Fractured Sandstone Research Articles

Mechanism study of preventing crack propagation of fractured rockunder dynamic loads

To deeply understanding dynamic fracture properties and preventing crack propagation of fractured rock mass under dynamic loads, impact experiments were conducted using TWSRC (tunnel with single radial crack) samples, and sandstone were selected as the raw material to manufacture fractured rock samples. The crack initiation, propagation and obstructing behavior were measured by using a drop hammer impact test device and crack propagation gauge measuring system. The mechanism of preventing crack propagation and failure behavior during dynamic fracturing process was focused, and then the corresponding numerical simulation was conducted by using the finite difference code, which can be used to accurately estimate the experiment result. The results indicate that the whole dynamic fracturing process of fractured rock under dynamic loads is composed of the cyclic process of crack initiation, high-speed crack propagation, slowly deceleration, preventing crack propagation. In addition, the period of crack obstruction was approximate the microsecond level. The ratio of transgranular (TG) fracture at the crack obstruction point of fractured rock was smaller than that of the crack initiation point, and the ratio of TG fracture of green sandstone during the dynamic fracturing process was larger than that of black sandstone. The fracture energy for crack initiation again after crack obstruction was much less than the fracture energy required for the initial initiation of pre-existing crack.

Research and application of profile control and water shutoff technology in fractured sandstone reservoir based on system analysis method

Huoshaoshan Oilfield is a sandstone reservoir with severe heterogeneity. After years of enhanced water flooding, the reservoir conditions have changed greatly. Based on the research results of fine reservoir description, the key to successful industrial application of large-scale water shutoff and profile control is to select suitable water shutoff and profile control agents and improve injection technology and evaluation technology. This technology has become a leading technology for improving water flooding recovery in the development of extra-high water cut stage in Huoshaoshan Oilfield, with remarkable economic benefits. In this paper, the main factors affecting the performance of pre-gel system and the flow experiment used to test its performance index are described by systematic analysis method, and the construction effect of well group H1194 is briefly introduced.

Dynamic Splitting Behavior and the Constitutive Relationship of Frozen Sandstone Containing a Single Fissure

Fractured sandstone is widely distributed in mining areas throughout western China where the artificial freezing method is extensively adopted to construct vertical shafts. Blasting and excavation generate stress waves and break frozen fractured sandstone. Among the failure modes of frozen fractured rocks, tensile failure is very common. In this study, the dynamic tensile strength of fractured sandstone samples with four crack inclination angles (0°, 30°, 60°, and 90°) is tested by using a split Hopkinson pressure bar at four subzero temperatures (−5, −10, −15, and −20°C). Accordingly, a damage constitutive relationship that considers the effect of fissure angle and freezing temperature is established. The results show the following: (1) the fissure angle does not significantly affect the dynamic tensile strength of frozen fractured sandstone but mainly affects the failure mode of the sample. (2) The dynamic tensile strength of fractured sandstone has a negative linear correlation with the freezing temperature. (3) When the fissure angle is small, only tensile cracking occurs; when the fissure angle is large, tensile cracking occurs along both the loading direction and the fissure; and shear cracking occurs along the fissure as well. (4) Regardless of the fissure angle, tensile cracking is initiated at the stress-concentration zone and then propagates towards the loading end. Fissure ice provides both resistance to deformation and resistance to crack propagation which affects the crack propagation and coalescence mode. A dynamic constitutive relationship is established by considering the effects of fissure angle and freezing temperature on the dynamic properties of frozen fractured sandstone, which is proven to be highly reliable and provides a reference and basis to study the dynamic mechanical properties of similar rock types.

Stress-sensitive permeability of matrix cores and artificially fractured cores with nonproppant-filled fractures under high-pressure conditions

Stress-sensitive permeability (SSP) influences gas well productivity and is a crucial element influencing gas reservoir development. SSP for high-pressure fractured gas reservoirs with an initial reservoir pressure of more than 20 MPa has never been comprehensively evaluated to the best of our knowledge. SSP experiments with special procedures were designed by adopting the variable confining pressure (VCP) and variable internal pressure (VIP) methods. VCP is a test method in which the confining pressure is altered and a constant internal pressure is maintained for the experimental core holder. VIP is a test method in which the internal pressure is changed and a constant confining pressure is maintained. A four-stage curve analysis method was developed to perform regressions on semilogarithmic curves and exponential curves of experimental data. A method to evaluate the SSP was developed using stress sensitivity coefficients obtained via regressions. A calculation approach for determining the degrees of permeability damage and permeability recovery also was evaluated. In total, six matrix cores and six cores with artificial fractures from a high-pressure fractured sandstone gas reservoir were tested using the two methods. The SSP curves for high-pressure reservoirs were characterized by four-stage variation trends, which show differentiation with low-pressure reservoirs with an initial reservoir pressure less than 20 MPa. The stress sensitivity of the VCP method was stronger than that of the VIP method. The core samples mainly showed a “medium”/“medium-strong” stress sensitivity under low/high effective stress conditions. Compared with matrix cores, fractured cores showed stronger stress sensitivity owing to strong plasticity and weak elasticity. The maximum permeability damage degree reached 99.67%, and the minimum permeability recovery was only 6.9%. Our method of experimental design, four-stage curve analysis, stress sensitivity evaluation, and our overall findings can provide references for future studies on SSP in high-pressure fractured sandstone gas reservoirs.

