Experimental investigation on the mechanical behaviours of a low-clay shale under water-based fluids

Abstract

The interactions between drilling/fracking fluids and shale formations have great effect on shale gas exploitation. Previous studies have shown that water/brine saturation will cause clay-rich shale swelling and strength decreasing. However, little research has been done to investigate such effect on resource shales which mainly have low clay contents. This paper presents an experimental study of the impact of water-based fluids on the mechanical properties of a low-clay shale. Uniaxial compressive strength (UCS) tests were performed on shale samples soaked in NaCl, KCl and CaCl2 solutions with saline concentrations of 0%, 10%, 20% and 25.4%. The crack propagation and the failure mechanism of the shale samples were recorded using acoustic emission (AE) sensors in combination with Digital Image Correlation (DIC) technique. Scanning electron microscopy (SEM) analysis was used to understand the micro scale variation of shale samples after soaking in saline solutions. According to the experimental results, water-saturated samples have the highest swelling increasing and the largest strength reduction. The swelling potential and UCS values decrease with increased saline concentration. Potassium ions have a significant effect on shale strength enhancement when compared to sodium ions and calcium ions. The failure pattern of the samples is mainly splitting breakage. NaCl and CaCl2 saturated samples present more axial fractures than the KCl saturated samples when failure occurs. The SEM results show that crystals of NaCl, KCl and CaCl2 were observed on the surface, and the amount of crystals increases with increasing saline concentrations. The AE results also show that saline saturation could increase the cumulative AE energy.