Fractured Sandstone Research Articles

Frictional behavior of sandstone fractures under forward-backward pre-peak cyclic loading

The objective of this study is to determine the effect of cyclic loading under pre-peak condition on shearing behavior of fractures in Phra Wihan sandstone, primarily to improve an understanding of the stability conditions of slope embankments along roadways and railways in the north of Thailand where seismic activities have often occurred. The fractures were artificially made in the laboratory by tension inducing method and cyclically sheared under forward to fully backward loading paths with constant normal stresses between 1 and 4 MPa. Two cyclic loading conditions were used on separate sets of specimens: constant stress amplitude and constant displacement amplitude. Fifty shear cycles were applied. The results indicated that the pre-peak cyclic loading under both loading conditions slightly reduce shear strengths and stiffness of the sandstone fractures due to the degradation of the second order fracture asperities. Fracture roughness notably reduced particularly under large shear stress and displacement amplitudes due to the deposition and packing of rock power. The subsequent monotonic loading performed after cyclic loading showed further reduction of fracture roughness due to the degradations of the first order asperities.

Experimental study of the permeability of fractured sandstone under complex stress paths

AbstractOil and gas are main storage resources in the pores and fractures of rocks. Under a complex stress environment, the permeability evolution of the reservoir rock fracture directly affects the flow of oil and gas, which is important in oil and gas exploration and development. The permeability test of a single sample in complex stress paths was performed on intact sandstone, single‐fractured sandstone, and double‐fractured sandstone using high‐precision hydro‐mechanics‐coupled triaxial experimental equipment to study the permeability evolution of fractured sandstone. The experimental schemes of the permeability tests are as follows: (a) under increasing confining pressure; (b) under increasing liquid pressure; (c) under cyclic loading and unloading deviatoric stress; and (d) under increasing confining pressure and deviatoric stress synchronously. Results show that liquid flow on fractured sandstone can be regarded as laminar flow with low velocity. The permeability and stress sensitivity levels of the fractured sandstone are higher than those of the intact sandstone. The permeability decreases in a negative exponential manner when the confining pressure increases but increases in a positively exponential manner when the liquid pressure increases. The increase in the deviatoric stress decreases the permeability, whereas unloading increases the permeability. The entire evolution of permeability is irreversibly reduced. As the confining pressure and deviatoric stress increase synchronously, the permeability decreases, and the decrease rate tends to be stable. These results can provide an important basis for prediction of the permeability evolution of fractured sandstone and for efficient oil and gas exploitation.

Experiment study of physical and mechanical properties of sandstone after variable thermal cycles

To study the effects of temperature cycles on the mechanical properties, thermal damage, and fracturing of sandstone, uniaxial compression tests and Brazilian splitting tests were performed using a hydraulic pressure servo rigidity rock mechanics experiment system. Before the tests, the sandstone samples were treated with cyclic (repeated from 0 to 8 cycles) temperatures from 100 to 600 °C. In addition, the chromaticity (color parameters L*, a*, and b*), thermal conductivity, and P wave velocity of the sandstone samples were measured using a color meter, thermal conductivity tester, and acoustic wave detection device, respectively. The results show that the peak strength, tensile strength, thermal conductivity, and P wave velocity decrease rapidly when the samples are exposed to higher temperatures (> 400 °C). Below 300 °C, the decrease is mainly due to the escaping of the adhered, bound, and structural water. Between 300 and 600 °C, the thermal response of the minerals in sandstone increases the development of microcracks and weakens the sandstone. At these same temperatures, when the number of cycles increases, the peak strength and tensile strength of the sandstone decrease, which is more obvious at higher temperatures. After multiple cycles, the plastic characteristics of the sandstone increase. The results obtained in this paper are good for predicting the thermo-physical properties of sandstone when exposed to high temperatures.

