Effect Of Fracture Surface Roughness Research Articles

A permeability model for the fractal tree-like fracture network with self-affine surface roughness in shale gas reservoirs

The complex natural fracture network with self-affine rough surface and branching characteristics significantly impacts the gas transport in shale gas reservoirs. However, its effects on the permeability have not been studied so far. This study proposes an analytical permeability model for the fractal tree-like fracture network with self-affine surface roughness and branching characteristics. Firstly, the self-affine rough profiles of fracture surface are generated at different fractal dimensions by the Weierstrass–Mandelbrot function and a rough fractal tree-like fracture network is constructed with these surface profiles and branching characteristics. Then, an analytical permeability model is proposed to consider the effects of fracture surface roughness and tree-like branching characteristics on gas flow. This analytical model is verified by numerical simulations. Finally, the velocity distribution of the fracture network and the sensitivity of its structure parameters are analyzed. It is found that eddy flow is more easily formed on rougher fracture surfaces with larger fractal dimension when their fracture aperture is at millimeter scale. The eddy flow disappears when the fracture aperture is at micron scale. Bigger gas flow resistance and more energy loss are observed for smaller fracture aperture and rougher fracture surface. The gas velocity in rough fractures decreases by 60% at micron scale, but decreases by 50% at millimeter scale. Gas flow resistance also increases with the increase of branch angle, branch level and length ratio, but decreases with aperture ratio. As a result, permeability decreases with fractal dimension, branch angle, branch level and length ratio, but increases with aperture ratio.

The Effect of Fracture Surface Roughness and Mechanical Aperture on the Onset of Nonlinear Flow Behaviors in 3D Self-Affine Rough Fractures

Equivalent hydraulic aperture and fracture surface roughness are two significant factors affecting the fluid flow behaviors in rock fractures. To understand the role of fracture surface roughness and aperture in the fluid flow through 3D self-affine rough fractures, roughness fracture surfaces with joint roughness coefficients equal to 2.5, 7.5, 12.5, and 17.5 were established, and the Navier–Stokes equation was used to compute the fluid flow in these 3D self-affine rough fractures with a mechanical aperture increase from 0.2 mm to 0.8 mm with a gradient of 0.2 mm. The results show that when the fracture mechanical aperture is 0.2 mm, the impact of fracture surface roughness on fluid flow is considerable, while this effect decreases obviously with the increase of fracture mechanical aperture. Comparing the permeability obtained by the Navier–Stokes equation with the cube law under different hydraulic gradients, we found that their deviation increased with the increase of hydraulic gradients. This allows for the definition of a critical hydraulic gradient ( J c ), below which the permeability can be properly predicted using the cubic law for its simplicity, and above which Forchheimer’s equation should be applied, and Forchheimer’s coefficients A , B , and J c can be calculated by the prediction equations established in this study.

Digital Image-Based Stress\u2013Permeability Relationships of Rough Fractures Using Numerical Contact Mechanics and Stokes Equation

Flow in fractures is sensitive to their geometrical surface characteristics. The surface can undergo deformation if there is a change in stress. Natural fractures have complex geometries and rough surfaces which complicates the modelling of deformation and fluid flow. In this paper, we present a computational model that takes a digital image of a rough fracture surface and provides a stress–permeability relationship. The model is based on a first-principle contact mechanics approach at the continuum scale. Using this first principle approach, we investigate numerically the effect of fracture surface roughness and shifting of surfaces on the permeability evolution under applied stress and compare the results with laboratory experiments. A mudrock core fracture surface was digitalized using an optical microscope, and 2D cross sections through fracture surface profiles were taken for the modelling. Mechanical deformation is simulated with the contact mechanics based Virtual Element Method solver that we developed within the MATLAB Reservoir Simulation Toolbox platform. The permeability perpendicular to the fracture cross section is determined by solving the Stokes equation using the Finite Volume Method. A source of uncertainty in reproducing laboratory results is that the exact anchoring of the two opposite surfaces is difficult to determine while the stress–permeability relationship is sensitive to the exact positioning. We, therefore, investigate the sensitivity to a mismatch in two scenarios: First, we assess the stress–permeability of a fracture created using two opposing matched surfaces from the rock sample, consequently applying relative shear. Second, we assess the stress–permeability of fractures created by randomly selecting opposing surfaces from that sample. We find that a larger shift leads to a smaller drop in permeability due to applied stress, which is in line with a previous laboratory study. We also find that permeability tends to be higher in fractures with higher roughness within the investigated stress range. Finally, we provide empirical stress–permeability relationships for various relative shears and roughnesses for use in hydro-mechanical studies of fractured geological formations.

