Characterization Of Fractured Rock Research Articles

Pore pressure effects on the anisotropic joint elastic-electrical properties of artificial porous sandstones with aligned fractures

Fractures widely exist in rocks and joint interpretation of elastic and electrical survey data can improve the characterization of fractured rocks. However, the cross-property relationship between the elastic and electrical rock properties, which is the key to the successful joint elastic-electrical interpretation, remains poorly understood in fractured rocks with varying pore pressure. This knowledge gap was by developing a method for making calcite-cemented sandstones with penny-shaped fractures and by simultaneously measuring the anisotropic elastic and electrical properties of the artificial sandstones with and without fractures as a function of pore pressure through the development of an anisotropic joint elastic-electrical measurement system. We find that elastic velocities and electrical resistivities of the intact and fractured samples reduce with increasing pore pressure, and the reduction in the physical properties of the fractured rock is less significant than that of the intact sample. We also find that elastic and electrical anisotropic parameters decrease in the sample without fractures but decrease first and then increase with increasing pore pressure in the sample with fractures. Most interestingly, the anisotropic elastic and electrical properties are found to exhibit a strong linear correlation in the intact and fractured samples with varying pore pressure and a difference exists in the slopes between the samples with and without fractures. The experimental data are explained and interpreted through the contrary contributions of the dilation of the random microcracks and the aligned fractures with increasing pore pressure to the anisotropic elastic and electrical properties. The results reveal the pore pressure effects on the anisotropic joint elastic-electrical properties of sandstones with fractures and provide a potential for the detection of overpressure in fractured rocks through integrated elastic and electrical surveys.

Analysis of thermal dilution experiments with distributed temperature sensing for fractured rock characterization

Thermal dilution experiments with Fiber-Optic Distributed Temperature Sensing (FO-DTS) were conducted at the In-situ Stimulation and Circulation (ISC) rock laboratory, at the Grimsel Test Site (GTS) in Switzerland. The experiment consists in replacing the total volume of a borehole with a warmer water and monitoring the rate of temperature increase and decrease during the heating and cooling periods, respectively. The changes in temperature monitored in depth and time under various hydraulic conditions and in different boreholes, are used to investigate the information provided by thermal dilution experiments in terms of groundwater flow and thermal properties in low-permeable fractured crystalline rock. The data analysis, and the use of analytical and numerical solutions for reproducing this data in the context of pure diffusion and advection–diffusion scenarios, lead to the following improvements and conclusions. (i) The formation thermal conductivity is estimated along the borehole by inverting the data collected under ambient conditions with a simple analytical solution. The estimated values are consistent with laboratory estimates. The method presents the advantage of requiring much shorter experiments than existing methods based on standard active-line-source (ALS) experiments, i.e., several hours versus the traditional 1–2 days. (ii) Hydraulically active fractures connecting boreholes are detected from experiments conducted under cross-borehole forced hydraulic conditions. (iii) The formation thermal conductivity and fracture flow velocity have a distinguished impact on the temperature anomalies for some ranges of these property values, implying that both properties can be estimated from well-parametrized experiments.

Thermal Experiments for Fractured Rock Characterization: Theoretical Analysis and Inverse Modeling

AbstractField‐scale properties of fractured rocks play a crucial role in many subsurface applications, yet methodologies for identification of the statistical parameters of a discrete fracture network (DFN) are scarce. We present an inversion technique to infer two such parameters, fracture density and fractal dimension, from cross‐borehole thermal experiments data. It is based on a particle‐based heat‐transfer model, whose evaluation is accelerated with a deep neural network (DNN) surrogate that is integrated into a grid search. The DNN is trained on a small number of the heat‐transfer model runs and predicts the cumulative density function of the thermal field. The latter is used to compute fine posterior distributions of the (to be estimated) parameters. Our synthetic experiments reveal that fracture density is well constrained by data, while fractal dimension is harder to determine. Adding nonuniform prior information related to the DFN connectivity improves the inference of this parameter.

DTSGUI: A Python Program to Process and Visualize Fiber-Optic Distributed Temperature Sensing Data.