Geomechanical simulation of low-order fracture of tight sandstone

Abstract The core factors in the study of fractures within tight sandstone reservoirs are tectonics, geomechanics and fluid mobility, which are not independent but are coupled to each other. In this paper, taking the upper Paleozoic gas reservoir within the low-amplitude tectonic zone in the Sulige gas field of the Ordos Basin as an example, the developmental characteristics and geomechanical model of fractures and their relationships with the productivity of tight gas sandstone reservoirs were systematically studied. The results show that this low-amplitude structural zone develops mainly low-order ruptures. The paleotectonic stress field of the target layer in the Yanshanian period was restored using a 3D finite element method. The horizontal principal stresses exhibited stress diffusion and were affected by local low-amplitude structures. There was a positive correlation between σH and σh, and σH/σh was distributed primarily between 1.35 and 1.95 with an average of 1.62. The strain energy density (U) was calculated to quantitatively characterize the microfracture distribution of the target layer based on the principle of energy conservation. There are certain positive correlations between U and the horizontal principal stresses. The geostresses of high-U regions underwent a certain degree of stress torsion, which manifested as a stress deflection reaching up to 30° during stress propagation. Finally, we proposed the concept of U-value geomechanical sweet spots in a tight gas sandstone. Based on this criterion, commercial gas wells can be effectively identified.

Salt Solution Attack–Induced Freeze–Thaw Mechanical Degradation and Its Correlation with Strength Characteristic of Mode-I Fracture Sandstone

Abstract The safety and durability of rock mass engineering face rigorous testing in cold regions, prompting experimental study of the effect of chemical corrosion and freeze–thaw (F–T) cycles. By ...

Laboratory investigation of the mode-I fracture of sandstone caused by a combination of freeze-thaw cycles and chemical solutions

The mechanical parameters and deterioration mechanisms of sandstone specimens obtained from the hydrofluctuation belt in the Three Gorges Reservoir Region of China were investigated under a combination of freeze-thaw cycles and different chemical solutions. The degradation in several mechanical characteristics was discussed, and the degree of freeze-thaw deterioration of the mechanical behaviors was quantitatively analyzed based on the crack propagation radii and a damage variable that reflects the changes in porosity. The results showed that the mechanical parameters of sandstone had a significant damage deteriorating trend under freeze-thaw cycles and different chemical solutions. And the degree of damage deterioration of fracture toughness KIC was the greatest, followed by that of tensile strength; that of compression strength was the smallest. There was obvious consistency among fracture toughness KIC, tensile strength, and compression strength of sandstone under a combination of freeze-thaw cycles and different chemical solutions; those were obvious linear relationship. The crack propagation radii for the various effects of freeze-thaw cycles and chemical solutions were smaller than those of the natural state and the radii generally decreased with increasing freeze-thaw cycles.

Direct Tensile Behavior of Limestone and Sandstone with Bedding Planes at Different Strain Rates

To investigate the tensile behavior of rock bedding planes, a series of direct tensile tests were conducted on cylindrical limestone and sandstone specimens with a distinct bedding plane perpendicular to the axial direction. Different tensile strain rates of the bedding plane areas within the magnitude range from 10−6 to 10−2 s−1 were considered. The results indicate that the limestone fracture surfaces maintain the original roughness of bedding planes with few grain fractures. However, the sandstone fracture surfaces display many intergranular and transgranular fractures and are rough at the grain scale. The stress–strain curves of limestone specimens can be divided into elastic deformation stage and yield stage (the nonlinearity of yield stage is weaker with the increase of strain rate), while that of sandstone specimens develop in a nonlinear way accompanied by plastic deformation throughout the loading process. The strain rate significantly affects the tensile deformation and strength of the tested specimens (except the average deformation modulus of sandstone specimens). The elastic modulus, average deformation modulus, peak strain and tensile strength increase as strain rate goes up and have a linear relation with the logarithm of strain rate under the tested strain rate range (namely, under a relatively low strain rate range).

Effect of Stress and Water Pressure on Permeability of Fractured Sandstone Based on Response Surface Method

In order to study the effects of stress and water pressure on the permeability of fractured sandstone, an ultra-deep gas layer of fractured tight sandstone in WengFu Mine in GuiZhou province, was selected for experimental design and research, and the effects of confining pressure, internal pressure, effective stress and water pressure on gas permeability were studied. The results show that the effective stress and slippage effect compete with each other when the confining pressure is kept fixed and the internal pressure is increased in the fractured tight sandstone reservoir. At the beginning of the experiment, the gas slippage effect is strong while the rock stress sensitivity is weak. The gas permeability increasingly decreases with the increase of internal pressure.The effective stress is stronger than slippage effect, and the gas permeability increases with the increase of internal pressure when crossing the low point of permeability.The greater the confining pressure is, the greater the internal pressure needed to reach the low permeability point will be.With the increase of axial stress, horizontal contact hydraulic hole increasingly closed. Horizontal hydraulic holes are more sensitive to axial stress than vertical ones.

