Abstract
The direct mechanical effects of seepage force on the behavior of hydrate-bearing porous media (HBPM) have often been neglected in previous studies, which may lead to inaccurate predictions of the mechanical behavior of HBPM under seepage conditions. Here, we propose an extended three-phase physical model for unsaturated HBPM, including gas, capillary water, and generalized solid skeleton (GSS). Based on the model, the force balance equations for the three phases are formulated. Performing a force analysis of generalized solid particles under seepage conditions, we find that the tangential force acting on the generalized solid particle is the seepage force in HBPM. By combining this finding with the formulated balance equations, we derive the expression for the seepage force, which can distinguish the mechanical effects between the tangential force and normal force. The stresses, induced by the tangential force (i.e., seepage force), are divided into two types: one acts on the cross sections of generalized solid particles and the other on the contacts between particles. Neither of them is transmitted through the GSS. The former mainly causes the particles themselves to compress, whereas the latter primarily influences the sliding of the particles at contacts. Based on the mechanisms of these two stresses, their effects on the mechanical behavior of HBPM are quantified, which provides a new insight to evaluate the direct mechanical effects of seepage force.
Highlights
Seepage is an important flow phenomenon in hydrate-bearing porous media (HBPM), and it refers to the flow of water in saturated HBPM or of gas and water in unsaturated HBPM under the gradient of total head [1]
Some researchers have performed a large number of experiments by macroscopic and microscopic techniques in the laboratory to investigate the seepage characteristics of HBPM, such as the variation of gas/water relative permeability with different hydrate habits and saturations [8,9,10], the impact of various pressure gradients on gas production rate and water relative permeability [11, 12], and the evolution mechanism of relative permeability associated with capillarity and drying-wetting cycles [13, 14]
Under saturated or unsaturated seepage conditions, the forces acting on the fluids and on the solid particles are composed of normal and tangential forces
Summary
Seepage is an important flow phenomenon in HBPM, and it refers to the flow of water in saturated HBPM or of gas and water in unsaturated HBPM under the gradient of total head [1]. The mechanical effects of Geofluids seepage force are generally considered in such a way that the reaction of seepage force results in the variation of pore-fluid pressure, which causes the variation in the effective stress and mechanical response of HBPM [5, 21,22,23]. For this objective, we first analyze the interactions between hydrates, solid particles, gas, and water and build an extended threephase physical model including the gas phase, capillary water phase, and GSS phase. The stresses, induced by the seepage force, acting on the GSS are analyzed, and their direct effects on the mechanical behavior of HBPM are quantified
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