Laboratory hydraulic fracturing in layered tight sandstones using acoustic emission monitoring

Abstract

Understanding the propagating mechanism of complex fracture network is essential for optimizing the hydraulic fracturing design scheme. Laboratory hydraulic fracturing experiments were performed on three tight sandstone specimens with bedding plane perpendicular, oblique and parallel to the axial direction, respectively. Before hydraulic fracturing, three specimens were first loaded close to the critical stress state under 22.5 MPa confining pressure. The entire hydraulic fracturing process was monitored by acoustic emission (AE) and ultrasonic measurement. The experimental results show that the hydraulic fracturing behaviors were significantly influenced by bedding structure and water infiltration. The bedding plane plays an important role on breakdown pressure, distribution characteristics of AE activities and hydraulic fracture networks. The bedding-oblique specimen exhibits lower breakdown pressure than those of the bedding-parallel and bedding-perpendicular specimens. AE activities in the bedding-oblique specimen is characterized by lower b-value, while higher b-value is found in other two specimens. X-ray computed tomography images of the broken specimens show an excellent agreement between AE hypocenters and macroscopic fractures, indicating geometrical morphology of macroscopic fractures is closely related to the bedding plane and well imaged by AE hypocenters. A bedding-parallel shear fault formed in the bedding-oblique specimen. Complex shear fault linking several bedding-parallel echelon arrays or branches formed in the bedding-parallel and bedding-perpendicular specimens. Water infiltration affects the hydraulic fracturing behaviors by inducing AE events initiate at transition region of wet-dry boundary around the central borehole. In view of the formation of fracture network and the risk of induced seismicity, the influence of bedding structure and water infiltration should be taken into consideration under given in-situ stress regime during the hydraulic fracturing design scheme.