Experiment on the relationship between the characteristics of flowback liquid ions and the complexity of hydraulic fractures in shale

Abstract

The evaluation of the effect of hydraulic fracturing is the focus of shale gas safety and economic exploitation, and evaluation of the complexity of hydraulic fractures is a major difficulty. In order to better evaluate the complexity of shale gas fracturing fractures, based on the characteristics of ion exchange between fracturing fluid and shale formation, the flowback fluid ion changes, and a core immersion experiment that simulates the flowback of fracturing fluid is carried out. The immersion experiment simulates the continuous immersion process of cores representing different levels of complex fractures in deionized water under formation temperature conditions, and tests the ion types and concentrations and hydrogen and oxygen isotopes of the aqueous solution at different times. The ion concentration changes of the aqueous solution under different fracture complexity conditions are compared and analyzed, and the ion source is analyzed based on the results of core mineral and element analysis. The results showed that the Na <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> content is the highest in the aqueous solution, and the concentration change over time is the most obvious, and the Na <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> concentration in complex slits is higher than that in branch slits and single slits. The concentration of Si <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> and B <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3+</sup> is lower than that of Na <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> , and their change over time is similar to that of Na <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> . The total salinity of aqueous solution changes stepwise with time, and the total salinity in complex fractures is higher than that of branched and single fractures. Various trace elements in the aqueous solution mainly come from the dissolution of the elements adsorbed on the clay minerals, and some of them come from the dissolution of minerals. By comparing and analyzing the characteristics of Na <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> and Si <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4+</sup> plasma changes in the flowback fluid, combined with other means of monitoring fractures, the complexity of the fractures can be better evaluated and provide guidance for safe and economic exploitation of shale gas.