Permeability Of Fractured Rocks Research Articles

Analysis of Fractured Rock Permeability Evolution Under Unloading Conditions by the Model of Elastoplastic Contact Between Rough Surfaces

Analysis of Fractured Rock Permeability Evolution Under Unloading Conditions by the Model of Elastoplastic Contact Between Rough Surfaces

Laboratory Investigation on the Permeability Variation of Fractured Inada Granite by Multiple Transient Axial Stress Disturbances

A persistent increase in the permeability of rock mass caused by transient stress disturbances could explain the variation in groundwater level caused by earthquakes in the far-fields, increase in petroleum production due to artificial vibrations, induction of small earthquakes by seismic waves in intermediate and far-fields, etc. However, the effect of transient stress disturbances on rock permeability has not yet been fully clarified. In this study, the permeability of triaxially fractured Inada granite under multiple transient disturbances in axial stress was measured to clarify the effects of the transient stress disturbances on the fractured rock permeability. In the experiments, the permeability of fractured Inada granite decreased with time. However, the permeability increased with each series of axial stress disturbances whose amplitude was 3 MPa or larger. The degree of increase in permeability increased with the axial stress disturbance amplitudes. The increased permeability decreased with time and the duration in which the permeability decreased to its value before the disturbances was longer for larger axial stress disturbance amplitudes. The increase in permeability could be employed to enhance oil and gas productions, prevent large earthquakes, reroute underground water flow, etc.

Permeability evolution of grout infilled fractures subjected to triaxial compression with low confining pressure

Fractures in rock masses not only are main flow conduit for groundwater, but also provide planes of weakness on which further displacement can more readily occur. Grouting has been widely used to reduce the permeability of fractured rocks, as well as to improve the stability of fractured rocks. Therefore, fractures infilled with grout appear extensively in practice, and may be subjected to different in-situ stress conditions during the construction and operation life of tunnels and underground spaces in fractured rock masses. To examine the stress induced changes in permeability of grout infilled fractures, we performed triaxial compression tests on fractured granite samples infilled with grout, during which continuous permeability measurements were also conducted. Both planar and rough fractures with different apertures were tested. The permeability of infilled fractures slightly decreased with increasing deviatoric stress from 0 to the crack damage stress, followed by drastic enhancement in permeability with further increasing deviatoric stress. The fracture permeability increased by about 5–10 times at the peak deviatoric stress compared with that at the deviatoric stress of 5 MPa. Both CT techniques and particle discrete element models were used to investigate the changes in the microstructures of grouts and granites, as well as the interface between them. The results show that compression induced cracks in grouts and grout-granite interfaces are the main sources for the enhancements in permeability of grout infilled fractures under triaxial compression with low confining pressures.

Increased Fractured Rock Permeability After Percolation Despite Limited Crack Growth

AbstractFractured rock permeability can increase by crack extension that creates additional flow pathways or by increases in crack openings that increase crack transmissivity. Understanding the partitioning between these two mechanisms during rock deformation is critical for conceptual models of fluid flow and transport in crystalline rocks and sedimentary layers with low matrix permeability and for the hydromechanics of crustal rocks. Using ice as a model for rock, new systematic experiments reveal that when subject to uniaxial loading at a constant strain rate, crack density remains nearly constant after the onset of percolation even while permeability increases, indicating that after the onset of percolation the increase in permeability is primarily due to the opening of existing cracks rather than the extension of cracks. These observations have implications for conceptual models of fractured rock permeability that often focus on the evolution of permeability with changing fracture density rather than changing fracture transmissivity and for attempts to link fractured rock permeability to seismic properties that often dominantly consider changes in crack density rather than crack apertures.