Investigation on Creep Behavior and Permeability Evolution Characteristics of Sandstone under Different Pore Pressures

To investigate the influence of pore pressure ( σ w ) on the creep behavior and permeability of red sandstone, triaxial creep test with permeability test under different pore pressures was conducted using MTS 815 testing system. The experimental results demonstrate that water has significant weakening effect on the long-term mechanical properties of sandstone, and the long-term strength of sandstone gradually decreases with increase in pore pressures. All permeability-time curves demonstrate a “decreasing-increasing” trend, but two different permeability evolution trends during the steady creep stage are observed, which are related to deviatoric stress. The permeability of both intact ( k 0 ) and fractured sandstone ( k f ) samples increases with the increase in pore pressure, which are in consistent with the failure mode analysis of fractured sandstone samples. However, while the relationship between k 0 and σ w is positive linear, it is a positive exponential function relationship between k f and σ w .

Improved box-counting methods to directly estimate the fractal dimension of a rough surface

Rough surfaces exist widely in nature, and the measurement of these surfaces’ fractal dimension is beneficial for understanding their formation mechanism and effect. The basic principles of previous cubic covering methods are described, and then the stable cubic covering method (SCCM) and stable differential cubic covering method (SDCCM) are proposed based on the box-counting method. Afterward, a real sandstone fracture surface and Takagi surfaces are adopted to compare these methods’ differences. The results show that different measurement methods are needed to measure surfaces with different theoretical fractal dimensions. Meanwhile, the SCCM and SDCCM can obtain more stable measurement results than previous methods. Then, the recommended methods and their corresponding scale factors are summarized. Finally, the potential of these cubic covering methods to measure a thick film is described. The research is of great significance to the fractal dimension measurement of a rough surface or a thick film.

Experimental Research on Permeability of Coal and Rock Mass considering Postpeak Failure

In most mining areas of China, coal seams are characterized by low gas pressure, poor permeability, and high gas adsorption capacity, all of which have brought considerable difficulties to coal seam mining and coalbed methane (CBM) extraction. According to the multiyear scientific research and production practice of China, gas is migrated in quantity only after the coal body is directly mined, and the surrounding rocks deform and fracture under the mining influence. Thus, the key to effective control of gas migration and the coal and CBM comining technology lies in investigating the gas resolution, permeation, migration, and accumulation laws in the coal seams under the unloading confining pressure during mining. The MTS815.02 rock mechanics testing system and its supporting equipment are combined to test the permeability characteristics of coal and rock mass (postpeak fractured coal and sandstone specimens) under the loading and unloading of confining pressure using the steady method, and then, the permeation laws of the fractured coal and rock mass are obtained. Results show that after the postpeak rock crack propagation reaches a stable state, the confining pressure gradually increases, and the gas permeability presents an approximately linear reduction; in the postpeak unloading phase, the opening and coalescence degree of rock cracks gradually increase as the deformation extends. Thus, permeability reaches a peak value. The strain softening phase follows, where the cracks are closed and permeability declines to a certain extent. Moreover, the unloading step size of confining pressure has bearing on gas permeability. Specifically, as the unloading step size of confining pressure decreases, the change of gas permeability increases in stability.

Natural fractures in tight gas sandstones: a case study of the Upper Triassic Xujiahe Formation in Xinchang gas field, Western Sichuan Basin, China

AbstractThe Upper Triassic Xujiahe Formation is a typical tight gas reservoir in which natural fractures determine the migration, accumulation and production capacity of tight gas. In this study, we focused on the influences of natural fractures on the tight gas migration and production. We clarified characteristics and attributes (i.e. dips, apertures, filling degree and cross-cutting relationships) of the fractures based on image logging interpretations and core descriptions. Previous studies of electron spin resonance, carbon and oxygen isotopes, homogenization temperature of fluid inclusions analysis and basin simulation were considered. This study also analysed the fracture sequences, source of fracture fillings, diagenetic sequences and tight gas enrichment stages. We obtained insight into the relationship between fracture evolution and hydrocarbon charging, particularly the effect of the apertures and intensity of natural fractures on tight gas production. We reveal that the bedding fractures are short horizontal migration channels of tight gas. The tectonic fractures with middle, high and nearly vertical angles are beneficial to tight gas vertical migration. The apertures of fractures are controlled by the direction of maximum principal stress and fracture angle. The initial gas production of the vertical wells presents a positive correlation with the fracture abundance, and the intensity and aperture of fractures are the fundamental factors that determine the tight gas production. With these findings, this study is expected to guide the future exploration and development of tight gas with similar geological backgrounds.

