Insights into Salt Ion Diffusion Behavior in Imbibed Fluids and the Source of Salt Ions in Clay-Rich Shale from Upper Cretaceous Basin

Shale oil/gas is a type of unconventional energy and an important supplement to the energy structure worldwide. Hydraulic fracturing plays a necessary role in exploiting this kind of resource, and a higher salt ion concentration exists in flowback fluid, which is totally different from conventional reservoirs. To understand the special mechanism of high salt ion concentration, the effect of testing conditions on salt ion diffusion behavior by a Mettler SevenExcellence multiparameter conductivity instrument and then salt ions in soaked solution are analyzed. The study samples are selected from lacustrine shale from the Upper Cretaceous Qingshankou Formation. The testing conditions include measuring sites, stirring or not, particle size, sample weight, solution volume, and initial solution concentration. The experiment has perfect repetition as long as 238 days. The conductivity measured at the upside is lower than the one measured at the downside before 10 days and becomes similar after 10 days. Stirring before measurement increases the salt ion diffusion rate obviously before 10 days, which has less influence on the late period. The smaller the particle size is, the higher the conductivity is. The ratio between conductivity and sample weight decreases with sample weight increase. The higher the liquid volume is, the higher the total salt ion in liquid is at last. The initial salt ion concentration has less influence on the salt ion diffusion behavior, which is mainly controlled by its own physical property. Salt ion diffusion needs a long time to approach stability, and the conductivity in some samples is still increasing after 218 days. The main anions in soaked solution are SO42-, Cl-, and NO3-, and the main cations are Na+, Ca2+, Mg2+, K+, Si4+, and Sr2+. Cl- and Sr2+ are mainly from precipitated salt ions in pores, and other salt ions are partially from precipitated salt ions in pores. Our study contributes to understanding the influencing factors for evaluating the salt ion diffusion behavior in the shale reservoir and clarifying the salt ion component in soaked solution, which is conducive to the explanation of flowback fluid with a higher salt ion concentration in clay-rich shale reservoirs. Our research contributes to the selection of high productive shale oil formation.