Experimental Investigation on the Post-Peak Short-Term and Creep Behavior of Fractured Sandstone

Short-term and creep tests of fractured sandstone with different degrees of damage prepared using pre-peak and post-peak unloading tests on intact sandstone were carried out using a servo-controlled rock mechanics system. Based on our experimental results, the influence of confining pressure and damage on short-term mechanical behavior of fractured sandstone with different degrees of damage was first analyzed. The results show that the peak strength, residual strength, elastic modulus, and secant modulus of fractured sandstone increase linearly with increasing confining pressure, but decrease with increasing damage. The short-term failure modes depend on the damage and change from typical shear failure modes to multiple shear failure modes with increasing damage. Then, the influence of the differential stress, confining pressure, and the degree of damage on the creep mechanical behavior of fractured specimens was further investigated. The axial instantaneous strain and creep strain increase linearly with increasing differential stress, and the specimens exhibit significant time-dependent behavior under high stress. The steady creep rate increases with increasing stress, but it decreases with increasing confining pressure and damage. However, the long-term strength and creep failure strength of fractured specimens increase linearly with increasing confining pressure, but they decrease linearly with increasing damage. The creep failure modes of fractured specimens are also the main shear failure modes, which are similar to the short-term failure modes.

Natural fractures at depth in the Lower Cretaceous Kuqa Depression tight sandstones: identification and characteristics

AbstractThe Kuqa Depression in the northern Tarim Basin, NW China, is characterized by fault-controlled anticlines where natural fractures may influence production. Natural fractures in the Lower Cretaceous tight sandstones in the depression have been studied using seismic profiles, borehole images, cores and thin-sections. Results show that thrust faults, two types of opening-mode macrofractures and two types of microfractures are present. Thrust faults were generated during Cenozoic N–S-directed tectonic shortening and have hydraulically linked Jurassic source rocks and Cretaceous sandstones. Opening-mode fractures can be subdivided on the basis of sizes, filling characteristics and distribution patterns. Type 1 macrofractures are barren or mainly calcite-lined. They have straight traces with widths (opening displacements) that are of the order of magnitude of 10 μm, suggesting that their primary role is that of migration channels. Type 2 macrofractures are calcite-filled opening-mode fractures. They have an elliptical or tabular shape with sharply tapering tips. Transgranular microfractures are lens-shaped and open or filled mostly by calcite; maximum widths range between 0.01 mm and 0.1 mm. Intragranular microfractures are the most common microfracture type. They are filled by calcite, feldspar or quartz. The macrofractures and transgranular microfractures have regular distributions, while most intragranular microfractures are irregularly distributed owing to their inherited origin. The results imply that natural fractures in the tight sandstones were formed as tectonic, diagenetic and natural hydraulic origins. In situ stress and cementation analyses suggest that Type 1 macrofractures and their genesis-related microfractures have controlled the present flow system of the tight sandstones.

Experimental study on characteristics of surface potential and current induced by stress on coal mine sandstone roof

Rocks generate voltage and current when subjected to load. Hence, the study of their characteristics is critical to the prevention and management of mine dynamic disasters and their secondary impacts. To investigate the characteristics of surface potential and current induced by stress on coal mine sandstone roof, an experimental scheme for determining the mechanical–electrical characteristics of rocks is constructed. The aforementioned system tests the evolution rules of current and voltage generated by the coal mine sandstone roof at different loading rates; the formation mechanism and coupling rules with mechanical properties of sandstone are also analysed. Experimental results show that although loading rates may differ, the variations of pressure stimulated current (PSC) and pressure stimulated voltage (PSV) curves are similar. Throughout the loading process, the PSC is divided into three stages: rapid growth, slow growth, and peak; on the other hand, the PSV curve exhibits a ‘double peak’ characteristic. The loading rate changes the current and voltage growth rates and peak values. When combined with mechanical properties, it can be observed that the PSC is related to the effective elastic modulus, whereas the PSV is related to micro-crack and macro-crack events in rocks. The current and voltage generated by the coal mine sandstone roof subjected to load are consistent with stress. The mechanical–electrical properties of sandstone indicate that the change processes in the PSC and PSV result from the coupling effects of various mechanisms. In the compaction stage, these are mainly caused by the piezoelectric effect of sandstone. In the elastic–plastic deformation stage, they are primarily generated by the electric dipoles formed by micro-cracks. In the sandstone fracture deformation stage, the change processes are mainly caused by the crack tip discharge. This study provide an experimental basis and practical results that reflect the microscale process and electrical precursors of sandstone fracturing.