Failure mechanism of fractured rock and associated acoustic behaviors under different loading rates

Mechanical and associated temporal-spatial microcracking behaviors of fractured sandstones, a hard and stiff rock, were tested and observed under different uniaxial loading rates with the aid of acoustic emission (AE) monitoring. Based on the principle of microcracking nucleation and multiple locked-patches failure theory borrowed from seismology, some experimental phenomena during crack propagation process, AE generated mechanisms as well as the static loading rate effects of loaded fractured rock were illustrated semiquantitatively. Results show that there are significant static loading rate effects on the mechanical, acoustic and microcracking process of fractured hard rock. With the loading rates increase, the dominant failure modes of fractured sandstones change from mode-II shear to mode-I tensile fracture. Acoustic signals gradually increase with the applied axial stress. Especially, in the meta-instable stage, AE counts show repeated “suddenly increasing then quiet” features which can be used for the early-warning of rock catastrophic failure. The failure of rock mass can be considered as the continuous brittle fracture of locked-patches inside. And each locked-patch rupture corresponds to a drop of stress and an extremum value of AE counts.

Fracture characterization in oil-based mud boreholes using image logs: example form tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area, Kuqa Depression, Tarim Basin, China

The natural fractures are dominated factors for the gas yield in deeply buried tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area, Kuqa Depression. For the high temperature and pressure environment, boreholes were drilled in oil-based mud. To characterize fractures in ultra-deep formation, electrical imagers (High-definition Formation MicroImager (FMI-HD), Earth Imager (EI), and Oil Base MicroImager (OBMI)) and acoustic imagers (Circumferential Borehole Imaging Log (CBIL) and Ultrasonic Borehole Imager (UBI)) were run in five oil-based mud openholes in KS5 well area. In this paper, fractures in different classifications were interpreted and compared in various borehole images. Opened and partially opened fractures were, respectively, continuous and discontinuous bright sinusoidal traces in electrical images. Closed fractures should be recognized by electrical and acoustic images cooperatively. In Bashijiqike Formation, opened and partially opened medium-angle, high-angle, and network fractures were well developed, while low-angle fractures were infrequent. Bashijiqike Formation might be divided into fractures well-developed, no-developed, and less-developed intervals upwards vertically. The strikes of fractures concentrated in nearly NWW-SEE, paralleled the strikes of major faults. This paper might help in understanding the complete vertical distributional characteristics of fractures in tight sandstones of Lower Cretaceous Bashijiqike Formation of KS5 well area and in turn on the exploration potential of petroleum systems.

Cyclic Drying–Wetting Effect on Shear Behaviors of Red Sandstone Fracture

Cyclic Drying–Wetting Effect on Shear Behaviors of Red Sandstone Fracture

Natural fracture opening preservation and reactivation in deep sandstones of the Kuqa foreland thrust belt, Tarim Basin

In deep to ultra-deep sandstones, natural open fracture patterns strongly influence the directions, magnitudes, and heterogeneities of fluid flow. We present an analysis of the natural fracture patterns, critically-stress state, and productivity of 35 wells in the Keshen and Dabei gas fields of the Kuqa foreland thrust belt. Our results show that reservoir performance is governed by the critical stress that acts on existing fractures to enhance their permeability. There is a good correspondence between the critically-stressed fractures and the natural open fractures in cores. These open fractures have patterns of barren cement, quartz bridges, and slickensides. Barren cement and quartz bridges indicate the preservation of permeable fractures, while the slickensides indicate shear reactivation. Barren fractures developed due to extension at the late stage of structural activities. Quartz bridge can prop up fracture walls to preserve porosity and permeability when compression. Shear reactivations consist of strike-slip and dip-slip, which broke the gypsum and calcite seal. Shear-reactivated fractures could reflect stress state transition from thrust regime to strike-slip regime during rapid burial. Combined with the analysis of fracture morphologies and kinematics, we indicate that open fractures development may relate to folding and fault reactivation. Besides, our case indicates that the E-W open fractures in the Keshen field and NE-SW open fractures in the Dabei field are not consistent with the direction of modern-day maximum horizontal compressive stress (σHmax). We conclude that the quartz bridge and shear reactivation govern the persistence of open fractures in tight and ultra-deep sandstones, even if the open fractures are perpendicular to σHmax.

