Fractures hydraulic conductivity

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. 

In weakly consolidated, stratified sediments, the injection pressure must be controlled so that the surrounding formation is not fractured. If fracturing occurs, there is usually a severe loss in hydraulic conductivity because the bedding planes are disturbed. Pressures that will cause fracturing range from a low of 0.5 psi/ft of depth for poorly consolidated coastal plain sediments, to 1.2 psi/ft depth for crystalline rock. For most recharge wells in unconsolidated sediments, the injection pressure should be carefully controlled so that the positive head (in psi at the surface) does not exceed 0.2 x h, where h is the depth (in ft) from the ground surface to the top of the screen or filter pack. 

Geological information of the rock sequence and tectonic settings may be instructive but not definitive, as it is hard to translate field data into hydraulic conductivity values a shale bed may be fractured and let water flow through in one case, and a clay bed may be weathered and act as an aquiclude in another. In addition, a variety of processes lower the local water conductance, occasionally preventing lateral flow. An example of such a process is chemical clogging (Goldenberg et al., 1983). 

The hydrogeology of the Aspo site is controlled by flow in fractures (Figure 15). The hydraulic conductivity of the rock mass is 10 -10 ° m s while the hydraulic conductivity of the fractures is 10 " -10 m s (Rhen et al., 1997). The irregular water table is locally controlled by sparsely distributed, poorly interconnected fractures. At depth, the hydraulic pressure head distribution becomes more regular due to control by fracture zones and larger... 

This model assumes laminar flow between two perfectly smooth and parallel plates. However, Experiments have shown that the real mechanical aperture (E) and the hydraulic conducting aperture (e) are not equal. The cubic law (e = E) is only valid for very open fractures and/or for fractures with smooth fracture surfaces (low JRC). The mechanical aperture E can be converted into the hydraulic conducting aperture e, by using Eq. (2) (Barton et al., 1985) ... 

The normal stress acting on such a fault plane approaches the lowest values of the stress tensor. In the writers experience, the least effective stress in North Sea fields is typically less than 10 MPa. The hydraulic conductivity of many open natural fractures is reduced to that of the unfractured rock at approximately 10 MPa (see, for example, data presented by Gale (1982)). Consequently, we may readily infer that at least any unfilled fractures which are subparallel to the main fault plane, within or adjacent to that plane, may be conductive. Although fractures may not always be present (e.g., if the fault plane is filled with salt), this provides one mechanism which is broadly consistent with the observation of a minimum of sealing faults striking parallel to the maximum horizontal stress. 

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. 

The hydraulic conductivities and porosities of common rocks are compared in Table 8.1. Clay has the highest porosity, but is among the least permeable of materials. Its low permeability is why clay is used to line the bottom of waste ponds, for example. Basalt and limestone may have low total porosities, but because groundwater flow in basalt and limestone may occur in large fractures and also in cavernous zones in limestone, these rocks often have high permeabilities. 

For low-permeability injection and prodnction weUs, hydraulic fracturing improves the polymer injectivity and the productivity of liquid. Daqing practices show that the improvement was more effective when fracturing was conducted during the early low water-cnt periods (Nin et al., 2006). 

Behavior attributable to TNE has been observed in aggregated, macro-porous, heterogeneous (with respect to hydraulic conductivity), and fractured... 

The approach for unsaturated conductivity outlined in previous sections was extended to modeling the unsaturated hydraulic conductivity of rough fracture surfaces (Or Tuller, 2000). Flow on rough fracture surfaces is an essential component required for deriving constitutive relationships for flow in unsaturated fractured porous media (Or Tuller, 2001). The detailed derivations are obtained by consideration of a dual porosity model (matrix - fracture) and the proportional contributions to flow from these different pore spaces. 

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). 

Or, D., and M. Tuller. 2000. Flow in unsaturated fractured porous media Hydraulic conductivity of rough surfaces. Water Resour. Res. 35 1165-1177. 

For various aquifer minerals, porosity varies over a fairly narrow range (ca. 0.3 to 0.5) but hydraulic conductivity varies over many orders of magnitude.2 Even for a specific type of aquifer material, ranges of 1-4 orders of magnitude are common (e.g., 10 8 5 to 10 4 m/s for fractured rock, 10-5 to 10 3 m/s for well-sorted sand). The lowest hydraulic conductivities are found for crystalline rock (10 14 to 10 10 m/s) and the highest for well-sorted gravel (10-2 to 1 m/s) and clean sand or cavernous limestone (10 6 to 10 2 m/s). 

One of the most important factors in the effectiveness of the hydraulic fracturing treatment is the ability to predict the settling velocity of proppant under fracture conditions. The transport of proppant and the final distribution of proppant in the fracture highly depends on the accurate estimation of settling velocity of proppant. The length of the propped fracture, the conductivity of the propped fracture, and height of the propped fracture are consequently affected by the settling velocity. 

Numerous hydraulic tests were carried out, during the various project phases, either in an isolated borehole or between boreholes. Hydraulic experiments consisted of pulse tests, constant head injection and/or extraction and constant rate injection/extraction using borehole intervals delimited in FBX 95001, FBX 95002, BOUS 85.001 and BOUS 85.002 (Figure 1). Both water flow and piezometric level measurements for borehole intervals are available. Continuous pressure monitoring has allowed detecting crosshole responses during pulse testing these have then been used to calibrate the hydraulic conductivities of fractures. 

This paper focuses on a repository located in sparsely fractured rock with a hydraulic conducting horizontal fracture intersecting the vertical deposition hole (Figure lb). The analysis for the case of a homogenous intact rock (Figure la) is presented in Millard et al., (2003). This paper... 

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