Experimental Investigation of Fracture Orientation in Rocks Using Shear Wave Splitting

Abstract

ABSTRACT The understanding of the orientation of pre-existing discontinuities is crucial for subsurface engineering applications, such as drilling, well completion, and hydraulic fracturing. A variety of methods have been developed to identify fracture orientation, including wellbore imaging and monitoring of microseismic events in reservoirs. Recent research has utilized shear waves to evaluate fracture orientation and density in porous media by analyzing the shear wave splitting (SWS). The aim of this study is to assess the fracture orientation in two rock types, synthetic Hydrostone and Eagle Ford shale, under triaxial loading conditions using the SWS phenomenon. Shear wave velocities were measured using two orthogonal transducers at each end of the fractured sample for transmitting and receiving shear wave travel times. The results showed that SWS, the velocity difference between the two shear waves, was the highest at fracture orientations of 0° and 90°, when one of the transducers was parallel with the fracture. Whereas SWS was the lowest for the sample with fracture at 45°, when the split shear waves attenuated during re-orientation with their preferred propagation directions. The findings of this study could help identify fracture orientation near the wellbore and contribute to minimizing various drilling and completion issues. INTRODUCTION Natural fractures are abundant mechanical structures that exist in most rock types, especially in carbonates and shale (Moore and Wade, 2013). These pre-existing discontinuities could cause several drilling issues including mud losses (Razavi et al., 2017) and drilling breaks (Narr et al., 2006). In production engineering, the presence of natural fractures may enhance the local permeability, and hence the production rate (Luffel et al., 1993; Sakhaee-Pour and Bryant, 2011; Ben et al., 2012; Al-Rubaye et al., 2020). However, in the meantime, it can also increase the gas-oil ratio and water cut impairing the hydrocarbon productivity (Aguilera, 1980; Wennberg et al., 2016). Furthermore, in hydraulic fracturing applications, these natural fractures are known to have significant impact on the hydraulic fracture trajectory and the Stimulated Reservoir Volume (SRV) depending on their relative orientation to the hydraulic fracture propagation direction (Gale, et al., 2007; Zhou et al., 2008; Bahorich et al., 2012; Lee et al., 2015). The orientation of natural fractures is determined by the paleo-stress field of the most active tectonic period (Maerten and Maerten, 2006; Zhang et al., 2022). Nevertheless, this orientation may not always align with the current stress field (Abul Khair et al., 2013; Abul Khair et al., 2015; Likrama et al., 2019). Thus, characterizing the orientation of natural fractures is crucial for effective design of various subsurface engineering applications.