The effect of chemical erosion on mechanical properties and fracture of sandstone under shear loading: an experimental study

In order to study the effect of water-rock interactions on shear strength characteristics, we performed shearing tests under varying hydrochemical environments. Moreover, a custom meso-shear test equipment for coal rock was used for the tests. Through 3D scanning of the shear fractures and scanning electron microscope imaging, we studied the effect of different pH chemical solutions on the shear strength and fracture characteristics of sandstones. We obtained three main results. With increasing solution acidity or alkalinity, water-hemical solution corrosivity increases. Moreover, the shear strength of sandstones reduces almost linearly and the fracture surfaces become smoother. The erosive effect is evidenced by the decrease in fracture surface fluctuations, roughness and the high-order microbulges, and scaling of the grain structure. A collection of characteristic parameters, including the maximum height Sh, the root mean square deviation Sq, the area ratio SA, and the slope root mean square S∆q, can be used to quantitatively describe the rough and irregular texture of the fracture surface.

Influence of surface roughness on fluid flow and solute transport through 3D crossed rock fractures

The influence of surface roughness on fluid flow and solute transport through three-dimensional (3D) crossed rock fractures are investigated by numerical simulations. Three crossed fracture models with different degrees of surface roughness are established by two intersecting rough-walled fractures with four branches. The fracture surface morphological data are measured from three natural fractures in sandstone and granite rock samples. The fluid flow is simulated by solving the Navier-Stokes equations and solute transport is simulated by solving the advective-diffusion equation. By rotating one fracture plane while fixing the other, series of intersection models with different angles between the two crossed fractures are established to investigate the influence of the intersecting angle. Simulation results of the rough-walled fractures are compared with the smooth parallel-plate model, showing that the surface roughness significantly enhances channeling and mixing for fluid flow and solute transport at fracture intersections. The mechanism is that the complex geometry of the intersection for rough-walled models results in reallocation of fluid pathways at the intersection, which consequently affect the mixing behavior depending on the Peclet number. The intersecting angle affects the channeling and mixing behavior because it influences the geometrical structure of the fracture intersection. The correlation between the mixing ratio and the geometrical characteristics of intersections is quantified by a relative roughness parameter. The results reveal that the widely adopted smooth parallel-plate model may lead to significant uncertainty in predicting the solute transport in crossed fractures, especially at intersections with unmated fracture surfaces. The correlation between the mixing ratio and the roughness parameter developed in this study can be incorporated into discrete fracture network models to improve their performance in estimating solute transport in fractured rocks.

Experimental study on repair of fractured rock mass by microbial induction technology.

The surrounding rock mass is often required to have good strength and impermeability in underground engineering. Some grouting methods, such as the chemical grouting method and the cement grouting pressure pump method, are often applied to reform underground environment and improve the engineering performance of rock mass. However, the application of some traditional grouting materials would destroy the original environment in which the project is located. This paper focuses on the repairing effect of Bacillus pasteurii composites in fractured rocks. The repairing effect of microbial materials on fractured sandstone is analysed through nuclear magnetic resonance (NMR) and unconfined compression-shearing equipment. The result shows that the longer the repairing time is, the better the effect will be. After 42 days of repairing, the porosity of fractured sandstone decreases by 36.41%, the impermeability increases by 94.62%, and the compressive strength increases by 30.52%. Through the study of reaction mechanisms, this technology has the advantages of mild reaction conditions, no pollution and good environmental compatibility. The application of this technology to the maintenance of geotechnical engineering can provide new ideas for the research and development of new environmental grouting materials and underground space reconstruction technology.