Factors Affecting Permeability Reduction of MICP for Fractured Rock

Microbial induced Calcite Precipitation (MICP) has shown great potential to reduce the permeability of fractured rock, but its permeability reduction characteristic affected by solution concentration and the geometric morphology of fracture rock is not presented. Then MICP grouting and seepage test device were designed to study the effect of factors including urea and calcium chloride concentration, fracture roughness and aperture on permeability reduction of MICP on fracture rock. During the test, the same total volume was injected into each sample with the same injection times. The results showed that MICP grouting reduced the Darcy permeability coefficient of fractured rock to 3-5×10-5 m/s by 4 orders of magnitude smaller than that of non-grouted fractured rock. And with increasing of fracture roughness, the urea and calcium chloride concentrations and decreasing of fracture aperture, MICP grouting ratio in fractured rocks increased and the Darcy permeability coefficient decreased. And the permeability reduction of MICP on fracture rock increased with decreasing of the four factors. The permeability coefficient and the permeability reduction were the most sensitive to urea concentration. The permeability reduction could be fitted well by power function consider the above factors. The effects of fracture aperture larger than 2.5 mm or roughness larger than 0.873 mm need further research. The research results can provide important guidance and reference significance for microbial grouting to improve fractured rock performance.

Interactive roles of geometrical distribution and geomechanical deformation of fracture networks in fluid flow through fractured geological media

In this study, the combined effects of geometrical distribution and geomechanical deformation of fracture networks on fluid flow through fractured geological media are investigated numerically. We consider a finite-sized model domain in which the geometry of fracture systems follows a power-law length scaling. The geomechanical response of the fractured rock is simulated using a hybrid finite-discrete element model, which can capture the deformation of intact rocks, the interaction of matrix blocks, the displacement of discrete fractures and the propagation of new cracks. Under far-field stress loading, the locally variable stress distribution in the fractured rock leads to a stress-dependent variable aperture field controlled by compression-induced closure and shear-induced dilatancy of rough fractures. The equivalent permeability of the deformed fractured rock is calculated by solving for the fracture-matrix flow considering the cubic relationship between fracture aperture and flow rate at each local fracture segment. We report that the geometrical connectivity of fracture networks plays a critical role in the hydromechanical processes in fractured rocks. A well-connected fracture system under a high stress ratio condition exhibits intense frictional sliding and large fracture dilation/opening, leading to greater rock mass permeability. However, a disconnected fracture network accommodates much less fracture shearing and opening, and has much lower bulk permeability. We further propose an analytical solution for the relationship between the equivalent permeability of fractured rocks and the connectivity metric (i.e. percolation parameter) of fracture networks, which yields an excellent match to the numerical results. We infer that fluid flow through a well-connected system is governed by traversing channels (forming an “in parallel” architecture) and thus equivalent permeability is sensitive to stress loading (due to stress-dependent fracture permeability), whilst fluid flow through a disconnected system is more ruled by matrix (linking isolated clusters “in series”) and has much less stress dependency.

The impact of stress orientation and fracture roughness on the scale dependency of permeability in naturally fractured rocks

The equivalent permeability of layered fractured rocks plays an important role in hydrocarbon recovery, underground energy storage, waste disposal management, groundwater hydrology, and subsurface contaminant transport. Borehole data contain some uncertainties/sampling bias during collection and interpretation. This sampling effect may lead to an inaccurate characterization of the fractured media. Studies show that long fractures with high permeability, which are rarely seen in borehole images, dominate the flow pattern and affect the overall permeability of the fractured system. This means that samples taken at any scale smaller than the scale of interest result in imprecise permeability upscaling.To understand this sampling problem, we have established an efficient sampling method to study the existence of the representative elementary volume (REV) in naturally fractured rocks. We selected a collection of outcrop data represented by discrete fracture and matrix (DFM) models in which fracture apertures are mechanically constrained. A finite-element-finite-volume approach is utilized to characterize the flow behavior of DFM models. Multiscale random sampling is combined with flow-based upscaling to determine the equivalent permeability tensor and its anisotropy by considering the variable orientation of the stress state and fracture roughness.Our findings indicate a convergence towards a scale-invariant equivalent permeability and fracture density with increasing sample size, and the equivalent permeability itself has a multimodal distribution. The spatial variation of the permeability tensor and the change in the degree of anisotropy with sample size reflect the inhomogeneity of the fracture patterns.

