Analysis of Hydraulic Fracture Propagating Performance with Geomechanical Characteristics in Naturally Fractured Shale Formations

Abstract

In order to predict fracture propagation in naturally fractured reservoir, several fracturing models have been proposed. Especially, for analyzing interaction between hydraulic fracture and natural fracture, previous models have limits in representing fracture propagation, taking into account multiple planar fracture only with opening mode in fracture mechanics. When hydraulic fracture non-orthogonally encounters natural fracture, hydraulic fracture tip shears along natural fracture interface and then propagates into matrix once it meets the boundary of elastic region. Therefore, not only opening mode but sliding mode also needs to be considered in fracture propagation analysis. In this study, we proposed hydraulic fracture propagation model considering geomechanical factors as functions of Poisson's ratio and Young's modulus using multiple planar fracture with mixed mode by linearly superposing opening and sliding modes.

The proposed model was used to analyze the fracture propagating behavior for different shales of Marcellus, Barnett, and Eagle Ford with respect to geomechanical properties. From the results of sensitivity analysis for Poisson's ratio, Young's modulus, and stress anisotropy on crossing angle of hydraulic fracture into natural fracture, it indicated that Young's modulus is mostly sensitive among geomechanical properties on fracture crossing angle. As Poisson's ratio and stress anisotropy increase, or as Young's modulus decreases, hydraulic fracture can easily penetrate into natural fracture. In the aspects of fracture mechanics, as Young's modulus increases, the implementation of sliding mode is more significant, whereas, almost no effect regardless of magnitude of Poisson’ ratio or stress anisotropy. Based on the results of sensitivity analysis, the proposed model was run for Marcellus, Barnett, and Eagle Ford shales having different geomechanical characteristics for examining the importance of sliding mode. From the results, since Poisson's ratio is low and Young's modulus is high in Barnett shale, the largest effect of sliding mode was yielded due to its highly brittle characteristics, in which this shale presents larger deformation in longitudinal direction than transverse. Meanwhile, in Marcellus shale, hydraulic fracture more easily crosses natural fracture with low value of crossing angle, because its Young's modulus is lower than Barnett shale, consequently, the implementation of sliding mode is less dominating. Furthermore, since Eagle Ford, representing higher Poisson's ratio and Young's modulus than Barnett shale, shows higher deformation in transverse direction by local stress change, the fracture crossing angle was estimated as higher value than Barnett shale. That is, hydraulic fracture is difficult to cross natural fracture, and thereafter, propagating direction of fracture is highly deviated. Therefore, particularly in Eagle Ford, the model with mixed mode was found to be extremely important on fracture propagating behaviors. As overall result, hydraulic fracture propagates diversely corresponding to shale characteristics as well as the existence of natural fracture, the stimulated reservoir volumes were estimated quite differently.