Fiber-optic distributed temperature sensing (FO-DTS) has proven to be a transformative technology for the hydrologic sciences, with application to diverse problems including hyporheic exchange, groundwater/surface-water interaction, fractured-rock characterization, and cold regions hydrology. FO-DTS produces large, complex, and information-rich datasets. Despite the potential of FO-DTS, adoption of the technology has been impeded by lack of tools for data processing, analysis, and visualization. New tools are needed to efficiently and fully capitalize on the information content of FO-DTS datasets. To this end, we present DTSGUI, a public-domain Python-based software package for editing, parsing, processing, statistical analysis, georeferencing, and visualization of FO-DTS data.

Effects of aligned fractures on the dielectric properties of synthetic porous sandstones

Understanding the dielectric properties of reservoir rocks has important applications in various aspects in the petroleum industry. Fractures are common features in sedimentary porous rocks that not only provide space for the hydrocarbon to accumulate and reside but also are important pathways for the hydrocarbon to flow and therefore have a controlling impact on the formation and distribution of the hydrocarbon reservoirs. However, the dielectric properties of fractured reservoir rocks are still poorly understood. We investigated, through dedicated laboratory experiments, the dielectric behaviors of synthetic porous sandstones with aligned fractures and their relationships with the salinity of the electrolyte saturating the samples, the measurement frequency of the applied electrical field and the directions relative to the fractures. It showed that the fractures have significant effects on the dielectric properties of porous sandstones and their dependence on the brine salinity and measurement frequency reveals the various polarization mechanisms occurring at different frequency ranges. The results offer a preliminary basis for the characterization of fractured rocks using the dielectric properties, which has great potential applicability in oil and gas exploration and exploitation.

Dynamic acousto-elastic testing with an ultrasonic tomography probe: Application to fractured rock characterization

We study nonlinear elastic phenomena at the laboratory scale to help interpret the subtle velocity changes observed in the Earth’s crust, for instance, during strong ground motion, earthquake slip processes or Earth tides. Dynamic Acousto-Elastic Testing (DAET) provides unprecedented details on the nonlinear elastic response of consolidated granular media (e.g., rocks, concrete), including tension/compression asymmetry, hysteretic behaviors as well as conditioning and relaxation effects. Such technique uses a pump-probe scheme where a high frequency, low amplitude wave probes the state of a sample that is dynamically disturbed by a low frequency, large amplitude pump wave. While previous work typically involved a single pair of probing transducers, here we use two dense arrays of ultrasonic transducers to image a sample of Westerly granite with a complex fracture. We apply double beamforming to disentangle complex arrivals and conduct ray-based and finite-frequency tomography using both travel time and am...

Experimental observation of water saturation effects on shear wave splitting in synthetic rock with fractures aligned at oblique angles

Fractured rocks are known to exhibit seismic anisotropy and shear wave splitting (SWS). SWS is commonly used for fractured rock characterization and has been shown to be sensitive to fluid type. The presence of partial liquid/gas saturation is also known to affect the elastic properties of rocks. The combined effect of both fractures and partial liquid/gas saturation is still unknown. Using synthetic, silica-cemented sandstones with aligned penny-shaped voids, we conducted laboratory ultrasonic experiments to investigate the effect fractures aligned at an oblique angle to wave propagation would have on SWS under partial liquid/gas saturation conditions. The result for the fractured rock shows a saturation dependence which can be explained by combining a fractured rock model and a partial saturation model. At high to full water saturation values, SWS decreases as a result of the fluid bulk modulus effect on the quasi-shear wave. This bulk modulus effect is frequency dependent as a result of wave-induced fluid flow mechanisms, which would in turn lead to frequency dependent SWS. This result suggests the possible use of SWS for discriminating between full liquid saturation and partial liquid/gas saturation.