Supercritical CO2 Foam Stabilized by a Viscoelastic Surfactant in Fractured Porous Media: The Effect of Fracture Surface Roughness

Supercritical CO₂ Foam Stabilized by a Viscoelastic Surfactant in Fractured Porous Media: The Effect of Fracture Surface Roughness

A pore–scale investigation of surface roughness on the evolution of natural fractures during acid dissolution using DBS method

Natural fractures are the essential pathways by which water, hydraulic fracturing fluids, oil and gas, reactants, and contaminants can travel in the porous media. To fully understand subsurface processes relevant to the fluid transport in natural fractures, the Darcy–Brinkmann–Stokes (DBS) pore–scale method was employed to investigate the effects of fracture surface roughness on the reactive transport and dissolution mechanisms of porous media with natural fractures. The advection–diffusion equation described the transport of reactants, and the incorporated mass balance equation dynamically tracked the fracture evolution during acid dissolution. The model has been developed based on the OpenFOAM platform, a CFD open–source code. The partial differential equations were discretized by the Finite Volume Method (FVM) on the structured grids. We studied the dissolution mechanisms, effective surface area, and normalized porosity–permeability relationship on different surface roughness. An Indiana limestone sample with different calcite content was investigated to elucidate the impacts of unreacted minerals on acidizing process. We categorized the solid dissolution of fractures into three dissolution patterns regarding the second Damkohler number and the Peclet number—face dissolution, front dissolution, and edge dissolution—, where the front dissolution pattern and the face dissolution pattern had the shorter time to consume up the rock in the fractured system. The quantified pore–scale parameters obtained from this study are expected to improve the accuracy of Darcy–scale models. The microscopic insights from the present study can guide effective operating parameters and enhance hydrocarbon production by optimizing the acidization process. • Pore–scale reactive transport and dissolution model in fractured porous media by DBS method. • Three dissolution and transport patterns in fractures were investigated. • The fracture evolution, dissolution rate, porosity–permeability relationship, effective surface area were elucidated. • μCT scan image model with different mineralogical content were analyzed.

Experimental investigation on the diffusion of carbon fibre composite grouts in rough fractures with flowing water

Although grouting has been widely used in tunnels to seal groundwater inrush and reinforce rock masses, dynamic water grouting in fractured rock remains challenging. Carbon fibre and an additive were added to a traditional cement grout to enhance the sealing efficiency and dynamic water resistance in dynamic water grouting. A series of orthogonal tests and contrast tests were conducted to investigate the effect of the additive and carbon fibre on the solidification time and dynamic water resistance of carbon fibre composite grouts. The results demonstrate that the grout solidification time is shortest when the additive comprises 60% sodium aluminate, 20% sodium silicate, 14% calcium chloride, and 6% cellulose, which is favourable to enhancing the efficiency of dynamic water grouting in tunnels. The dynamic water resistance rapidly increases with the carbon fibre concentration when the carbon fibre concentration is less than 0.4%. Barton Curves were introduced to highlight the effect of fracture surface roughness on the grouting and improve the reliability of the simulation experiment. Physical simulation experiments were conducted to investigate the effects of the initial water flow speed, aperture width (roughness), cement water ratio, and additive concentration on the grout diffusion in rough rock fractures in tunnels using a self-designed simulation device. The results indicate that the diffusion distance of the carbon fibre composite grout increases with the initial water flow speed and decreases with an increase in aperture width, cement water ratio, and additive concentration. The most influential factor is the initial water flow speed. A fitting equation for grout diffusion is obtained to provide references for practical grouting operations in tunnels. The distribution of residual grout suggests that the cement-water ratio and additive concentration should be reduced to promote the diffusion of grout in rock fractures in tunnels when the water flow speed is low.

Numerical investigation of fracture morphology effect on heat transfer characteristics of water flow through a single fracture

The understanding of fluid flow and heat transfer properties in fractured rock is still insufficient. This study numerically investigates the effect of fracture morphology on convective heat transfer between fracture surface and flowing fluid. Water flow and heat transfer in a single fracture with different degrees of surface roughness are simulated by a numerical model and compared with experimental data. Simulation results show that a rough surface enhances the heat transfer intensity compared with a smooth fracture. The water temperature at the outlet increases as the surface roughness increases. Water temperature in both rough fracture and smooth fracture increases nonlinearly along the flow direction. The temperature at the inner surface of the fracture fluctuates drastically where the fracture surface suddenly fluctuates. A morphology condition factor (MCF) is introduced to take into consideration the fracture morphology effect on heat transfer characteristics. A linear relationship between normalized MCF and normalized profile length can be observed. A model is proposed to describe variation of the MCF. The promoting effect of fracture surface roughness on heat transfer intensity is gradually reduced with the increase of flow rate.