Dolomitization of shallow-water, mixed silicilastic-carbonate sequences: The Lower Triassic ramp succession of the Transdanubian Range, Hungary

The Lower Triassic succession of the Transdanubian Range in Hungary comprises limestones, dolomite, marl, sandstones and siltstones, deposited in tidal flat, lagoon and ooid shoal environments on the marginal ramp of the western Neotethys. Seven cores were chosen for petrographic and stable isotope investigations aiming to reconstruct the paragenetic sequence with special regard to the dolomitization and hydrothermal events. Five lithotypes were differentiated: (i) dolomite, (ii) sandy, silty, clayey dolomite, and dolomarl, (iii), dolomitic siltstone and sandstone, (iv) dolomitic limestone, and (v) limestone. In these lithotypes, three types of dolomites are present: non-ferroan replacive, ferroan replacive, and ferroan cement. Fabric retentive and fabric destructive non-ferroan replacive dolomitization are interpreted to have occurred by seepage reflux. Supporting evidence includes the presence gypsum and anhydrite in the Lower Triassic beds. Stable isotope values of the ferroan dolomite (δ18O of −10.7 to −4.2‰ and δ13C of −4.8 to 4.7‰) suggest dolomitization by fluids of relatively high temperature. The similar stable isotope values (δ18O of −9.3 to −5.8‰ and δ13C of −1.9 to 2.5‰) of the non-ferroan dolomite phase suggest that the reflux dolomite was overprinted by this second dolomitization event. Traces of exotic minerals, such as barite, chalcopyrite, galena and sulphosalts were found as fillings of vugs and fractures in the dolomite-cemented sandstone. The metals could have been sourced from the underlying Permian red sandstone beds. The heterogeneous sediment composition had profound impact on the diagenesis of these sedimentary successions.

Natural fractures in tight-oil sandstones: A case study of the Upper Triassic Yanchang Formation in the southwestern Ordos Basin, China

Natural fractures are important storage spaces and fluid-flow channels in tight-oil sandstones. Intraformational open fractures are the major channels for fluid flow in tight-oil sandstones. Small faults may provide fluid-flow channels across different layers. According to analogous outcrops, cores, and borehole image logs, small faults and intraformational open fractures are developed in the tight-oil sandstones of the Upper Triassic Yanchang Formation in the southwestern Ordos Basin, China. Among them, high dip-angle intraformational open fractures are the most abundant. Northeast-southwest–trending fractures are the principal fractures for fluid flow because that is the present-day maximum horizontal compressive stress direction. Combined with production data, horizontal wells, striking normal to or at a large angle relative to the major flow pathways, are beneficial for tight-oil production improvement. Fractures with high dip angles are the main factor that influences initial oil production. Linkage and tip damage zones are more favorable for oil production improvement than wall damage zones. This study provides an example of natural fracture characterization and unravels fracture contributions to reservoir physical properties and oil production of tight-oil sandstones, which could provide a geological basis for oil exploration and development in tight sandstones.

The evolution of sandstone microstructure and mechanical properties with thermal damage

AbstractThe physical and mechanical properties of rocks at high temperatures change considerably with geothermal exploitation, underground coal gasification, and nuclear engineering construction, posing a threat to the safety of underground engineering. To investigate the effect of temperature on micro‐ and macroscale damage of sandstone, a series of uniaxial compressive strength (UCS) tests were conducted using an MTS 815 mechanical testing system. Acoustic emission (AE) monitoring, scanning electron microscopy (SEM), and nuclear magnetic resonance (NMR) were also employed. Macroscopically, it was found that the physical and mechanical properties of sandstone change with treatment temperature, but these changes do not follow a monotonic trend. In addition, the brittle‐ductile transition occurs at approximately 600°C, which is further confirmed by AE monitoring. Regarding the microstructural evolution of sandstone, the percentage of micropores shows a monotonically decreasing trend with increasing treatment temperature. The change in mesopores decreases slightly first, then shows a gradual increase, and finally decreases. The macropores first decrease and subsequently increase with increasing temperature. The decreasing trend of the meso‐ and macropores is attributed to thermal expansion at a relatively low temperature. However, the decrease in mesopores is due to their coalescence into macropores at higher temperatures. Furthermore, the integral value of the NMR spectrum first decreases and then increases with increasing treatment temperature, corresponding to the decrease in porosity from 25°C to 200°C, and then increases with temperature to 900°C. Finally, a constitutive model for the deformation and fracture of sandstone is established based on the effective medium theory and AE energy. The present study is helpful for improving the understanding of the process of thermal damage sandstone from both micro‐ and macroscale perspectives.

