Numerical Calculation of Equivalent Permeability Tensor for Fractured Vuggy Porous Media Based on Homogenization Theory

A numerical procedure for the evaluation of equivalent permeability tensor for fractured vuggy porous media is presented.At first we proposed a new conceptual model,i.e.,discrete fracture-vug network model,to model the realistic fluid flow in fractured vuggy porous medium on fine scale.This new model consists of three systems:rock matrix system,fractures system,and vugs system.The fractures and vugs are embedded in porous rock,and the isolated vugs could be connected via discrete fracture network.The flow in porous rock and fractures follows Darcy’s law,and the vugs system is free fluid region.Based on two-scale homogenization theory,we obtained an equivalent macroscopic Darcy’s law on coarse scale from fine-scale discrete fracture-vug network model.A finite element numerical formulation for homogenization equations is developed.The method is verified through application to a periodic model problem and then is applied to the calculation of equivalent permeability tensor of porous media with complex fracture-vug networks.The applicability and validity of the method for these more general fractured vuggy systems are assessed through a simple test of the coarse-scale model.

Review of permeability evolution model for fractured porous media

The ability to capture permeability of fractured porous media plays a significant role in several engineering applications, including reservoir, mining, petroleum and geotechnical engineering. In order to solve fluid flow and coupled flow-deformation problems encountered in these engineering applications,both empirical and theoretical models had been proposed in the past few decades. Some of them are simple but still work in certain circumstances; others are complex but also need some modifications to be applicable. Thus, the understanding of state-of-the-art permeability evolution model would help researchers and engineers solve engineering problems through an appropriate approach. This paper summarizes permeability evolution models proposed by earlier and recent researchers with emphasis on their characteristics and limitations.

Applicability of cement-based grout for ground heat exchanger considering heating-cooling cycles

The applicability of cement grout （or cement-based grout） has been considered as an alternative to bentonite grout commonly used to backfill closed-loop vertical ground heat exchangers. In a geothermal heat pump system, repeated heating-cooling cycles may cause adverse effects on the integrity of cement grout in the ground heat exchanger. To account for the temperature cycling effect, the strength degradation of cement grout due to temperature cycling has been examined by measuring the unconfined compression strength of cured specimens in a humidity-temperature controlling chamber with applying temperature cycles between -5℃ and 50℃. There is a tendency that the unconfined compression strength decreases with an increase in the number of temperature cycles. On the other hand, an equivalent hydraulic conductivity of a pipe-embedded cement grout specimen was evaluated by carrying out a modified flexible wall permeameter test equipped with a water circulating system to control temperature inside the pipe section. The applied operating temperature range was from 5 to 35℃. After three cycles of heating-cooling circulation, the equivalent hydraulic conductivity becomes asymptotic to a constant value, which implies there is no severe detachment of the pipe from the cement grout.

Gas leakage mechanism in bedded salt rock storage cavern considering damaged interface

During the long-time operation of salt rock storage cavern,between its formations,damaged interfaces induced by discontinuous creep deformations between adjacent layers will possibly lead to serious gas leakage.In this paper,damaged interfaces are considered as main potential leakage path:firstly in meso-level,gas flow rule along the interface is analyzed and the calculation of equivalent permeability is discussed.Then based on porous media seepage theory,gas leakage simulation model including salt rock,cavity interlayers and interface is built.With this strategy,it is possible to overcome the disadvantage of simulation burden with porous-fractured double medium.It also can provide the details of gas flowing along the damaged zones.Finally this proposal is applied to the salt cavern in Qianjian mines(East China).Under different operation pressures,gas distributions around two adjacent cavities are simulated;the evolvement of gas in the interlayers and salt rock is compared.From the results it is demonstrated that the domain of creep damage area has great influence on leakage range.And also the leakage in the interface will accelerate the development of leakage in salt rock.It is concluded that compared with observations,this new strategy provides closer answers.The simulation result proves its validity for the design and reasonable control of operating pressure and tightness evaluation of group bedded salt rock storage caverns.