Hydraulic Conductivity Of Fractured Rocks Research Articles

On the onset of nonlinear fluid flow transition in rock fracture network: Theoretical and computational fluid dynamic investigation

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.

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

Field measurements of hydraulic properties of the rock matrix and in low permeability hard rocks generally require highly accurate and sensitive technical equipment and special methods of measurement. Hydraulic conductivity of intact rocks is mostly measured in the laboratory conditions; field measurements are less common. The Czech geological survey developed equipment, which allows measurements of very low consumption of water during WPT (water pressure tests). This device enables to implement the same testing methodology for all sections of the borehole and subsequently compare the obtained results from low-permeability as well as more permeable sections. Hydraulic conductivity of eight different granitoids was studied in the Bohemian Massif in the Czech Republic. Mean values of hydraulic conductivity of fractured rock are on the order of 10-8 to 10-7 m·s-1, while hydraulic conductivity of rock matrix was most frequently on the order of 10–11 to10-12 m·s-1. The difference of values measured in laboratory and field is caused by scale effect. The significance of scale effect in rock matrix is determined mainly by the connectivity of a network of microfissures. Length and aperture of microfissures govern mainly the connectivity of fissure net in rock matrix. Coarse-grained granitoids have greater length and aperture of microfissures, higher connectivity and hydraulic conductivity, and the scale effect is less significant. The fracture network appearance and hydraulic conductivity of rock matrix is similar and characteristic for different types of granitoid 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

Comprehensive information on vertical variations in hydraulic conductivity along a borehole is indispensable for characterizing the complexity of the hydraulic properties of fractured bedrock aquifers or for use as input data for groundwater modeling. This study presents a practical method for addressing various engineering concerns (e.g., project budget, completion time, manpower, and direct measurement of hydraulic properties from rock core specimens) when obtaining detailed and continuous hydraulic conductivity data along a borehole. Based on in-situ hydrogeological data collected from 26 boreholes located in the Choshui River Basin of Central Taiwan, eight individual and six composite geological indices were investigated to determine their correlations with the hydraulic conductivity of fractured rocks through bivariate analysis. The correlation analysis results indicate that all composite geological indices have higher correlations with the hydraulic conductivity than individual indices do. Moreover, the greater the number of individual geological indices integrated into one composite index, the higher the correlation. Based on the correlation results, quantification models for predicting the hydraulic conductivity using the composite geological indices were developed using regression analysis techniques. The performances of various statistical models were evaluated and compared. The regression analysis results for all six predictive models show that a power law relationship exists between each composite geological index and the hydraulic conductivity, and that the coefficient of determination ranges from 0.77 to 0.88. Therefore, the newly developed models can serve as an alternative for obtaining vertical hydraulic conductivity data when the budget for onsite hydraulic test is limited. • Models for predicting the hydraulic conductivity in boreholes are developed. • Eight individual and six composite geological indices are used. • All composite indices have high correlations with hydraulic conductivity. • The developed models are proven to be reliable.

Variation in hydraulic conductivity of fractured rocks at a dam foundation during operation

Characterizing the permeability variation in fractured rocks is important in various subsurface applications, but how the permeability evolves in the foundation rocks of high dams during operation remains poorly understood. This permeability change is commonly evidenced by a continuous decrease in the amount of discharge (especially for dams on sediment-laden rivers), and can be attributed to fracture clogging and/or hydromechanical coupling. In this study, the permeability evolution of fractured rocks at a high arch dam foundation during operation was evaluated by inverse modeling based on the field time-series data of both pore pressure and discharge. A procedure combining orthogonal design, transient flow modeling, artificial neural network, and genetic algorithm was adopted to efficiently estimate the hydraulic conductivity values in each annual cycle after initial reservoir filling. The inverse results show that the permeability of the dam foundation rocks follows an exponential decay annually during operation (i.e. K / K 0 = 0.97e −0.59 t + 0.03), with good agreement between field observations and numerical simulations. The significance of the obtained permeability decay function was manifested by an assessment of the long-term seepage control performance and groundwater flow behaviors at the dam site. The proposed formula is also of merit for characterizing the permeability change in riverbed rocks induced by sediment transport and deposition.

Estimating the Hydraulic Conductivity of Deep Fractured Rock Strata from High-pressure Injection Tests

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.