Strategies for characterization of fractured rock using cross-borehole electrical tomography

There is an increasing need to characterize fractured rock systems and to monitor the movement of fluids in these systems. Fractured rock aquifers are increasingly exploited for water resources, and are subject to contamination from industrial activities at the Earth's surface. Deep rock repositories of hazardous waste must be carefully characterized in terms of fracture transport characteristics. More recently, there has been a surge in technologies designed to increase permeability of shale reservoirs by creating fractures to promote fluid removal. However, fractured rock systems present unique challenges for characterization and monitoring technologies. Fracturing typically generates highly heterogeneous and anisotropic systems, making the evaluation of the distribution of physical properties from sparse subsurface measurements particularly problematic. Geophysical imaging technologies are increasingly applied in an effort to overcome the limitations of sparsely located direct observations of subsurface properties. However, the application of geophysical technologies to fractured rock systems presents challenges for imaging as appropriate regularization model constraints for complex, heterogeneous systems are hard to define without additional subsurface information.

Field evaluation of the hydromechanical behavior of flat-lying fractures during slug tests

Summary Slug tests were conducted at depths of 20–50 m in a fractured gneiss and axial displacements were measured along the borehole with a portable extensometer in order to evaluate the feasibility of combining pressure and displacement measurements to improve the characterization of fractured rock. Displacements on the order of microns occurred during slug tests when maximum head changes were on the order of meters. Maximum displacement lagged behind the maximum head, and the signals at a given location were repeatable to within reasonable tolerances. Tests were conducted at 12 packed-off depth intervals in a borehole and the signals from each interval were distinctly different from those at other depths. Parameter estimation techniques were used with a hydromechanical model to interpret the field results, and the findings show that there are three transmissive zones in the borehole and the compliance of the fractures generally decreases with depth. The inversion also highlights the importance of considering heterogeneities in the vicinity of the well bore when interpreting hydromechanical tests. Zones of leakage and blockage in the fractures were predicted in the vicinity of the well bore, and their presence had a significant effect on the displacement signals.

Trends, prospects and challenges in quantifying flow and transport through fractured rocks

Among the current problems that hydrogeologists face, perhaps there is none as challenging as the characterization of fractured rock (Faybishenko and Benson 2000). This paper discusses issues associated with the quantification of flow and transport through fractured rocks on scales not exceeding those typically associated with single- and multi-well pressure (or flow) and tracer tests. As much of the corresponding literature has focused on fractured crystalline rocks and hard sedimentary rocks such as sandstones, limestones (karst is excluded) and chalk, so by default does this paper. Direct quantification of flow and transport in such rocks is commonly done on the basis of fracture geometric data coupled with pressure (or flow) and tracer tests, which therefore form the main focus. Geological, geophysical and geochemical (including isotope) data are critical for the qualitative conceptualization of flow and transport in fractured rocks, and are being gradually incorporated in quantitative flow and transport models, in ways that this paper unfortunately cannot describe but in passing. The hydrogeology of fractured aquifers and other earth science aspects of fractured rock hydrology merit separate treatments. All evidence suggests that rarely can one model flow and transport in a fractured rock consistently by treating it as a uniform or mildly nonuniform isotropic continuum. Instead, one must generally account for the highly erratic heterogeneity, directional dependence, dual or multicomponent nature and multiscale behavior of fractured rocks. One way is to depict the rock as a network of discrete fractures (with permeable or impermeable matrix blocks) and another as a nonuniform (single, dual or multiple) continuum. A third way is to combine these into a hybrid model of a nonuniform continuum containing a relatively small number of discrete dominant features. In either case the description can be deterministic or stochastic. The paper contains a brief assessment of these trends in light of recent experimental and theoretical findings, ending with a short list of prospects and challenges for the future.

Numerical simulation of wave propagation in media with discrete distributions of fractures: effects of fracture sizes and spatial distributions

