Abstract

The dynamic failure behaviors of gas shale is a fundamental knowledge for well drilling and completion, and drilling fluid is a nonnegligible factor involved on rock failure. Thus, the aim of this paper is to understand the dynamic rock properties of gas shale and the effects of drilling fluid (Air, Oil, and Water) on the dynamic failure. The Split Hopkinson Pressure Bar (SHPB) testing was conducted on thirty-two shale samples from Ordos Basin, China. Those samples were divided into three groups with treatment of water saturation, oil saturation, and drying respectively. The time-dependent strains and images were recorded by high dynamic strain indicator and high speed camera. The dynamic properties of gas shale demonstrate some similar results to some other rock materials like sandstone. For example, higher strain rate impacting leads to higher Young's Modulus and specific energy, while smaller cutting size. However, the stress-strain responses of saturated samples are quite different. We found that the water-saturated shale performs the lowest dynamic Young's Modulus, followed by oil-saturated and thirdly dried ones. However, the oil-saturated shale consumes the least specific energy to be broken comparing with the water-saturated and dried samples. The saturated samples, by water or oil, hold bigger cutting size than the dried ones. The crack density, analyzed by high speed imaging technique which captures the crack initiation, is positively correlated with the cutting size. It can be concluded that the dynamic rock properties of gas shale follow the common results, meanwhile, the drilling fluid affects the properties significantly.

1. Introduction

Shale is a tight and hard rock, it is difficult to break it only by means of conventional pure shear-grinding method, which is in low efficiency, and also, along with serious wear of bits. In recent years, as the representatives of the dynamic-static coupling rock breaking tools, the air hammer, torque impactors, hydraulic impactors, etc., which break rocks mainly by high strain rate impact, have shown huge advantages in deep well drilling, and hard rock breaking. Therefore, a method of impact-shear is proposed on shale breaking.

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