The Influence of Pre-Existing Weakness Planes in Rock on Hydraulic Fracturing

Abstract

Many weakness planes (such as faults, joints, and micro-fractures, etc.) are usually pre-existed in rock. However, the stress statement changed by hydraulic fracture (HF) propagation may have an important impact on hydraulic fracturing, which is closely related to stress statement of weakness planes. Firstly, the rock samples containing the pre-existing weakness planes were analyzed according to the curves of volumetric strain versus stress difference acquired by laboratory experiment. Secondly, the effective normal stress and shear stress of weakness planes were calculated by the tensor transformation method. And then, weakness planes were divided into four kinds according to the relationship between stress statements of weakness planes and failure lines of Mohr diagram and the kinds of weakness planes were visually described in the Mohr diagram. Finally, it was respectively discussed that pre-existing weakness planes did have an influence on hydraulic fracturing under different stress statements. The research results show that when the effective stress is more than zero, with the effective stress decrease, weakness planes are the more easily inclined to become the dilatation phenomenon where the self-propping effect can improve the reservoirs permeability due to surface asperities of weakness planes. However, there are very complex mechanical phenomena induced by weakness planes under higher effective stress. When hydraulic fractures encounter the pre-existing weakness planes under the approximate stress statement, it may be easy to occur shear slipping of weakness planes or it is difficult to be opened by hydraulic fractures. The latter is extremely beneficial to not become the maximum simulated reservoir volume (SRV) and should be avoided by fracturing operation as early as possible. It is somewhat different that the influence of different mechanical phenomena on hydraulic fracturing, which has a certain guidance for improving hydraulic fracturing stimulation.

1. Introduction

Compared to the conventional oil and gas resources, there is usually not natural production in unconventional oil and gas resources, and it needs to rely on hydraulic fracturing to improve development effectiveness (Shrivastava and Sharma, 2018). The complex fracture networks may be created in hydraulic fracturing, which is a combination of shear and tensile failures (Lin et al., 2018). The shear failure of weakness planes (such as faults, joints, and micro-cracks, etc.) resulted in long-term geological tectonic movement is anticipated to dominate in hydraulic fracturing. However, it is a fact that rock dilatation may be caused by rock plastic behavior that the horizontal stress is balanced by the pressure near the fractures tip in hydraulic fracturing (Alko and Economides, 1995). With the development of unconventional oil and gas resources, these weakness planes are a double-edged sword that they can act as a good oil and gas flowing channel, but they can also lead to hydraulic fracturing failure (Ye, 2017). Therefore, it is necessary that considering the role of weakness planes in hydraulic fracturing will be used to optimize the hydraulic fracturing design.