Shale hydration is an important factor affecting the productivity and flowback rate of shale gas wells. In this paper, we conduct a set of experiments on shale sample from the Longmaxi Formation (LF) located in the Sichuan Basin. The experiment of shale hydration was carried out, and the low field nuclear magnetic resonance (NMR) technique was used to realize the on-line monitoring of fluid migration in shale pores. The experimental results shown that the shale samples from LF in Sichuan has the characteristics of multi-scale pores. The T2 spectrum curves of shale samples before water hydration shown bimodal characteristics. Matrix pores were the main contributors to imbibition capacity, and micro-fractures induced by shale hydration provided channels for water imbibition, both of which determined water imbibition capacity in organic shale. The water hydration equilibrium time of shale sample was 10-15 days, which plays an important guiding role in optimizing the shut-in-time of shale gas wells.The microfractures induced by shale hydration are mainly lamellation, with obvious directionality. The micro-fractures 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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