Elasticity fracture permeability

Figure 7 presents the permeability change with the increasing k ratio simulated by increasing the horizontal boundary stress from 2.5 MPa to 25 MPa with the fixed vertical boundary stress of 5 MPa (the k ratio changes from 0.5 to 5). To evaluate the effect of shear dilation, the results are compared with the pure elastic fracture model that does not consider the failure and dilation. 

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. 

Radiolytic oxidation alters most of the important properties of graphite, including strength, elastic modulus, work of fracture, thermal conductivity, permeability, and diffusivity but does not affect the thermal expansion coefficient or Poisson s ratio. The effects of radiolytic oxidation on the properties of a wide range of graphites have been studied in the U.K. [7,73,74] where it was found that, to a first approximation, they can be described by similar relationships ... 

The fact that this model considers elastic and inelastic fracture aperture or closure has already been discussed in the literature. For instance, Renner et al (2000) investigated the behaviour of fractured argillaceous rocks including permeability variations induced by changes in confining pressures. In this work crack dimensions and permeability are correlated by means a model that takes into account elastic crack closure and crack closure controlled by inelastic processes. This later is explained by asperity indentation when rough crack walls contact each other. 

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 ... 

The second mechanism is dilation/contraction induced permeability change during shear (Figure 1 (b)). Fractures dilate when fracture failure occurs under high differential stresses. This mechanism can play a major role in the flow field since the dilated fracture tend to have much higher transmissivity than the fracture under elastic deformation. 

Both pure elastic and elastoplastic models show similar response until the k value reaches approximately 2.5. With the increase of horizontal stress, the anisotropy in the permeability becomes significant. This is because the sub-vertical fractures are more vulnerable to the horizontal stress and its effect on y-directional permeability is larger. The difference between x- and y-directional permeability is about a factor of 2 for k ratios of 0.5 and 2. 

When the k ratio is beyond 2.5 some fractures start to fail with continuous dilation and notable differences from elastic models are observed. Interesting aspects from these results are 1) the increase of permeability with the increase of differential stresses, and 2) dilation induced channelling effect. 

Measure the elastic modulus of coal and rock, the permeability and other basic parameters the use of specialized software the simulation for perforating fracture distribution formed by perforating in the operating environment. This information will all serve as reference for the trial. 

Muller-Huber (2013) investigated low porous carbonate rocks from Austria. Figure 2.23 shows a plot of permeability versus porosity. The measured permeabilities range from slightly less than 0.05 up to 1190 md. In elastics, permeability depends strongly on porosity, fri craitrast, the permeability of carbonates is controlled not solely by porosity, but also by the pore and fracture geometry. 

---