Mechanism Analysis on Hydraulic Fracture Initiation under the Influence of Induced Stress in Shale Gas

Abstract

To study the fracture network propagation mechanism in shale gas reservoirs and determine the influence of induced stress from the growth of multiple fractures, this paper describes the crack initiation pattern in a shale reservoir based on a core laboratory experiment and volume fracturing concept. In accordance with linear elastic fracture mechanics and based on the mathematical model of induced stress field with single fracture as reference, the rock surrounding stress equation of multiple fractures in a horizontal well was deduced. Prediction models of fracture pressure under different initiation patterns were established. Induced stress correction factor was proposed to simplify and correct the prediction models. Results demonstrate that the mechanical parameters of rock directly affect the fracture initiation pattern in shale reservoirs. Tensile failure on the bedding surface primarily occurs, along with shear slippage damage for brittle rocks, whereas shear was mainly observed in plastic rocks. The morphology and distribution of fractures are closely related to induced stress field. Simulation results show that induced stress is positively correlated with fracture height and negatively correlated with fracture interval. Dimensionless fracture interval between one to two is the "gold window" to create a fracture network in fracturing design. Minimum induced stress occurs at 30° and 150° with the minimum horizontal stress direction. The study significantly contributes in the research of crack initiation law and optimization of fracture design.