝、蚓孔及基质多重滤失效应的酸液流动反应模型,以现场实例井的酸化压裂改造为例,使用所建模型模拟前置液酸压注酸过程中裂缝内酸液渗滤过程及酸岩动态刻蚀形态,通过酸液沿水力裂缝长度方向的浓度变化和残酸极限浓度综合确定酸蚀裂缝有效缝长,并与压力恢复试井的解释结果进行了对比。结果表明:(1)裂缝—孔隙型储层酸化压裂过程中,沿裂缝长度方向的酸液滤失速度非定值,滤失速度曲线呈锯齿状波动变化,在酸蚀蚓孔与天然裂缝相遇时酸液滤失速度普遍大于基质处的滤失速度;(2)裂缝—孔隙型储层酸化压裂过程中酸液滤失严重,酸液有效作用距离大幅度降低;(3)验证井酸化压裂解堵增产效果显著,该井试井解释结果与所建立的数学模型模拟解释结果相吻合,说明所建立的前置液酸化压裂数学模型可靠。结论认为,考虑多重滤失效应的前置液酸化压裂数学模型更适用于裂缝—孔隙型储层的酸化压裂模拟。

Simulation of effective fracture length of prepad acid fracturing considering multiple leak-off effect

For the purpose of improving the accuracy and effectiveness of acid fracturing design for carbonate reservoirs with developed natural fractures, an acidizing fluid flow and reaction model taking the multiple leak-off effect of natural fracture, wormhole and matrix into account was established according to the liquid phase reaction equilibrium principle and the local reaction equilibrium principle after the dynamic change of fracture geometry in the process of fracture creating by prepad fluid was simulated in the classical pseudo-three dimensional mathematical model of fracture propagation. Then, the acid fracturing stimulation of a case well on site was taken as an example. The newly developed model was used to simulate the filtration process of acidizing fluid in fractures and the dynamic etching morphology of acidic rocks during the acidizing fluid injection of prepad acid fracturing. The effective length of etched factures was determined by analyzing the concentration change of acidizing fluid along the direction of hydraulic fracture length and the threshold concentration of residual acidizing fluid comprehensively, and then it was compared with the interpretation result of pressure buildup test. And the following research results were obtained. First, in the process of acid fracturing in fractured–porous reservoirs, the acidizing fluid filtration velocity along the direction of fracture length is not constant and the filtration velocity curve fluctuates in a serrated shape. And the acidizing fluid filtration velocity where etched wormholes meet natural fractures is commonly higher than that in matrix. Second, acidizing fluid is lost seriously and the effective distance of acidizing fluid gets short significantly during the acid fracturing of fractured–porous reservoirs. Third, acid fracturing in the verification well is remarkable in blockage removing and stimulation, and its well test interpretation results are consistent with the simulation interpretation results provided by the newly developed mathematical model. It is indicated that this newly developed model is reliable. In conclusion, the mathematical model of prepad acid fracturing which considers multiple leak-off effect is more suitable for acid fracturing simulation of fractured–porous reservoirs.