Comparison of hydraulic conductivity of rock matrix and fractured blocks of granitic rocks

This paper proposes a new application of the rock mass classification concept on the estimation of hydraulic conductivity of fractured rocks. The new rock mass classification system called as based on the following four parameters: rock quality designation (RQD), depth index (DI), gouge content designation (GCD), and lithology permeability index (LPI). HC-values can be calculated from borehole image data and rock core data. To verify rationality of the defined HC-system, the study collected data from the results of two hydrogeological investigation programs in three boreholes to determine a relationship between hydraulic conductivity and HC. Regression analysis was performed to estimate the dependence of HC on hydraulic conductivity. The regression results indicated that a power law relationship exists between the two variables with a coefficient of determination of 0.866. The regression equation provides a useful tool to predict hydraulic conductivity of fractured rocks based on measured HC-values. By using this regression equation, hydraulic conductivity data in a given site can be directly acquired, which removes the cost on hydraulic tests. For in-situ aquifer tests, the HC-system is a valuable new rock mass classification system for preliminary assessment of the degree of permeability in a packed-off interval of a borehole.

IntroductionUndertaking engineering tasks such as tunnel construction, dam construction, mine development, the abstraction of petroleum, and slope stabilization require the estimation of hydraulic conductivity for fractured rock mass.The understanding of hydraulic properties of fractured rock mass, which involves the fluid flow behaviour in fractured consolidated media, is a critical step in support of these tasks.To obtain hydraulic properties of fractured rock mass, double packer systems can be adopted (NRC 1996).They can be used to determine the hydraulic conductivity in a portion of borehole using two inflatable packers.Although this type of test can directly measure the hydraulic parameter, costs of the testing are fairly high.Several studies (Snow, 1970;Louis, 1974;Carlsson & Olsson, 1977;Burgess, 1977;Black, 1987;Wei et al., 1995;) have proposed the estimation of rock mass hydraulic conductivity using different empirical equations, which were based on the concept that rock mass permeability decreases with depth, as shown in Table 1.These empirical equations provide a great feature for characterizing rock mass hydraulic properties quickly and easily.However, the applicability of these equations is very limited because depth is not the only significant variable on the prediction of rock mass permeability.Hydraulic properties of rock mass may vary with geostatic stress, lithology and fracture properties, including fracture aperture and frequency, fracture length, fracture orientation and angle, fracture interconnectivity, filling materials, and fracture plane features (Lee & Farmer, 1993;Sahimi, 1995;Foyo et al., 2005;Hamm et al., 2007).Thus, a more applicable empirical equation for estimating hydraulic conductivity of rock mass possibly must include the aforementioned factors.This chapter proposes two empirical models to estimate hydraulic conductivity of fractured rock mass.The first empirical model was based on the rock mass classification concept.The study developed a new rock mass classification scheme for estimating hydraulic conductivity of fractured rocks.The new rock mass classification system called as "HCsystem" based on the following four parameters: rock quality designation (RQD), depth index (DI), gouge content designation (GCD), and lithology permeability index (LPI).HCvalues can be calculated from borehole image data and rock core data.The second empirical model was simply based on results of borehole televiewer logging, flowmeter logging and packer hydraulic tests.Three borehole prospecting techniques for fractured rock mass hydrogeologic investigation were performed to explore various hydrogeologic characteristics, such as fracture width, fracture angle, flow velocity and hydraulic www.intechopen.com

Hydraulic conductivity (K) and scale effects in basalt in the dam area of Xiluodu hydroelectric station were investigated by three kinds of field hydraulic tests with different test scale, 2608 water pressure tests in single borehole, 54 water seepage tests in adit and groundwater tracer test. Statistical results show the high heterogeneity of fractured rock and K difference between two neighboring test intervals are often more than two orders of magnitude. However, there is a strong decreasing trend of hydraulic conductivity with the increase of vertical depth. Moreover, these three kinds of hydraulic test results demonstrate that hydraulic conductivity increases with the increase of test scale in heterogeneous basalt and the heterogeneous degree of K decreases with the increase of test scale. K from water seepage test in adit, with the test scale of 1-2 m, is dispersed from 0.00024 m/d to 3.46 m/d. K from water pressure test in single borehole, with the test scale of 4-7 m, is 0.0002-1.04 m/d. K from groundwater tracer test, with the test scale of 70-145 m, is concentrated between 0.46 m/d and 2.1 m/d. High heterogeneity of fractured rock and multi-level of fractures are thought as the major reason resulted in scale effects of hydraulic conductivity.