An Investigation of CO2-Responsive Preformed Particle Gel for Conformance Control of CO2 Flooding in Reservoirs With Fractures or Fracture-Like Channels

Summary Gel treatment is an important technique to solve early CO2 breakthrough and excess–CO2–production problems, caused by the low viscosity and low density of CO2, as well as the heterogeneity of reservoirs with fractures or fracture–like channels. However, there is no reported work on gel that increases its volume after reacting with CO2 (termed CO2–responsive gel) for the conformance control of CO2 flooding. In this paper, the intrinsic properties of a CO2–responsive preformed particle gel (CR–PPG) were evaluated in a water/supercritical–CO2 (scCO2) environment in high–pressure vessels. Continuous scCO2 injection and CR–PPG treatment were conducted in fractured sandstone cores, to probe their plugging performance to scCO2 flow in a high–permeability–contrast system. The volumetric swelling ratio (VSR) of the CR–PPG increased by approximately two times in the presence of scCO2, compared with a sample under similar conditions in the absence of scCO2. The CR–PPG swelling ratio decreased with increasing NaCl concentration. Under the same conditions, the temperature did not have an apparent effect on the swelling ratio after 31 days of swelling. In coreflooding experiments, the placed CR–PPG resisted a considerable pressure up to 617.0 psi before breakthrough. After a shut–in process, CO2–breakthrough pressure was detected at 437.2 psi. It is observed that the shut–in process improved the plugging performance of CR–PPG to CO2 as revealed by the increase in the residual resistance factor. Controlling the shut–in time was found to be effective in augmenting the increase in the residual resistance factor, by increasing the VSR of placed CR–PPG. Resistance of CR–PPG to some real field challenges, including a high pressure gradient and long–term exposure to CO2, was also reported for field–applicability concerns.

Seismic Waves Could Decrease the Permeability of the Shallow Crust

AbstractSeveral experiments and field observations have demonstrated seismic waves‐induced permeability increases. However, a few experiments have revealed significant decreases in the permeability of fractured sandstone after shaking produced by dynamic stresses. In this paper, we analyze the tidal responses of the water level in two wells (Junan and Dingyuan) situated on and near, respectively, the Tan‐Lu fault in China. The results sampled by a single well indicate that seismic waves could both increase and decrease the permeability. In addition, our results indicate that the earthquakes that decreased the permeability in a single well were distributed in certain azimuthal zones. Given the azimuthal distribution of distant earthquakes, the observed permeability decrease could be attributed to the seismic wave‐induced clogging of fractures that compose the flow paths in the shallow crust. Our research contributes to an enhanced overall understanding of the effects of earthquakes on the permeability of the shallow crust.

Shear failure modes and AE characteristics of sandstone and marble fractures

Abstract To study the shear failure modes of rock fractures with different morphologies, rock types and test conditions, direct shear tests were conducted with the aid of an acoustic emission test system to obtain the characteristics of acoustic emissions in the shear process. The test results indicated that sandstone fractures experienced brittle shear failure under higher constant normal loads (15-30 kN) and plastic shear failure under lower normal loads (5 kN and 10 kN), while the marble fracture experienced plastic shear failure under each normal load. Dropping of shear stress, sharp change from shear dilatation to shear shrinkage and sudden release of high AE energy could be found when brittle shear failure occurred in sandstone fracture under a higher normal load. Thus, combining the location evolution characteristics of acoustic emission, it can be concluded that the entire shear dislocation of sandstone fracture in brittle shear failure mode happened just after the peak under higher normal loads. However, the entire shear dislocation of sandstone fracture under lower normal loads and marble fracture under all normal loads occurred just at the beginning of the shear process.