SUMMARY We model seismic wave propagation in media with discrete distributions of fractures using the pseudospectral method. The implementation of fractures with a vanishing width in the 2-D finite-difference grids is done using an effective medium theory (that is, the Coates and Schoenberg method). Fractures are treated as highly compliant interfaces inside a solid rock mass. For the physical representation of the fractures the concept of linear slip deformation or the displacement discontinuity method is used. According to this model, the effective compliance of a rock mass with one or several fracture sets can be found as the sum of the compliances of the host (background) rock and those of all the fractures. To first order, the background rock and fracture parameters can be related to the effective anisotropic coefficients, which govern the influence of anisotropy on various seismic signatures. We test the validity of the method and examine the accuracy of the synthetic seismograms by a comparison with theoretical ray traveltimes. We present three numerical examples to show the effects of different fracture distributions. The first example shows that different spatial distributions of the same fractures produce different wavefield characteristics. The second example examines the effects of variation of fracture scale length (size) compared with the wavelength. The final example examines the case of fractures with a power-law (fractal) distribution of sizes and shows how that affects the wavefield propagation in fractured rock. We conclude that characterization of fractured rock based on the concept of seismic anisotropy using effective medium theories must be used with caution. Scale length and the spatial distributions of fractures, which are not properly treated in such theories, have a strong influence on the characteristics of wave propagation.

Characterization of fractured rock in the vicinity of tunnels by the swept impact seismic technique

The diversity of the applications of high-resolution seismics requires investigations in the range of hundreds of meters to be performed in very diverse conditions. We found the swept impact seismic technique (SIST) to be a viable solution for high-resolution surveys in hard rocks. Swept impact seismic technique combines the Vibroseis swept frequency and the Mini-Sosie multi-impact ideas. Several variations of the method have been studied leading to improved resolution and efficiency. The development was partly funded by the Finnish Center for Technical Development (TEKES) and partly by the French National Agency for the Management of Radioactive Waste—ANDRA. A test programme was carried out at the Grimsel test site, operated by NAGRA—the Swiss National Cooperative for the Disposal of Radioactive Waste. Measurements were also carried out with single-pulse sources, but data of acceptable quality could not be obtained. Surface and tunnel-wall, as well as borehole SIST sources have been developed and tested. The ability of the seismic techniques to detect and characterize rock discontinuities was proven by investigating a rock block delimited by two parallel boreholes and a tunnel perpendicular to them, involving source–detector distances of 100–200 m. The characterization included the determination of the 3-D positions and orientations of rock features and the tomographic mapping of seismic velocities.

A multidisciplinary fractured rock characterization study at Raymond field site, Raymond, CA

A dedicated field site was developed and a suite of experiments were conducted in the Sierra Nevada foothills, near the town of Raymond, CA to develop and test a multi-disciplinary approach to the characterization of groundwater flow and transport in fractured rocks. A wealth of geologic, hydrologic and geophysical data was collected at the site using a variety of unique tools. A cluster of nine approximately 90 m deep boreholes were drilled at the site in a V-shaped pattern with an angle of 60°. The boreholes are spaced 7.5, 15, 30 and 60 m from the central borehole. Various geophysical and hydrologic tests were conducted in and between these boreholes. Integration of cross-hole radar and seismic tomography, borehole flow surveys and images from a new digital borehole scanner indicated that groundwater flow is mainly confined to a few sub-horizontal fracture zones. A unique suite of hydraulic tests were conducted, in which three to four intervals in each of the nine boreholes were isolated using pneumatic packers. Some 130 injection tests were conducted, and more than 4100 cross-hole transient pressure measurements were obtained. A computer algorithm was developed to analyze such massive interference data systematically. As a result of the analysis, an image of the fracture connections emerged, which is consistent with the geophysical data. High precision tiltmeters were effective in remotely characterizing the preferential flow path. Several radial convergent tracer tests were conducted by injecting a mixture of several conservative tracers and one sorbing tracer: deuterium, fluorescein, lithium bromide and polystyrene micro-spheres. Some differences between the breakthrough curves are observed, which may be due to possible differences among so-called “conservative” tracers. Some characterization tools were found to be more effective than others in locating flowing fractures. However, no single tool was almighty. Characterization of fractured rock is extremely challenging and requires a stepwise and well-thought approach, which is basically a good old scientific approach. Prediction of transport based on the characterization results is even more challenging and one should always bear in mind that it is virtually impossible to uniquely characterize a fractured rock.

