Fracture permeability controls

SOME THMC CONTROLS ON THE EVOLUTION OF FRACTURE PERMEABILITY... 

Fracture permeability from this model calculation is controlled by porosity and the square of the fracture dimensicm. This is comparable to the architecture of Eq. (2.53) with square of pore radius. 

R. C. Navarrete and J. P. Mitchell. Fluid-loss control for high-permeability rocks in hydraulic fracturing under realistic shear conditions. In Proceedings Volume, pages 579-591. SPE Prod Oper Symp (Oklahoma City, OK, 412-414), 1995. 

The PF technology also has several potential limitations. Fractures do not always propagate in the direction or to the distances expected. Fractures may open new pathways for the unwanted spread of contaminants. Pockets of low permeability may remain after fracturing. Surface heave and stress resulting from the process can create hazards for buildings or other structures at a site. If the moisture content of the contaminated media is not controlled, the formation may swell and close the fractures. PF is not applicable at sites with high natural permeabilities. Fractures will close in soils with low clay content. In addition, PF should not be used in areas of high seismic activity. 

For apexes below 3500 m, i.e., the deepest subcompartments of the north Hank, there is an increased possibility of actual breaching (vertical fracturing) of the Spekk Formation cap rock. This could cause direct coupling between the Jurassic reservoirs (overpressured to the extent controlled by the Spekk Formation prior to breaching) and the overlying less overpressured Lower Cretaceous semi-permeable silty (occasionally sandy) claystones. Once attained, this situation might be expected to cause relatively dramatic pulses of vertical leakage (e.g., Mandl and Harkness, 1987). 

The key to a successful acid fradure stimulation is to control the fluid leak-off (23). Most fluid loss additives control leak-off by building up a low-permeability filter cake against the fracture face, thereby deating a wall-building mechanism. The three mechanisms that govern fluid loss as described by Howard and Fast (24) are... 

Investigations to determine the leak-off control mechanisms of foam have shown (26—29) that the effective permeability of a porous medium is greatly reduced in the presence of foam. Some basic assumptions were used during the testing to determine the leak-off control mechanisms of foamed fracturing fluids. The first assumption was that the liquid or continuous phase moves freely, and permeability reduction is a function of the liquid saturation. The other assumption was that the gas or discontinuous phase flows only by rupture and reformation of the foam film. The resistance of foam to flow through porous media is a function of the stability of the foam. 

Controls of permeability. Differences in permeability and hence degree of infiltration may relate to a number of factors including (1) the size, number, distribution and orientation of fractures, (2) connected porosity, (3) stress, (4) fluid pressure, and... 

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. 

The thermo-hydrological calculations have indicated that it is possible to choose appropriate hydrological parameters in order to obtain a distribution of saturation similar to the one prevailing in the in situ test. Intrinsic permeability was taken from the fractures and retention curve was taken from the matrix. Relative permeability for gas and for liquid had to be modified. None of the functions valid for the matrix or the fracture were appropriate. The problem in fact, is that relative permeabilities are controlled by degree of saturation in the fracture and this model used a global degree of saturation. Therefore, relative permeability functions should undergo variations near full saturation because the fractures desaturate for low capillary pressures compared to the matrix. 

Changes in fracture transport characteristics that result from the removal or redistribution of mineral mass within a fracture may be constrained by calculating the evolution of permeability with the concurrent loss or redistribution of mineral mass. Mass loss is unambiguously recorded by the effluent flux, but redistribution may only be discerned by imaging. Independent measurements of fluid and mineral flux, concurrent with non-invasive imaging, are used to constrain the processes controlling mass redistribution within the fracture. 

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