A Complex Network Model for Analysis of Fractured Rock Permeability

Fractured rocks exist widely in nature. The fracture network is an effective storage space and main seepage channel of low‐permeability oil and gas reservoirs, which controls the seepage system of low‐permeability oil and gas reservoirs. The connection characteristics of fracture networks are complex and evolve dynamically with time. The rise of complex network research can provide reliable analysis for the relationship between network structures and network behaviors. In this work, the fracture network is considered as a hierarchical network with self‐similarity, and complex network theory is applied to analyze the permeability of fractured rocks. According to the power‐law relationship of degree distribution of network nodes, the number of nodes is corresponding to the number of network edges and a new power‐law distribution relationship of edges with degree of nodes is proposed. Eventually, the permeability model of fractured rocks is derived and it is found that permeability of fractured rocks is a function of degree of maximum node kmax, self‐similarity index γ, power index dk, and other structural parameters. Compared with the existing numerical simulations, the validity of the model is verified. By calculating the influence of model parameters on the permeability, the following results are obtained: (1) fracture porosity is directly proportional to permeability; (2) fracture surface density is linearly increasing with permeability; (3) power index is inversely proportional to permeability; and (4) permeability is exponentially increasing with the maximum degree of a node.

Disentangling the Simultaneous Effects of Inertial Losses and Fracture Dilation on Permeability of Pressurized Fractured Rocks

AbstractHow fluids flow through pressurized fractured rocks is relevant to many engineering applications and geophysical processes including fault rupturing, hydraulic fracturing, induced seismicity, fluid extraction, and contaminant transport. With increasing fluid pressure and concomitantly elevated hydraulic gradient, the permeability of fractured rock is reduced because of inertial losses within the fluid. There is an accompanying flow regime change when this happens. On the other hand, increasing pressure causes fracture dilation which enhances permeability. In this case, the fracture geometry changes. These two competing consequences of increasing pressure had always been studied independently. Here we present an analytical expression for fractured rock permeability where flow regime and medium geometry simultaneously co‐evolve. The theory was applied to core and field flow tests. With continuously increasing fluid pressure, the inertial effect on permeability first dominates over that of fracture dilation and this dominance theoretically reverses at Forchheimer number = 1/3.

EFFECT OF FRACTAL FRACTURES ON PERMEABILITY IN THREE-DIMENSIONAL DIGITAL ROCKS

The fracture has great impact on the flow behavior in fractured reservoirs. Fracture traces are usually self-similar and scale-independent, which makes the fractal theory become a powerful tool to characterize fracture. To obtain three-dimensional (3D) digital rocks reflecting the properties of fractured reservoirs, we first generate discrete fracture networks by stochastic modeling based on the fractal theory. These fracture networks are then added to the existing digital rocks of rock matrixes. We combine two low-permeable cores as rock matrixes with a group of discrete fracture networks with fractal characteristics. Various types of fractured digital rocks are obtained by adjusting different fracture parameters. Pore network models are extracted from the 3D fractured digital rock. Then the permeability is predicted by Darcy law to investigate the impacts of fracture properties to the absolute permeability. The permeability of fractured rock is subject to exponential increases with fracture aperture. The relationship between the permeability and the fractal dimension of fracture centers is exponential, as well as the relationship between permeability and the fractal dimension of fracture lengths.

Experimental characterization and dependence of rock fracture permeability on 3D stresses

The effectiveness of transmitting underground fluid in fractured rock mass is significantly influenced by stresses. This paper intends to study the hydraulic behavior of rock fractures under 3D stresses. First, a fracture permeability model has been developed for predicting permeability of rough fractures subjected to 3D stresses on the basis of minimum potential energy principle. Then, a series of transient flow tests are conducted on three fractured shale samples. The test results show that pressure response deviates the exponential behavior at early time, thus the late time data is recommended for permeability calculation. The permeability of fractured rocks are related to 3D stresses, specifically, permeability decreases with the increasing of confining pressure. Variation of curve for fracture deformation versus flow time follows the same trend as the curve for differential pressure versus time (Δp-t), indicating that the pressure decay has a principal dependence on fracture deformation. At last, a comparison between with experimental permeability and the proposed model is tackled, which demonstrates that the proposed model is capable of matching the experimental data for 3D stresses.

