Fractures hydraulic properties

The results for flow on a single fracture surface are incorporated in the derivation of hydraulic properties of unsaturated fractured rock mass. Liquid retention and hydraulic conductivity in partially saturated fractured porous media are modeled in angular pores and slit-shaped spaces representing rock matrix and fractures, respectively. A bimodal distribution of pore sizes and apertures accounts for the two disparate pore scales and porosity. These considerations provide a framework for derivation of retention and hydraulic conductivity functions for fractured porous media (Or Tuller, 2001). 

We discuss the suspensions used in well stimulation and hydraulic fracturing processes. The following sections pertain to various types of suspensions used in well stimulation and fracturing processes, their rheological characterization and hydraulic properties, behavior of suspensions in horizontal wells, a state-of-the-art review of proppant settling velocity and proppant transport in the fracture, presently available measurement techniques for suspensions and their merits, and, finally, a summary and conclusions on the use of suspensions in well stimulation. Future industry needs for better understanding of the complex behavior of suspensions are also mentioned in this section. 

Hydraulic properties, that is, friction-loss calculations of proppant-laden fluids or slurries, are very important not only in the design of any hydraulic fracturing treatment but also in real-time monitoring of fracturing treatments. Recent advances (27, 28) in real-time fracture analysis have necessitated an accurate knowledge of bottomhole treating pressure (BHTP). To estimate BHTP, an accurate prediction of friction pressures of fluids in the flow conduit is required. It is possible to obtain the BHTP from the surface pressure with the following equation ... 

It is obviously not possible to make experiments with duration of hundreds of thousands of years and over distance of hundreds of meters in the tight rock formations of interest. It is therefore essential to understand the key processes so well that credible predictions can be made using models based on well-established laws of nature. The models must be supported by experiments that can credibly be extrapolated. The models used are based on the laws of mass and energy conservation and on laws of thermodynamics. The difficulties in applying these laws arise mainly from the fact that the rock mass cannot be described in detail. The location, orientation and detailed hydraulic properties of the fractures cannot be measured in detail. The diffusion and sorption properties of the interior of the rock mass under natural stress cannot be readily measured. Mixing processes of different water packages in fractures and at intersections are not fully understood. All this makes it difficult to build models that account... 

For the Yucca Mountain site, incorporation of stress effects into hydraulic properties is based on a conceptual model of a highly fractured rock mass that contains three orthogonal fracture sets, as shown in Figure 2b. Porosity correction factor (F,) and permeability correction factors (Fu, Ft, FtJ calculated from the initial and current apertures (bii, b i, bsi and b , b , bj, respectively) in fracture sets 1, 2, and 3, according to ... 

Abstract A methodology for quantifying the contributions of hydro-mechanical processes to fractured rock hydraulic property distributions has been developed and tested. Simulations have been carried out on discrete fracture networks to study the sensitivity of hydraulic properties to mechanical properties, stress changes with depth, mechanical boundary conditions, initial mechanical apertures and fracture network geometry. The results indicate that the most important (and uncertain) parameters for modelling HM processes in fractured rock are fracture density and rock/fracture mechanical properties. Aperture variation with depth below ground surface is found to be of second order importance. 

Osnes, J. D., Winberg, A. and Andersson, J. 1988. Analysis of Well Test Data - Application of Probabilistic Models to Infer Hydraulic Properties of Fractures, Topical Report RSI-0338, RE/SPEC Inc., Rapid City, South Dakota. 

The DFN simulation cases were constructed to emulate the hydraulic behaviour of a typical Swedish granitic rock volume. In an earlier study (Gutters et al., 2(XX)), the geometric and hydraulic properties of the DFN model were based on the prototype repository and the TRUE Block Scale site at the Aspo Hard Rock Laboratory, Sweden. Since then, the DFN for the prototype repository was updated, Stigsson et al. (2001) carried out an extensive analysis of pump tests which has led to an improved understanding of the fracture set definition and their hydraulic properties their model was used in Gutters (2002). 

A third explanation is that the different rates could reflect differences in the pH of the waters. The rate of feldspar dissolution is enhanced at pH > 7 (e.g. Knauss Wolery 1986). This fact might explain the observation that the alteration is more intense at DH-3/26 m where the sampled water had a pH of 8.9-9.3, than at DH-4/80 m where the water had a pH of 6.8. Knauss Wolery (1986) found a rate of albite dissolution some five times faster at the higher pH levels. However, it follows from this hypothesis that waters in different fractures have acquired distinct pH values. Once again, this observation may reflect the differing natures of water/rock interactions in different fractures, which may in turn be linked to variable hydraulic properties of different fractures. 

Applications. The high heat tolerance and good salt compatibiUty of welan gum indicate its potential for use as an additive in several aspects of oil and natural gas recovery. Welan also has suspension properties superior to xanthan gum, which is desirable in oil-field drilling operations and hydraulic fracturing projects. It is compatible with ethylene glycol, and a welan—ethylene glycol composition that forms a viscous material useful in the formulation of insulating materials has been described (244). 

Relatively small quantities of a bacterial cellulose (0.60 to 1.8 g/liter) in hydraulic fracturing fluids enhance their rheologic properties [1425]. Proppant suspension is enhanced and friction loss through well casings is reduced. 

Hydraulic conductivity is one of the characteristic properties of a soil relating to water flow. The movement of water in soil depends on the soil structure, in particular its porosity and pore size distribution. A soil containing more void space usually has a higher permeability. Most consolidated bedrocks are low in permeability. However, rock fractures could create a path for water movement. 

The selection and preparation of sites for any of these gas stores is a fairly delicate process, because tightness can rarely be guaranteed on the basis of geological test drillings and modelling. The detailed properties of the cavity will not become fully disclosed until the installation is complete. The ability of the salt cavern to keep an elevated pressure may turn out not to live up to expectations. The stability of a natural rock cave, or of a fractured zone created by explosion or hydraulic methods, is also imcertain until actual full-scale pressure tests have been conducted. For the aquifers, the decisive measurements of permeability can only be made at a finite number of places, so surprises are possible due to rapid permeability change over small distances of displacement (Sorensen, 2004a). 

The sensitivity of the hydraulic conductivity and other transport properties of discontinua (fractured media) to normal stress is typically substantially greater than that of continua (unfractured media). The stress-sensitivity has been demonstrated in numerous studies of fracture flow (e.g.. Gale, 1982). Natural fractures are a suspected cause of anisotropic water-flooding with a maximum rate of flood front advance approximately in the direction of the maximum horizontal stress (Heffer and Dowokpor, 1990). Natural fractures were recognised as significantly contributing to Clair well productivity (Coney et al., 1993). These fractures are aligned with the present day direction of the maximum horizontal stress in at least one of the wells in the Clair Field. 

To design a successful hydraulic fracturing treatment for horizontal wells, accurate information on the transport properties of slurry in horizontal pipe is required. One must know the critical deposition and resuspension velocities of various fluids in horizontal pipe flow. 

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