Experimental study of the heat transfer by water in rough fractures and the effect of fracture surface roughness on the heat transfer characteristics

An accurate understanding of the heat transfer of water flow through rough fractures is important for geothermal energy extraction and utilization. In this study, we used 3D printing to manufacture ten samples with joint roughness coefficient (JRC) values of the fracture surface ranging from 0–2 to 18–20 and conducted experimental investigations of the heat transfer characteristics of the water flow through the ten samples. The effects of the volumetric flow rate, the outer wall surface temperature, the JRC, and the fracture apertures on the heat transfer characteristics were determined. The results showed that a rougher fracture surface resulted in greater heat removal during the flow of the fluid through the rock surface.

Experimental Study on the Shear-Flow Coupled Behavior of Tension Fractures Under Constant Normal Stiffness Boundary Conditions

This study experimentally investigated the effects of fracture surface roughness, normal stiffness, and initial normal stress on the shear-flow behavior of rough-walled rock fractures. A series of shear-flow tests were performed on two rough fractures, under various constant normal stiffness (CNS) boundary conditions. The results showed that the CNS boundary conditions have a significant influence on the mechanical and hydraulic behaviors of fractures, during shearing. The peak shear stress shows an increasing trend with the increases in the initial normal stress and fracture roughness. The residual shear stress increases with increasing the surface roughness, normal stiffness, and initial normal stress. The dilation of fracture is restrained more significantly under high normal stiffness and initial normal stress conditions. The hydraulic tests show that the evolutions of transmissivity and hydraulic aperture exhibit a three-stage behavior, during the shear process—a slight decrease stage due to the shear contraction, a fast growth stage due to shear dilation, and a slow growth stage due to the reduction rate of the mechanical aperture increment. The transmissivity and hydraulic aperture decreased, gradually, as the normal stiffness and initial normal stress increase.

Effects of fracture surface roughness and shear displacement on geometrical and hydraulic properties of three-dimensional crossed rock fracture models

Abstract While shear-flow behavior through fractured media has been so far studied at single fracture scale, a numerical analysis of the shear effect on the hydraulic response of 3D crossed fracture model is presented. The analysis was based on a series of crossed fracture models, in which the effects of fracture surface roughness and shear displacement were considered. The rough fracture surfaces were generated using the modified successive random additions (SRA) algorithm. The shear displacement was applied on one fracture, and at the same time another fracture shifted along with the upper and lower surfaces of the sheared fracture. The simulation results reveal the development and variation of preferential flow paths through the model during the shear, accompanied by the change of the flow rate ratios between two flow planes at the outlet boundary. The average contact area accounts for approximately 5–27% of the fracture planes during shear, but the actual calculated flow area is about 38–55% of the fracture planes, which is much smaller than the noncontact area. The equivalent permeability will either increase or decrease as shear displacement increases from 0 to 4 mm, depending on the aperture distribution of intersection part between two fractures. When the shear displacement continuously increases by up to 20 mm, the equivalent permeability increases sharply first, and then keeps increasing with a lower gradient. The equivalent permeability of rough fractured model is about 26–80% of that calculated from the parallel plate model, and the equivalent permeability in the direction perpendicular to shear direction is approximately 1.31–3.67 times larger than that in the direction parallel to shear direction. These results can provide a fundamental understanding of fluid flow through crossed fracture model under shear.

FRACTAL ASPECTS OF MISCIBLE DISPLACEMENT IN ROUGH FRACTURES: AN EXPERIMENTAL APPROACH

Experiments were performed to study the effect of fracture surface roughness on fluid distribution during miscible displacement. The transparent replicas of single fractures obtained from seven different rocks were prepared and the surface roughness of each sample was described by fractal dimensions using the variogram, power spectral, and triangular prism (TP) techniques. Then, the effect of flow rate and viscosity on the geometry of the displacement front during miscible radial injection was investigated experimentally. The fractal dimensions of the fronts were obtained using box counting fractal analysis at different time lapses. The fractal values of invasion front varied from lithology to lithology, due to different surface roughnesses controlled by the lithology of the rocks. Although fluctuations of fractal values were observed during the growth of the front, fractal dimensions typically yielded an increasing trend. Fractal dimension became more stable with increasing flow rate and developed modestly with increasing viscosity. Finally, relationships between the fractal dimensions of displacement fronts and fracture surfaces were quantitatively analyzed and correlated in order to improve the prediction of fluid distribution within a single fracture during miscible displacement. Overall, correlations were observed between the surface characteristics and front fractal dimension values with some exceptions. In summary, to determine the probable distribution of miscible fluid and development of the front, all parameters except power spectral density (PSD) fractal dimension can be applied in the case of high viscosity ratios. In the case of low injection rates, TP could be applicable. No fractal behavior was present at extreme injection and low viscosity ratios, thus no correlation can be determined for the miscible displacement.