Coupling stress and transmissivity to define equivalent directional hydraulic conductivity of fractured rocks

Quantifying the hydraulic properties of fractured rock masses along a borehole using composite geological indices: A case study in the mid and upper Choshui River Basin in Central Taiwan

This study conducted different borehole prospecting techniques for hydrogeological investigations of fractured rock at three active landslide sites in southern Taiwan. Borehole televiewer logging, flowmeter logging, and packer hydraulic tests were performed to quantify various hydrogeological parameters including fracture width, fracture angle, flow velocity and hydraulic conductivity. The dependence of hydraulic conductivity on fracture properties was evaluated by correlation analysis. While a high positive correlation was found between hydraulic conductivity and fracture width (r = 0.89), and flow velocity (r = 0.87), no correlation with fracture angle was observed. In addition, it is worthwhile to note that the product of fracture width and flow velocity showed a strong correlation with hydraulic conductivity (r = 0.97). The regression analysis also revealed that a power law relationship, with a coefficient of determination of 0.83, exists between the hydraulic conductivity and the product of fracture width and flow velocity. The rationality of this relationship was carefully verified by using another group of geophysical borehole measurements. The results demonstrated that it is capable of predicting the hydraulic conductivity of fractured rock based on borehole televiewer and flowmeter logging results.

Determination of the hydraulic conductivity components using a three-dimensional fracture network model in volcanic rock

Discrete fracture network and equivalent hydraulic conductivity for tunnel seepage analysis in rock mass

Conventional waterflooding in naturally fractured reservoirs can be uneconomic due to poor oil recovery performance. A technique to improve the oil recovery performance is pressure pulsing water-flooding. The technique is cyclic and consists of alternately pressuring and depressuring the reservoir. During the pressuring phase, water is injected and forced under high pressure to flow from the fractures into the matrix. Fluids are produced from the reservoir during the depressuring phase. The reservoir pressure is allowed to drop until either the oil production rate becomes too low or the producing gas-oil ratio becomes excessive. Cycles are repeated as long as the process is economic. This paper presents results of a simulation study to determine the important parameters which affect the success of pressure pulsing waterflooding. A dual porosity generalized compositional model was used for the simulations. The results show the success of pressure pulsing waterflooding is dependent on the interconnected fracture network, relative permeabilities and oil fluid properties. The fracture network allows injected water to move rapidly away from the wellbore throughout the reservoir system. Matrix relative permeabilities affect the amount of oil and water retained in the matrix rock during the depressuring phase. Gas collapse during the pressuring phase allows water to enter the matrix rock, and gas evolution in the depressuring phase drives oil from the matrix rock into the surrounding fractures. It is illustrated that the effectiveness of pressure pulsing waterflooding can be improved by conservation of gas in the reservoir and/or by supplemental gas injection in the pulsing cycles before water injection.

Fluid flow regimes affect the determination of hydraulic conductivity of fractured rocks, and the critical criteria for the onset of nonlinear fluid flow transitions in discrete fracture networks (DFNs) of rocks have yet to be established. First, the factors causing the fluid flow transition regime of fracture intersections and rough surface fractures are theoretically and numerically analyzed. This reveals that the fluid flow regime is governed by the fracture aperture, density of fracture intersections, surface roughness, and Reynolds number (Re). Then, these identified parameters are redefined in DFN models, and their influence on the onset of nonlinear fluid flow is further investigated by performing computational fluid dynamic analysis. The results show that the fracture intersection and aperture play a more significant role in the linear-to-nonlinear fluid flow transition than the fracture aperture heterogeneity. With the increase in the fracture aperture, unevenness of fracture surfaces, and connectivity of DFNs, the onset of the nonlinear fluid flow appeared at the lower flow velocity. With the Forchheimer equation, it is found that the critical hydraulic gradient Jc, defined as the hydraulic gradient at which inertial effects assume 10% of the total pressure loss, is highly correlated with the fracture aperture, fracture intersection, and roughness of the surface. Finally, the mathematical expression of Jc and the Forchheimer coefficients are formulated based on the regression analysis of fluid dynamic computation results, which provides an approach to determine whether the cubic law should be applied as governing equations for the computation of fluid flow in DFNs.

