Shale hydration is an important factor affecting the productivity and flowback rate of shale gas wells. Taking the shale of Longmaxi formation (LF) in the Sichuan Basin as the research object, the experiment of shale hydration was carried out, and the quantitative characterization method of rock softening degree of shale hydration was put forward. The rock mechanical parameters and the dynamic change law of induced microfractures were analyzed. The results shown that shale hydration has an important influence on rock mechanical parameters, in which rock compressive strength, peak strength, softening degree and elastic modulus were negatively correlated with hydration time. Under the experimental conditions, the equilibrium time of hydration was 15 to 20 days, which provides an important basis for optimizing the shut-in time of shale gas wells. The microfractures induced by shale hydration are mainly lamellation, with obvious directionality. The micro-cracks extend along the weak surface of mineral and rock cementation. The clay minerals content in the induced microfractures area can reach 53.6%, which plays a leading role in the process of shale hydration. The research results play an important guiding role in understanding the interaction mechanism between shale and fracturing fluid and optimizing the flowback system.

1. Introduction

The shale of Longmaxi formation in the Sichuan Basin is the main development zone of shale gas in China [1-3]. The shale rock is characterized by ultra-low water saturation, high salinity formation fluid, high clay mineral content and the development of nano-pores and micro-fractures [5-6], which makes the shale reservoir naturally have strong imbibition capacity [7-9]. By injecting tens of thousands of square fracturing fluid into the formation, the volume fracturing of horizontal wells is the key technology for the benefit development of shale gas reservoirs [10-13]. Large-scale water-based fracturing fluid and complex fracture network system make the interaction between shale and fracturing fluid more prominent [14-16].

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