Study on the Seepage Force-Induced Stress and Poroelastic Stress by Flow Through Porous Media Around a Vertical Wellbore

Fluid flowing through reservoir pores not only generates poroelastic stress but also exerts seepage force on rock skeleton. However, the mechanism of seepage force is not clear. Traditional methods of analyzing wellbore stability and hydraulic fracture initiation are mainly focused on the poroelastic stress without the effects of seepage force. Based on the linear elasticity and consolidation theory, this paper analyzed the mechanism of seepage force and poroelastic stress, and presented an analytical solution for seepage force-induced stress around a vertical wellbore. It also introduced how to calculate poroelastic stress by exerting hypothetical body force and surface force. Through comparison and superposition of stress fields, this paper studied the change characteristics of the poroelastic and seepage force-induced stress under different borehole pressures and the effects of seepage force on the wellbore tensile failure. Numerical simulation results show that when fluid flows through the rock, using traditional models without considering, the effect of seepage force to calculate the borehole pressure-induced stress will result in lower calculation results. Compared with the traditional model, seepage force-induced circumferential tensile stress is larger, and the seepage force significantly reduces the formation breakdown pressure. Rocks near the borehole wall with lower permeability and larger Poisson’s ratio have a greater action of seepage force. When fluid flows through the reservoir, the effects of seepage forces cannot be ignored in the analysis of hydraulic fracturing and wellbore stability.

Seepage Force and Its Direct Mechanical Effects in Hydrate-Bearing Porous Media

The direct mechanical effects of seepage force on the behavior of hydrate-bearing porous media (HBPM) have commonly been neglected in previous studies. In this work, the physical nature of seepage force in hydrate-bearing porous media under saturated and unsaturated seepage conditions was adequately explained, and then, the direct effects of seepage force on the mechanical behavior of HBPM were quantified. Therefore, the results obtained in this manuscript provide a new insight to evaluate the direct mechanical effects of seepage force.

Seepage through a Circular Geomembrane Hole when covered by Fine-Grained Tailings under Filter Incompatible Conditions

Experiments are conducted to quantify leakage through a circular geomembrane (GMB) hole using two types of underliner not meeting modern filter criteria. Test results show that a 1.9 and 5.0-fold increase in hole diameter cause a 1.8 and 6.2-fold increase in leakage, respectively; while a 2-fold increase in GMB thickness results in an approximate 10% decrease in leakage. For a constant effective stress, a 2 and 3-fold increase in water head above the GMB lead to a 1.8 and 2.5-fold increase in leakage, respectively. The effect of the intrusion of underliner materials through the hole is quantified. A numerical model, considering the localized concentration of head loss around the hole and the consequent non-homogeneity in hydraulic conductivity due to the seepage force, is presented. It is shown that percent of head loss within the hole and from the hole to any position above the GMB are independent of loading conditions, and are only influenced by the GMB thickness and hole diameter. Finally, empirical equations for predicting leakage through a circular GMB hole in tailings storage applications are proposed.

Theoretical Analysis of Safety Thickness of the Water-Resistant Rock Mass of Karst Tunnel Face Taking Into Account Seepage Effect

This paper took into account the adverse influence of the karst water seepage effect on the water-resistant rock mass. Based on the upper-bound theorem of limit analysis and the Hoek-Brown failure criterion, through a series of formula derivation, the expression of critical safety thickness of water-resistant rock mass of karst tunnel face was finally obtained. The paper carried out a feasibility analysis, an analysis of influencing factors and a comparative analysis with previous related research achievements of this method. The results showed that: (1) With the decrease of surrounding rock grade, the safety thickness of water-resistant rock mass gradually increased, and the safety thickness of surrounding rock at all grades remained within a reasonable range. (2) The safety thickness decreased as the compressive strength, the tensile strength and parameter A increased, and it increased as the karst water pressure, the tunnel excavation height, and parameter B increased. (3) The change trend of the safety thickness with the influencing factors was completely consistent under the two conditions of considering and without the seepage effect, and the safety thickness with considering the seepage force was greater than that without considering the seepage force. Taking the Yunwushan tunnel of Yiwan railway as an example, the critical safety thickness of the water-resistant rock mass was calculated and the calculated value was in good coincidence with the safety thickness adopted in the actual project. The research results are of great significance to prevent the occurrence of high pressure filling karst geological disasters such as water inrush in tunnels.

Effect of Seepage Force on the Wellbore Breakdown of a Vertical Wellbore

As a fluid flows through a porous media, a drag force, called seepage force in the paper, will be formed on the matrix of the media in the fluid flowing direction. However, the seepage force is normally ignored in the analysis of wellbore fracturing during hydraulic fracturing operation. In this paper, an analytical model for seepage force around a vertical wellbore is presented based on linear elasticity theory, and the effect of the seepage force on wellbore breakdown has been analyzed. Also studied are the effects of the two horizontal principal stresses and the reservoir permeability on the action of seepage force. The paper proves that seepage force lowers formation breakdown pressure of a vertical wellbores; the deeper a formation is, the greater action of the seepage force; seepage force contributes more to breakdown formation with small difference of the two horizontal stresses such as unconventional reservoirs; seepage force increases as rock permeability decreases, and it should not be ignored in hydraulic fracturing analysis, especially for low-permeability formation.

Karst Aquifer Water Inflow into Tunnels: An Analytical Solution

This study gives two new analytical solutions to the tunnel by high-pressure water in the cavern. Firstly, it deals with the analytical solution for the seepage inflow in unsupported karst aquifer tunnels considering the boundary condition. Secondly, it focuses on the study of the seepage force and gives the reduction coefficient of lining water pressure. A comparison of the analytical solution and the finite element software shows a curve relationship as the relevant permeability coefficient β increases. The results show that the analytical solution and numerical solution are consistent. As d increases, β decreases gradually. β increases as r w increases or the grouting circle and initial support become thinner or the secondary lining becomes thicker. In summary, the analytical solution of β can be used to predict the seepage inflow and the seepage force of the actual engineering.