Effects of Fracture Surface Roughness on Macroscopic Fluid Flow and Solute Transport in Fracture Networks

Effects of Fracture Surface Roughness on Macroscopic Fluid Flow and Solute Transport in Fracture Networks

Experimental Study of Fracture Permeability for Stimulated Reservoir Volume (SRV) in Shale Formation

“Stimulated reservoir volume”(SRV) makes shale gas production economic through new completion techniques including horizontal wells and multiple hydraulic fractures. However, the mechanism behind these treatments that provide sufficient permeability is not well understood. The effects of different stimulation treatments need to be further explored. To understand the effects of fracture surface roughness, fracture registration, confining pressure, proppant type and distribution mode, fiber and acidizing treatment on fracture permeability, a series of laboratory permeability experiments were performed on fractured cores from shale formation of Shengli Oilfield. The results of this study demonstrate that sedimentary bedding of shale has important influence on matrix permeability. At 35 MPa confining pressure, the permeability of aligned fracture (unpropped and without fracture offset) can increase about 1–3 orders of magnitude over shale matrix. The permeability of displaced fracture can increase about 1–2 orders of magnitude over the aligned fracture. The permeability of fracture propped with proppant can increase about 2–4 orders of magnitude over unpropped fracture. The greater the fracture surface roughness, the higher the permeability. The increasing degree of displaced fracture permeability is not proportional to the amount of fracture offset. In the microfracture of shale, the effect of ceramic proppant is still better than that of quartz sand, and the permeability of a centralized fairway distribution of proppant is about 1.2 times better than an even monolayer distribution of proppant. Under high pressure, proppant is easy to cause the break of fracture faces of brittle shale, and increase local fracture permeability to some extent. However, quartz sand are more easily broken to embed and block microcracks just made, which results in fracture permeability lower than that of ceramic proppant. At the same time, the argillation phenomenon is easy to happen on propped fracture faces of shale, which is one of the main factors that leads to a substantial decline in fracture permeability. The permeability of displaced fracture propped with proppant is greater than that of aligned fracture propped with proppant. Because of added fiber presence, the permeability of microfractures presented in SRV is greatly reduced. The pressure dependence of aligned fractures in shale obeys Walsh’s theory, but the pressure dependence of propped and displaced fractures in shale obeys Walsh’s law over a limited range of pressures. Deviations reflect proppant seating, proppant embedding and breaking. For shale formation with the high carbonate content, acidizing treatment should be carefully implemented. Experimental results may provide more valuable information for effective design of hydraulic fracturing in shale reservoir.

Measurement of Capillary Pressure Curve of DNAPL in a Water-Saturated Sandstone Fracture

This paper describes a methodology for the measurement of the capillary pressure curve of dense nonaqueous phase liquids (DNAPLs) in a water-saturated sandstone induced fracture. Details of specimen preparation, capillary barrier design, and capillary pressure measurement setup, procedure, and results are given. The aperture distribution of the induced fracture was estimated using the X-ray computed tomography (CT) technique. The primary drainage or invasion of DNAPLs in the water-saturated induced fracture was simulated using the invasion percolation (IP) approach. It was found that the invasion process was highly dependent on the spatial distribution of the fracture apertures. The IP model matched the experimentally measured entry pressure very well but deviated from the measured capillary pressure curve at the midrange and high nonwetting phase saturations. The IP model tended to yield an L-shaped function with a fairly flat portion at the midrange saturation and a sharp rise at the high end. The measured capillary pressure curve was observed to follow the well-known Brooks-Corey porous media function. These discrepancies between the measured and predicted results could be attributed to the fact that the IP model did not consider the effects of fracture surface roughness and undulation. For a fracture with small apertures, these two factors could make the fracture behave more as a porous medium than a parallel-plate channel.

Characterization of Fatigue Fracture in Ni-20 Pct Cr Alloys Using White Light Interference Microscopy and Scanning Probe Microscopy

Nanostructured and ultra–fine-grained metals and alloys are becoming of engineering interest. However, little is known about the influence of grain refinement on fatigue crack behavior. In this study, fatigue crack growth behavior and the key microstructural features controlling the fatigue fracture in nanocrystalline and ultra–fine-grain nickel alloys, processed using different techniques, were investigated. White light interference microscopy as well as the combination of atomic force microscopy (AFM) and ultrasonic force microscopy (UFM) were used to characterize the fractured surfaces of the metals. The role of grain size on the fatigue crack growth resistance and the effect of fracture surface roughness on the crack growth rate were evaluated. The combination of AFM and UFM is presented as a complementary tool to scanning electron microscopy in the fractography of metals.

