The reactivation degree of natural fractures is controlled by hydraulic fracturing, so as to improve the effect of hydraulic fracturing. In this paper, a large-size true three-axis simulation experiment is used to study the mechanism and factors of cyclic variable loading fracturing on the reactivation of the natural fracture. Placing the FBG stain sensor on the simulated natural fracture surface inside the artificial specimen, and observing the relationship between fracking load changes and natural fracture reactivation by dissecting the specimen after the experiment. The study shows that: (1) Cyclic variable load fracturing can aggravate the degree of natural fracture reactivation. (2) The process of cyclic variable load fracturing to reactivate natural fracture is divided into four stages: hydraulic fracture closes to the natural fracture stage, the stress shadow at the front end of the fracture causes the natural fracture to open, and the fracturing fluid permeates into the natural fracture; hydraulic fracture communication natural fracture stage, a large amount of fracturing fluid is injected into the natural fracture, and the natural fracture on one side of the displacement increased, causing the natural fracture to begin to open; the natural fracture slipping stage, the normal stress acting on the fracture surface and friction coefficient of the fracture decrease, and the natural fracture begins to slip; the accelerated expansion stage, fracture begins to expand rapidly; (3) The increase of the amplitude of cyclic variable fracturing load will increase the sliding speed and amount of natural fractures. This study provides a feasible method for controlling the reactivation of the natural fracture and the complexity of the hydraulic fracture through cyclic variable load fracturing.

1. Introduction

With the promotion and use of clean energy, natural gas has become a major energy industry. China's natural gas reserves reach 8.4 trillion cubic meters. As an important source of unconventional oil and gas reservoirs, shale gas has been widely valued in natural gas development. There are extremely complex natural fractures in shale reservoirs, which are generally closed and filled with cement. In the process of fracturing, the activation of natural fractures will affect the propagation of hydraulic fractures and the formation vibration.

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