Compilations of the hydraulic conductivity of fractured rock suggest an increase in conductivity between laboratory and field scales. This scaling is inconsistent with recent suggestions that natural fracture networks are near the percolation threshold as the effective conductivity of networks near the percolation threshold decreases with increasing scale. The predicted decrease in conductivity with increasing scale may not be apparent in laboratory data due to a systematic bias in laboratory scale samples; many laboratory scale samples may not contain fractures that are larger than the size of the sample. When this bias is accounted for, the conductivity of simulated networks near the percolation threshold is consistent with the compiled data and increases with the sample dimension. Therefore, observed scaling in conductivity can be interpreted as consistent with the idea that many fracture networks are near the percolation threshold.

A methodology for simulating groundwater flow in three-dimensional (3D) stochastic fracture rocks based on a commonly used finite-difference method is presented in this paper. Different realizations of fracture networks are generated by the fracture continuum method (FCM), in which appropriate 3D cuboids are used to describe the geometry of fractures. Then, the effects of different parameter distributions on the fracture networks indicated that the length, orientation, and density of fractures all play significant roles in the connectivity of fractures in this methodology. Greater length and density and wider orientation range of fractures lead to greater connectivity. The proper contrast in hydraulic conductivities between the fractures and matrix is found to be approximately 105 due to the contribution of fluid flow in the matrix which can be ignored. It is shown that the fracture density plays a key role in stabilizing the equivalent hydraulic conductivity (Ke) of the fracture networks. Furthermore, the greater length and closer orientation of fractures to the general flow direction, the larger Ke of the generated fracture networks possess. The findings of this study can help for a better understanding of the mechanism of FCM and the influence of geometry characteristics on the hydraulic conductivity of FCM models.

Groundwater in foothill and mountain watershed areas commonly occurs in fractured rock. The small well diameters and low apparent groundwater velocities in fractured rock require modification of normal techniques for the investigation of unconfined groundwater movement. The determination of hydraulic conductivity by the tracer dilution method normally employs injected radioisotope tracers. The dilution is determined by monitoring the isotope activity in the well with a scintillation probe. A modification of this method, using fluorescent dye tracers and physical sampling and analysis to determine dilution, has been applied in small wells with consistent results. Hydraulic conductivities of 0.02 to 0.5 ft/day have been determined in sixteen wells. Where comparison is possible, the values agree favorably with hydraulic conductivities determined by pumping tests. (Key words: Groundwater; permeability; tracers; wells)

Deep coal mining in the Yanzhou coalfield is threatened by a confined Ordovician limestone aquifer where water pressure exceeds 10 MPa. High-pressure injection tests are widely used to characterize the hydraulic properties of water-resisting fractured rock strata under such conditions, although estimating hydraulic conductivity remains an issue. This paper presents an approach to estimate it, using data from several high-pressure injection tests by accounting for the flow conditions. Typical P–Q curves obtained from the injection tests were summarized and divided into Darcian and non-Darcian flow phases, and an equation was proposed to estimate the hydraulic conductivity of fractured rocks. The hydraulic conductivity is pressure dependent and increased by injection pressure in the non-Darcian flow phase, due to hydraulic fracturing. As one would expect, the hydraulic conductivity estimated using the new equation was much greater than that estimated using Darcy’s law.

High‐pressure packer test (HPPT) is an enhanced constant head packer test for characterizing the permeability of fractured rocks under high‐pressure groundwater flow conditions. The interpretation of the HPPT data, however, remains difficult due to the transition of flow conditions in the conducting structures and the hydraulic fracturing‐induced permeability enhancement in the tested rocks. In this study, a number of HPPTs were performed in the sedimentary and intrusive rocks located at 450 m depth in central Hainan Island. The obtained Q‐P curves were divided into a laminar flow phase (I), a non‐Darcy flow phase (II), and a hydraulic fracturing phase (III). The critical Reynolds number for the deviation of flow from linearity into phase II was 25−66. The flow of phase III occurred in sparsely to moderately fractured rocks, and was absent at the test intervals of perfect or poor intactness. The threshold fluid pressure between phases II and III was correlated with RQD and the confining stress. An Izbash's law‐based analytical model was employed to calculate the hydraulic conductivity of the tested rocks in different flow conditions. It was demonstrated that the estimated hydraulic conductivity values in phases I and II are basically the same, and are weakly dependent on the injection fluid pressure, but it becomes strongly pressure dependent as a result of hydraulic fracturing in phase III. The hydraulic conductivity at different test intervals of a borehole is remarkably enhanced at highly fractured zone or contact zone, but within a rock unit of weak heterogeneity, it decreases with the increase of depth.