Hydraulic properties and energy dissipation of deep hard rock under H-M coupling and cycling loads

The permeability of deep rock is closely related to the stability and safety of underground engineering. The rocks in deep stratum are mostly with high stress and high osmotic pressure. Therefore, it is necessary to consider the coupling effect between porewater pressure and in situ stress on rock mass. A series of triaxial cyclic loading and unloading experiments under hydraulic-mechanics coupling conditions are carried out to studied the mechanical and hydraulic properties of granite in the depth of 1300 m to 1500 m. Especially, the effect of the disturbance on the permeability of fractured rocks are investigated by unloaded the confining pressure. Tests results presented that the stress-strain curves of deep granite showed typical brittle characteristics. The principal stress of granite exhibited a linear relationship under the high confining pressure of 34-40 MPa and high osmotic pressure of 13-15 MPa. Dissipated energy of the rock decreased to a relatively low level after 2-3 loading cycles and then slowly increased. Permeability showed a decreasing trend as the loading and unloading cycles increase. Finally, acoustic emission technology was used to monitor the fracture evolution in rocks, the acoustic emission signal released as the fractures develop and energy dissipated. The results would provide basic data for the exploitation and excavation in the deep galleries.

Variation in the Permeability of Intact and Fractured Rocks due to Transient Disturbances in Axial Stress or Pore Pressure

A persistent increase in the permeability of rock mass caused by transient stress disturbances, if present, could explain the variation in the groundwater level in far fields, increase in petroleum production caused by earthquakes or artificial vibrations in enhanced oil recovery, and induction of earthquakes by seismic waves in intermediate and far fields. However, it has not been verified whether the transient stress disturbances induce an increase or decrease in the rock permeability. In this study, the permeability values of intact and triaxially fractured Kushiro Cretaceous sandstone and Shikotsu welded tuff were measured before and after transient axial stress or pore pressure disturbances to clarify the effects of transient stress disturbances on rock permeability. According to the experimental results, the stress disturbances showed either decreasing or increasing effects on the permeability depending on the rock type and experimental conditions. However, when focusing on the fractured rocks rather than the intact ones, which would be more important in field applications, the argillaceous Kushiro Cretaceous sandstone mainly exhibited decreasing effects and the glassy Shikotsu welded tuff mainly exhibited increasing effects.

Relationship Between the Orientation of Maximum Permeability and Intermediate Principal Stress in Fractured Rocks

AbstractFlow and transport properties of fractured rock masses are a function of geometrical structures across many scales. These structures result from physical processes and states and are highly anisotropic in nature. Fracture surfaces often tend to be shifted with respect to each other, which is generally a result of stress‐induced displacements. This shift controls the fracture's transmissivity through the pore space that forms from the created mismatch between the surfaces. This transmissivity is anisotropic and greater in the direction perpendicular to the displacement. A contact mechanics‐based, first‐principle numerical approach is developed to investigate the effects that this shear‐induced transmissivity anisotropy has on the overall permeability of a fractured rock mass. Deformation of the rock and contact between fracture surfaces is computed in three dimensions at two scales. At the rock mass scale, fractures are treated as planar discontinuities along which displacements and tractions are resolved. Contact between the individual rough fracture surfaces is solved for each fracture at the small scale to find the stiffness and transmissivity that result from shear‐induced dilation and elastic compression. Results show that, given isotropic fracture networks, the direction of maximum permeability of a fractured rock mass tends to be aligned with the direction of the intermediate principal stress. This reflects the fact that fractures have the most pronounced slip in the plane of the maximum and minimum principal stresses, and for individual fractures transmissivity is most pronounced in the direction perpendicular to this slip.

