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.