The law of salt ion diffusion is an effective means to evaluate the complexity of volume fracture network in shale reservoir. In this paper, a new experimental apparatus for salt ion concentration testing under the conditions of formation temperature and confining pressure was designed. The quantitative evaluation method of salt ion diffusion ability was put forward to evaluate the law of shale salt ion diffusion. The results shown that the salt ion diffusion concentration of shale sample was positively correlated with the imbibition time. The micro-fracture induced by shale hydration increased the area of shale infiltration and ion interaction. The salt ion diffusion concentration was in dynamic equilibrium. The main types of salt ion diffusion were K+, Na+, Cl- and SO42-, the higher the complexity of the initial micro-fracture of shale was, the stronger the salt ion diffusion ability was. The hydration of shale can induce new micro-fractures, and its direction was parallel to the direction of shale bedding. Under the experimental conditions, the shortest shut-in time was 15-20 days. The research results play an important guiding role in understanding the interaction mechanism between shale and fracturing fluid.

1. Introduction

China is very rich in shale gas resources [1-2], in which the shale gas reservoir of Longmaxi formation in Sichuan Basin is the main development zone [3-7], and the volume fracturing technology of horizontal wells is the key technology for efficient development of tight oil and gas reservoirs [8-11]. The shale reservoir characteristics and fracturing technology are significantly different from those of conventional sandstone reservoirs [12-14]. Shale gas wells shown completely different flowback characteristics after volume fracturing, showing the characteristics of low flowback rate [15-18]. The Eagle Ford basin in the United States is less than 20% less than 50% in the Barnett basin, the flowback rate of Fuling shale gas wells in China is even less than 3% [19-23], and the salinity of the backflow fluid of some shale gas wells in North America is as high as 170000mg/L [24]. The law of shale salt ion diffusion is a key factor affecting the flowback rate of shale gas wells and evaluating the complexity of fracture network and imbibition capacity [25-27], so accurate testing of shale salt ion diffusion law is very important to evaluate the volume fracturing effect of shale gas reservoirs.

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