Stress-dependent shear wave splitting and permeability in fractured porous rock

It is well known that shear wave propagates slower across than parallel to a fracture, and as a result, a travelling shear wave splits into two directions when it encounters a fracture. Shear wave splitting and permeability of porous rock core samples having single fracture were experimentally investigated using a high-pressure triaxial cell, which can measure seismic shear wave velocities in two directions mutually perpendicular to the sample axis in addition to the longitudinal compressive wave velocity. A single fracture was created in the samples using a modified Brazilian split test device, where the cylindrical sample edges were loaded on two diametrically opposite lines by sharp guillotines along the sample length. Based on tilt tests and fracture surface profilometry, the method of artificially induced tensile fracture in the sample was found to create repeatable fracture surfaces and morphologies. Seismic velocities of the fractured samples were determined under different levels of stress confinement and fracture shear displacement or mismatch. The effective confining stress was varied from 0.5 MPa to 55 MPa, while the fractures were mismatched by 0 mm, 0.45 mm and 1 mm. The degree of matching of the fracture surfaces in the core samples was evaluated using the joint matching coefficient (JMC). Shear wave splitting, as measured by the difference in the magnitudes of shear wave velocities parallel (VS1) and perpendicular (VS2) to the fracture, is found to be insensitive to the degree of mismatching of the fracture joint surfaces at 2 MPa, and decreased and approached zero as the effective stress was increased. Simple models for the stress- and JMC-dependent shear wave splitting and fractured rock permeability were developed based on the experimental observations. The effects of the joint wall compressive strength (JCS), JMC and stress on the stress dependency of joint aperture were discussed in terms of hydro-mechanical response. Finally, a useful relationship between fractured rock permeability and shear wave splitting was found after normalization by using JMC.

Fractures in Volcanic Reservoir: a case study of Zhongguai uplift in Northwestern Margin of Junggar Basin, China

Fractures in Carboniferous volcanic rocks located at Zhongguai Area (China) highly influence the accumulation and productivity of oil and gas. As such, the study of development periods and genetic mechanisms of tectonic fractures could throw useful information regarding the evaluation and development of that reservoir. Their tectonic origins caused high-angle and oblique shear fractures. The primary orientation of those fractures appears close to EW (270°±10°), NW (300°±15°), NE (45°±15°), and SN (0°±10°). At least four fracture generations can be found in Carboniferous volcanic rocks at Zhongguai Area. Combined with a tectonic evolution, they are based on the segmentation relationship of the fracture fillings, the thermometry measurement of the fracture filling inclusion, and the acoustic emission, as well. Affected by a new horizontal principal stress, the opening and permeability of nearly EW fractures are the best. In this way, a priority in the development of well's patterns should be considered close to EW fractures. The pressure change in the process of exploitation may damage the reservoir permeability of fractured volcano rocks severely. Accordingly, well patterns should be adjusted to dynamic changes of permeability happened during the oilfield development since some differences have been detected in distinct fracture sets.

Evolution of permeability in sand injectite systems

Sand injectite complexes comprise kilometer-scale clastic intrusion networks that act as effective conduits for the migration, accumulation and then recovery of hydrocarbons and other fluids. An equivalent continuum model is constructed to represent a sand injectite reservoir, coupling stress and fluid flow in fractured rock using the continuum simulator TOUGHREACT coupled with FLAC3D to follow deformation and fluid flow. A permeability model, which uses staged percolation models, is proposed to improve permeability estimation of fracture networks by accommodating four different levels of fracture connectivity. This permeability model is confirmed against field and laboratory data, corresponding to the different connectivities of fracture networks. The new constitutive permeability model is incorporated into the coupled hydro-mechanical simulator framework and applied to sand injectites with the analysis of permeability evolution mechanisms and mechanical sensitivity. The results indicate that when the magnitudes of principal stresses increase in a constant ratio, normal closure is the dominant mechanism in reducing fracture aperture and thereby permeability. Conversely, the evolution of stress difference can accentuate aperture and permeability due to an increase in shear dilation for critically or near-critically oriented fractures. Also, the evolution of aperture and related permeability of fractured rock are more sensitive at lower stress states than at higher stress states due to the hyperbolic relationship between normal stress and normal closure of the fractures.