A Double Scale Methodology to Investigate Flow in Karst Fractured Media via Numerical Analysis: The Cassino Plain Case Study (Central Apennine, Italy)

A methodology to evaluate the hydraulic conductivity of the karst media at a regional scale has been proposed, combining pumping tests and the hydrostructural approach, evaluating the hydraulic conductivity of fractured rocks at the block scale. Obtaining hydraulic conductivity values, calibrated at a regional scale, a numerical flow model of the Cassino area has been developed, to validate the methodology and investigate the ambiguity, related to a nonunique hydrogeological conceptual model. The Cassino plain is an intermontane basin with outstanding groundwater resources. The plain is surrounded by karst hydrostructures that feed the Gari Springs and Peccia Springs. Since the 1970s, the study area was the object of detailed investigations with an exceptional density of water-wells and piezometers, representing one of the most important karst study-sites in central-southern Italy. Application of the proposed methodology investigates the hydraulic conductivity tensor at local and regional scales, reawakening geological and hydrogeological issues of a crucial area and tackling the limits of the continuum modelling in karst media.

Estimating hydraulic conductivity of fractured rocks from high‐pressure packer tests with an Izbash's law‐based empirical model

AbstractHigh‐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.

Fractal Analysis of Permeability of Unsaturated Fractured Rocks

A physical conceptual model for water retention in fractured rocks is derived while taking into account the effect of pore size distribution and tortuosity of capillaries. The formula of calculating relative hydraulic conductivity of fractured rock is given based on fractal theory. It is an issue to choose an appropriate capillary pressure-saturation curve in the research of unsaturated fractured mass. The geometric pattern of the fracture bulk is described based on the fractal distribution of tortuosity. The resulting water content expression is then used to estimate the unsaturated hydraulic conductivity of the fractured medium based on the well-known model of Burdine. It is found that for large enough ranges of fracture apertures the new constitutive model converges to the empirical Brooks-Corey model.

Using Borehole Prospecting Technologies to Determine the Correlation between Fracture Properties and Hydraulic Conductivity: A Case Study in 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 ([Formula: see text]), and flow velocity ([Formula: see text]), 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 ([Formula: see text]). 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.

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

This paper discusses the applicability of stochastic discrete fracture network (DFN) realizations for equivalent continuum hydraulic conductivity and tunnel seepage analysis in rock mass. Based on detailed geological investigation results, three DFN models were generated at divided equivalent domains showing the same statistics of fracture geometry. Two-dimensional hydro-mechanical modelling with UDEC was carried out to determine the amount of groundwater into a tunnel using DFN model from the equivalent hydraulic conductivity. Numerical results show that the hydraulic conductivity of fractured rocks decreases with using the disturbed aperture by an initial stress when the stress is not large enough to cause shear dilation of fractures, and that the flow rates increase with increasing the difference between the horizontal and vertical stresses. It is revealed from the study that equivalent hydraulic conductivity from DFN model can be widely used for tunnel seepage analysis including the effect of in-situ stresses to minimize uncertainties from complicated structure of rock fractures. (A) This paper was presented at Safety in the underground space - Proceedings of the ITA-AITES 2006 World Tunnel Congress and the 32nd ITA General Assembly, Seoul, Korea, 22-27 April 2006. For the covering abstract see ITRD E129148. Reprinted with permission from Elsevier.

Sample bias and the scaling of hydraulic conductivity in fractured rock

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.

Leaching of rock fractures: Laboratory and field tests for borehole heat stores

Abstract The aim of this research project was to increase the hydraulic conductivity of fractured rock by pumping a leaching fluid (NaOH) through rock fractures. A 16-week field test was carried out in a borehole heat store consisting of 19 vertical boreholes to a depth of 15 m in gneissic rock. The leaching process was studied simultaneously in a laboratory test where rock samples from core drillings of the test site were used. The hypothesis that NaOH-solution would leach and thereby widen the fractuers, was not fulfilled. On the contrary, the fractures were sealed as the leaching test went on. The explanation for this is that the leaching rate was higher than expected, the leaching fluid was saturated and the dissolved minerals precipitated. In principle, the minerals were dissolved and moved from one part of the fractures to precipitate at another part, causing clogging. The effect of the leaching field test has been simulated by means of a numerical model. The conclusion is that a leaching of rock as described in this paper should be combined with a deposition tank for the clogging material in order to avoid precipitation in the rock fractures. The results of this project have demonstrated a way of sealing rock fractures that has many more applications in engineering geology.

Tracer dilution sampling technique to determine hydraulic conductivity of fractured rock

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)