Abstract

During circular process of gas injection and withdrawal, the salt cavern for gas storage experience rapid temperature changes. The thermal effect coupling with the boundary conditions generates thermal stress, which induce the micro-fractures rock salt at the wall of underground cavity. Based on DEM, the Particle Flow Code is used to simulate the rock salt with interlayers. A novel hybrid DEM model is proposed, incorporating the rheological behavior of the pure rock salt, and the brittle character of interbedded mudstone. This model is capable of representing the macro-mechanical rock properties of laboratory observations. The high temperature decreases the compressive strength, makes the behavior of rock salt become more ductile instead of being brittle. The presence of interlayer induces more complex micro-cracking path, due to the heterogeneous heat transfer. Results illustrates the significant influence of temperature on the rock salt, resulting in the attenuation of the strength, induced thermal tensile cracking, and form weak zone around the interface of interlayers. The investigation of micro-mechanical response to the temperature influence can help us to predict the evolution of the damage zone around the interbedded salt cavern gas storage.

1.
Introduction

Rock salt is commonly accepted as host media for natural gas storage, as well as disposal of nuclear wastes, due to its characters, such as high solubility in pure water, very low permeability (Berest and Brouard 2003), creep behavior (Guessous et al., 1987), great potentiality of self-healing after damage (Chen et al., 2013), and relatively mechanical stability (Li et al., 2014; Zhu et al., 2016). The properties of non-halite evaporates varies different from one to another. (Jackson and Hudec, 2017), the behavior also changes when response to different temperature and confining pressure. High temperature changes the crystal structure of rock salt, which results in the variation of physical properties (Soppe et al., 1994; Cuevas 1997). Underground Gas Storage (UGS) is usually exposed to different temperature environment as the depth of its location varies. Therefore, an adequately capture and characterization of salt rock under different temperatures is essential for the design, construction, and operation of UGS.

As an alternative and promising simulation method, Discrete Element Method (DEM) can be applied to investigate the complexity of rock according to its discontinuum basis (i.e. the discrete element is independent to move from rock mass). DEM treats the rock material as an assembly comprising individual particles bonded at certain contacts modes, for simulating microscopic rock behavior including crack and deformation. Different from the conventional simulation methods, such as Finite Element Method (FEM), or Boundary Element Method (BEM), the cracks in PFC Modeling is the spontaneous consequence of breakage of bonds at which the bond strength is being exceeded by the motion of particles. The simulated fracture can be regarded as extensive microcracks to investigate its complex constitutive behavior. PFC modeling has advantage in study of micro-mechanical behavior of unconsolidated as well as complex non-elastic characters, according to direct application of Newton's second law (Cundall and Strack 1979; Jing and Stephansson, 2007; Martinez, 2012).

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