A Comparative Study on the Hydraulic Fracture Propagation Behaviors in Hot Dry Rock and Shale Formation with Different Structural Discontinuities

Abstract

Hydraulic fracture (HF) propagation behavior is significant when building an enhanced geothermal system for hot dry rock (HDR) and evaluating the simulated reservoir volume (SRV) for shale gas reservoirs. The HF propagation behaviors are closely related to geologically structural discontinuities (SDs), which differ significantly between HDR and shale. Granite, one of the most common HDRs, mainly possesses natural fractures (NFs), quartz veins (QVs), and lithological interfaces (LIs). The SDs in the shale are mainly bedding planes (BPs) and NFs. According to the physical and mechanical property differences regarding the rock matrix, SDs can be divided into discontinuous planes and discontinuous rocks. The physical simulation experiment of hydraulic fracturing is an effective way to assess the geometry and propagation behaviors of HFs. However, the HF propagation behaviors are not generally well understood, especially the influence of multiple SDs on the HF geometry. To clarify this further, a comparative study of hydraulic fracturing on granite and shale was conducted to investigate the intersection mechanism between HFs and different SDs. The results show that HF propagation behaviors are characterized by six basic patterns: along the SD, crossing without dilation, crossing and dilation, captured by the SD, branching, and deflection. The intersection behaviors are closely associated with the cementing strength of the SD and differences in the fracture toughness of discontinuous rocks. Moreover, the fluctuation degree of the pressure-time curve and complexity of the HFs seem positively correlated. HF propagation in a rock matrix or cross interference with multiple SDs would induce a higher injection pressure and frequent fluctuations. The acoustic emission (AE) energy in granite was higher than that observed in shale. In addition, the generation of new HFs in the rock matrix can induce more AE events than those created simply by propagating along SDs. Experimental investigations can provide a theoretical basis to optimize the engineering parameters of field fracturing.