Fracture permeability thermal deformation

To assess the thermal-mechanical effects on rock-mass permeability, a deformation-permeability relationship based on fracture normal stress was developed. 

EFFECT OF THERMAL DEFORMATION ON FRACTURE PERMEABILITY IN STRESSED ROCK MASSES... 

Abstract We analyse the effect of thermal contraction of rock on fracture permeability. The analysis is carried out by using a 2D FEM code which can treat the coupled problem of fluid flow in fractures, elastic and thermal deformation of rock and heat transfer. In the analysis, we assume high-temperature rock with a uniformly-distributed fracture network. The rock is subjected to in-situ confining stresses. Under the conditions, low-temperature fluid is injected into the fracture network. Our results show that even under confining environment, the considerable increase in fracture permeability appears due to thermal deformation of rock, which is caused by the difference in temperature of rock and injected fluid. However, for the increase of fracture permeability, the temperature difference is necessary to be larger than a critical value, STc, which is given as a function of in-situ stresses, pore pressure and elastic properties of rock. 

Abstract Modeling of the drift-scale heater test at the Exploratory Studies Facility at Yucca Mountain, Nevada, U.S.A. was performed. The objectives of the analysis were to investigate the (i) temperature effects on mechanical deformation surrounding the heated drift and (ii) thermal-mechanical effects on rock-mass permeability. The continuum representation of a deformation-permeability relationship based on fracture normal stress was developed to assess rock-mass permeability variations because of temperature changes. The estimated rock-mass displacements and permeability variations as a function of heating time were compared with field measurements. The estimated trend of permeability responses using a normal stress-based deformation-permeability relationship compared reasonably to that measured. 

In order to solve the coupled THM problem, mechanical properties should be considered for the rock and a constitutive law is also necessary to account for permeability changes induced by deformations. A simple way that permits coupled calculations is using the following properties for the rock Elastic modulus 36800 MPa Poisson ratio 0.2 and thermal expansion 0.00002. As heating of the rock takes place, volumetric expansion in the confined rock induces changes in porosity. In reality, fractures will tend to- close. These porosity/aperture changes can be transformed into permeability variations. A exponential law has been used to model permeability variations ... 

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