人工裂缝复杂程度的压裂液返排表征方法及应用

为评价非常规储层体积压裂后人工裂缝的复杂化程度，基于储层基质与压裂液存在离子交换现象，分析返排液中盐浓度差异的表示方法，建立了体积改造后返排液中盐浓度的饱和度分布数学模型，分析了不同返排液中盐浓度与裂缝复杂化特征关系。结果表明:压后返排液中盐浓度变化主要表现出“平稳型”和“升高-平稳型”，返排液初期流体主要以人工裂缝内的单相流为主，返排液中盐的浓度随着返排量的增加而增加，达到一定返排率后，出现2种特征:一是平缓型特征，即返排液中盐的浓度逐渐趋于平稳，表明储层改造后的人工裂缝与储层基质中的离子交换较少，人工裂缝趋向于单一裂缝为主的特征；二是升高-平稳型特征，即返排液中盐的浓度仍存在增加趋势，表明改造后人工裂缝与储层基质产生大量离子交换，并且交换的体积较大，盐浓度随返排液量增大而增加的时间越长，表明人工裂缝的复杂化程度越高。经现场应用并结合微地震等试验手段验证表明，该方法可判别非常规油气井压后人工裂缝的复杂化程度，为进一步优化体积压裂设计提供参考。

A characterization method on complexity degree of artificial fractures based on fracturing fluid flowback and its application

To evaluate the complexity degree of artificial fractures in unconventional reservoirs after volume fracturing, the method for representing the difference of salt concentration in flowback fluids was analyzed based on the phenomenon of ion exchange between reservoir matrix and fracturing fluids. Then, the mathematical model on the saturation distribution of salt concentration in flowback fluids after volume fracturing stimulation was developed. Finally, the relationships between the salt concentration of different flowback fluids and the complexity characteristics of fractures were analyzed. It is indicated that the salt concentration of flowback fluids after fracturing varies mainly in two patterns, i.e., steady type and rising-steady type. At the early stage, flowback fluids are dominantly existed in artificial fractures in the form of single phase flow, and the concentration of salt in flowback fluids increases with the increasing of flowback rate. When the flowback ratio reaches a certain number, two characteristics occur. One is the characteristic of steady type, i.e., the characteristic of artificial fractures tending to be simple fractures. In this pattern, the concentration of salt in flowback fluids approaches gradually to the steady value, indicating that the ion exchange between artificial fractures and reservoir matrix after reservoir fracturing stimulation decreases. The other is the characteristic of rising-steady type. And in this pattern, the concentration of salt in flowback fluidsstill rises, indicating that the ion exchange between artificial fractures and reservoir matrix is abundant after reservoir fracturing stimulation and the exchange volume is larger. The salt concentration rising period gets longer as the volume of flowback fluids increases, indicating that artificial fractures are more complex. Field application, combined with microseismic tests, demonstrates that this method can be used to discriminate the complexity degree of artificial fractures after the fracturing stimulation of unconventional oil and gas wells, and it provides the reference for optimizing the volume fracturing design further.