Investigation of effects of fracturing fluid on hydraulic fracturing and fracture permeability of reservoir rocks: An experimental study using water and foam fracturing

Hydraulic fracturing is a promising technique to enhance gas production from deep geological formations by enhancing reservoir rock permeability. However, permeability enhancement may vary with reservoir conditions and fracturing fluid. The main objective of this study is to investigate the effect of fracturing fluid on the fracture pattern and permeability of fractured rock. Fracturing experiments were conducted on siltstone specimens with water and foam (N2 + water) as the fracturing fluids, using a high-pressure triaxial apparatus. CT scanning and 3-D scanning technology were used to examine the fracture pattern and the fracture surface of the fractured specimens. The fractured specimens were then used to conduct permeability experiments for five different confining pressures (5 – 25 MPa) and six different injection pressures (1 – 6 MPa) with N2, and fracture permeability values were compared, based on the fracturing fluid type as well as the intact rock permeability. The results indicate that foam fracturing can induce a complex twisted fracture with greater surface area and the fracture is throughout the specimen. Moreover, foam fracturing emits more energy than water fracturing during the fracturing process, which is favourable for secondary fracture development. According to the measured permeability values, the fracture permeability of foam fractured specimens is about 5 × 105 times greater than that of intact and water-fractured specimens. With increasing confining pressure, N2 permeability decreases, regardless of the specimen (intact or fractured), and interestingly permeability decreases with increasing injection pressure. These observations are related to the effective stress principle and the Klinkenberg effect.

The permeability of fractured rocks in pressurised volcanic and geothermal systems

The connectivity of rocks’ porous structure and the presence of fractures influence the transfer of fluids in the Earth’s crust. Here, we employed laboratory experiments to measure the influence of macro-fractures and effective pressure on the permeability of volcanic rocks with a wide range of initial porosities (1–41 vol. %) comprised of both vesicles and micro-cracks. We used a hand-held permeameter and hydrostatic cell to measure the permeability of intact rock cores at effective pressures up to 30 MPa; we then induced a macro-fracture to each sample using Brazilian tensile tests and measured the permeability of these macro-fractured rocks again. We show that intact rock permeability increases non-linearly with increasing porosity and decreases with increasing effective pressure due to compactional closure of micro-fractures. Imparting a macro-fracture both increases the permeability of rocks and their sensitivity to effective pressure. The magnitude of permeability increase induced by the macro-fracture is more significant for dense rocks. We finally provide a general equation to estimate the permeability of intact and fractured rocks, forming a basis to constrain fluid flow in volcanic and geothermal systems.

Estimation of rock mass permeability using variation in hydraulic conductivity with depth: experiences in hard rocks of western Turkey

Determining permeability of fractured rocks is a focal point in engineering projects. Many methods have been developed for direct in situ determination of the hydraulic parameter; nevertheless, especially in the preliminary stages of rock mass characterization, empirical correlations are used to estimate the rock permeability. This study proposes a relationship between rock mass depth and permeability that may be useful for initial hydraulic characterization of fractured media. For this purpose, results of core drilling, injection tests, and geophysical prospecting carried out in fractured hard rocks—mainly andesites and secondly metamorphites—of western Turkey were considered. A decrease in rock mass permeability with depth is evident from the results of injection tests. Within the depth of investigation (the first 100 m), hydraulic conductivity shows an exponential declining trend with depth and depends on the surface value of the hydraulic parameter. No correlation is evident between discontinuity spacing and depth or rock quality designation (RQD) and depth. The applicability of the relationship found requires a hydrostructural homogeneity of the rock mass, which can be verified through inexpensive geological survey and geophysical prospecting.