Differentiating anisotropy and lateral effects using azimuthal resistivity offset Wenner soundings

Azimuthal resistivity surveys increasingly are being used by hydrogeologists in the identification and characterization of fractured rocks. In these investigations, electrical resistivity is measured as a function of azimuth about a fixed central point. In most recent published examples, any observed change in apparent resistivity with azimuth is interpreted as being indicative of fracture anisotropy. However, interpretation of rotational sounding data is actually more complicated, as azimuthal variations in apparent resistivity are also produced by the presence of dipping stratigraphy and other lateral changes in formation resistivity. Such effects are generally overlooked because the field techniques normally employed are incapable of detecting them. Consequently, it is quite probable that the results of many published surveys have been wrongly interpreted. An alternative field procedure and interpretation methodology has been developed to differentiate anisotropy, dipping layers, and lateral effects. This approach makes use of the offset Wenner technique and examines the different responses of the individual Wenner resistances above different geological structures. Analysis of data obtained using the azimuthal offset Wenner technique from sites in Britain and Ireland have successfully identified subsurface structures and determined the anisotropy where it is present.

Frequency-dependent seismic anisotropy in fractured rock

To examine the effects of multiple, aligned fractures in rock, we have developed a two-dimensional elastic finite difference code for fractured media. Fractures are incorporated into the model explicitly as displacement-discontinuity boundary conditions. The wavefield is computed using a 4th-order staggered grid scheme. Simulations were performed for a broadband explosion point source (center frequency 374 Hz) located at the center of the model. The model consisted of 90 horizontal fractures spaced approximately 1/8 of a wavelength apart. The normal and shear fracture stiffnesses were selected such that the normal incidence transmission coeficient is 0.6. The simulations show strong scattering attenuation of the P-wave in the vertical direction (⊥ to the fractures) and channeling of guided waves in the horizontal direction (II to the fractures). The same code was also used to model wave propagation in an anisotropic medium with equivalent effective moduli for the 90-fracture system. Significant differences between the amplitudes, velocities, and frequency content of the waves in the explicit and equivalent medium fracture models were observed. These differences result from frequency-dependent time delays and filtering across each fracture and channeling along fractures that are not included in the zero-frequency effective medium description. These effects are especially interesting because they illustrate that the dynamic properties of fractured rock include significant amplitude anisotropy that may prove useful in the characterization of fractured rock.

The influence of mechanical and geometrical variability in rock mass deformability

To examine the effects of multiple, aligned fractures in rock, we have developed a two-dimensional elastic finite difference code for fractured media. Fractures are incorporated into the model explicitly as displacement-discontinuity boundary conditions. The wavefield is computed using a 4 th -order staggered grid scheme. Simulations were performed for a broadband explosion point source (center frequency 374 Hz) located at the center of the model. The model consisted of 90 horizontal fractures spaced approximately 1/8 of a wavelength apart. The normal and shear fracture stiffnesses were selected such that the normal incidence transmission coeficient is 0.6. The simulations show strong scattering attenuation of the P-wave in the vertical direction (⊥ to the fractures) and channeling of guided waves in the horizontal direction ( II to the fractures). The same code was also used to model wave propagation in an anisotropic medium with equivalent effective moduli for the 90-fracture system. Significant differences between the amplitudes, velocities, and frequency content of the waves in the explicit and equivalent medium fracture models were observed. These differences result from frequency-dependent time delays and filtering across each fracture and channeling along fractures that are not included in the zero-frequency effective medium description. These effects are especially interesting because they illustrate that the dynamic properties of fractured rock include significant amplitude anisotropy that may prove useful in the characterization of fractured rock.

Laboratory characterization of fluid flow parameters in a porous rock containing a discrete fracture

A porous block of volcanic tuff with an embedded fracture was used to perform laboratory‐scale water and gas flow experiments for the purpose of obtaining unsaturated fractured rock characterization parameters. Rock matrix properties included porosity, hydraulic diffusivity and the gas‐phase diffusion coefficient. Fracture properties included apertures, transmissivity, hydraulic conductivity, and air‐entry pressure. Gas tracer experiments using argon and helium are presented for estimating fracture hydraulic conductivity, fracture apertures and rock matrix porosities.