Effects of Fractal Fracture Surface Roughness on Borehole Ballooning

Borehole ballooning or breathing is commonly observed during drilling through fractured zones. It refers to small, partial, and continuous mud losses and significant rapid mud gains due to annular pressure fluctuations resulting from mud circulation and noncirculation. Better understanding of the factors controlling borehole ballooning and breathing is needed for correct interpretation of the symptoms observed while drilling and to avoid mixing this phenomenon with lost circulation and well kicks. We have developed a two‐dimensional transient model of borehole ballooning or breathing. The model considers the effects of Newtonian fluid rheology, and fracture roughness on the fracture volume change as a function of transient wellbore pressure fluctuations inherent in typical drilling operations. Different types of fracture surface roughness that are commonly observed in sedimentary rocks and the degree of roughness identified by a wide variety of fractal dimensions were considered. The model was solved numerically to investigate the effects of fractures' natural geologic properties (fracture roughness, fracture width, and fracture grid size) on the fluid loss and gain rate between the borehole and the fractured formation. Analyses of the importance of fracture roughness and nonlinear deformation approximations were provided and situations were identified where the roughness starts to become an effective parameter in the process. Rough fractures yielded lower values of fluid loss rate than smooth fractures even though they had the same average aperture. The observations suggested that the scale used in data collection and generating fracture surfaces and apertures is a critical point. Increasing fracture aperture to twice its original value diminished the effects of fracture surface roughness on borehole ballooning.

Fatigue Behaviour in Fine Grained Aluminium Alloys

The effects of alloy production method on microstructure and hence fatigue crack growth rate and fracture mechanism have been examined for a variety of fine-grained/high dispersoid Al-Li-Mg-Zr and Al-Li-Cu-Mg-Zr alloys. Microstructures have been assessed by scanning and transmission electron microcopy, together with electron back scattered diffraction pattern assessment. In these fine-grained/high dispersoid materials, high crack growth rates are seen in comparison to the traditional DC cast alloys, excepting a material with high volume fraction of shearable precipitate. The effects of fracture surface roughness and closure levels in determining crack growth rates has been assessed.

Fatigue crack growth and closure in fine-grained aluminium alloys

The effects of microstructure on fatigue crack growth rate and fracture mechanism have been examined for several relatively fine-grained/high dispersoid aluminium alloys. Particular attention has been paid to the behaviour of spraycast Al–Li alloys, as such materials may present novel property combinations for light weight structural aerospace applications, with extended conventional composition ranges. Fracture resistance has been assessed in terms of both intrinsic and extrinsic factors, including roughness induced closure (RICC). In these fine-grained/high dispersoid materials, high crack growth rates are typically seen in comparison to traditional DC cast alloys, consistent with low fracture surface roughness levels and correspondingly reduced levels of RICC. The competitive influence of shearable precipitates on crack roughness is however noted in a high-Li alloy variant. The effects of fracture surface roughness and closure levels on long crack fatigue crack growth rates have been compared with recent novel modelling of closure levels.

Investigation of the grain boundary elemental composition of the low-alloy steel

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Experimental characterization of roughness-induced crack closure in Al-Li 2090 alloy

Fatigue crack propagation testing and a fractographic analysis for Al-Li 2090 alloy specimens were performed to experimentally characterize roughness-induced crack closure. The effects of fracture surface roughness on crack closure were quite significant. It was found that asperity contact inside of Al-Li alloy specimens can occur via different mechanisms which depend on the magnitude of load or crack length. In addition, sterelogical techniques were used to provide additional input required to characterize the three-dimensional nature of the fracture surface and to represent the most salient measurable features of the actual fracture surface in two-dimensional form. From this, relations between the roughness parameter and the nominal range of stress intensity factors were rationalized for different load ratios and load ranges. The degree of mixed mode fracture was characterized. Based on these studies, it was concluded that asperity contact can involve two components: (1) mechanical contact of asperities due to the roughness on the fracture surface and, (2) mixed mode crack tip displacements when the load is applied. Crack closure due to out-of--plane mixed mode fracture (i.e., Mode III) appeared to be more prominent compared to in-plane mixed mode fracture (i.e., Mode II) since the roughness dimension (mean height) is large inmore » the thickness section and the angle between asperity slip band facets is large (50{degree}).« less