Development mode of reverse fault‐associated fractures in deep tight sandstones: A case study in Xinchang Gas Field, Sichuan Basin, China

Evaluation of natural fractures is the key issue of the prediction of sweet spots in deep tight sandstones. In this article, taking the tight gas sandstone reservoir of the Xu 2 Member (second member of the Upper Triassic Xujiahe Formation) in Xinchang Gas Field, West Sichuan Depression as an example, we systematically studied the characteristics, formation periods, influencing factors, and developmental mode of fractures using testing results of cores, thin sections, rock mechanics, isotopes, fluid inclusions, and the structural curvature simulation method. The results show that the Xu 2 Member mainly develops vertical and high‐angle fractures. The fractures are highly effective and mainly developed in medium‐fine‐grained sandstone. The secondary minerals in the fractures have multi‐stage filling processes. The fluid inclusions are mainly gas–liquid two‐phase brine inclusions (secondary inclusions), and with the same crystal, the temperature difference between the brine inclusion and the organic inclusion is 23.7°C. Carbon and oxygen stable isotope tests show that calcite has two mixing stages. In the end, the formation periods of the fractures in the Xu 2 Member were divided into five stages. The influencing factors of fractures in the Xu 2 Member include lithology, fault, layer thickness, and structural deformation. According to geomechanical analysis, we finally established the development mode of reverse fault‐associated fractures in the Xu 2 Member.

Acoustic emission fractal characteristics of different prefabricated fractured sandstone under uniaxial compression tests

To study the strength characteristics, acoustic emission (AE) rules, and fractal characteristics of the single fractured sandstone under uniaxial compression tests, uniaxial compression strength tests were carried out on prefabricated single fractured sandstone samples with different angles and lengths. The results show that the maximum axial bearing capacity of the specimens decreases with the increase of the fracture length. The prefabricated fracture dip angle has a certain influence on the maximum axial bearing capacity of sandstone, but it is less than the influence of the fracture length on the bearing capacity of the sample. The existence of prefabricated fractures has a great influence on the correlation dimension of sandstone samples. When the prefabricated fracture is in the range of 31.6°–74.7°, the correlation dimension tends to decrease with the increase of the length of the prefabricated fracture. When the prefabricated fracture length l1/l of the sandstone sample is less than 0.26, the correlation dimension tends to increase first and then decrease with the change of the inclination angle of the prefabricated fracture. The experimental results can provide a reference for roadway support with strong joint and fracture development in the surrounding rock.

Study on the effect of water on electromagnetic radiation characteristics of fractured sandstone under load

A large number of electromagnetic radiation (EMR) signals are released during loading failure of rock mass, and these signals which contain complex and abundant fracture information can provides precursor of rock mass instability. In order to investigate the effect of water on the EMR characteristics of fractured sandstone, a uniaxial compression experiment was performed on the dried and saturated sandstone samples with prefabricated parallel double cracks. The EMR signals during loading failure of rock samples were acquired and their mechanical properties and EMR signal characteristics were analyzed. Besides, the precursor of rock sample instability was extracted based on the critical slowing down theory. The experimental results show that compared with the dried rock sample, the saturated rock sample has lower compressive strength, starts to crack earlier, and exhibits a more complicated failure mode. During the loading failure of rock samples, their EMR signals respond well to the fracturing and damage. The cumulative number of pulses of saturated rock sample grows uniformly, while that of dried rock sample show a “gentle-violent” rising mode. Moreover, the cumulative number of pulses of dried rock sample is greater than that of saturated rock sample. Water reduces the crack initiation stress level, and the saturated rock sample is the first to experience a sudden jump of EMR pulse. With the increase of the loading stress, the dominant frequency of EMR gradually transitions from low frequency to high frequency. Compared with dried rock samples, saturated rock samples have a lower proportion of high-frequency signals. Since the dominant mechanism of EMR varies with the change of failure stage, water first promotes the EMR signals of rock samples in the early stage of loading; then, it weakens the EMR signals in the middle and late stages of loading. Based on the critical slowing down theory, the EMR pulse signals were analyzed, which conduces to the extraction of potential precursor of rock sample failure and instability. Compared with dried rock samples, the precursory characteristic of saturated rock sample is more obvious. The research results provide some guidance for the stability of water-bearing fractured rock mass and the monitoring and early warning of related geological hazards.

Exploration advancement of the Stewardson Lake uranium project (Athabasca Basin, Canada) using an integrated geological and geochemical methodology and academic–industry collaboration

ABSTRACTKurt Kyser contributed significantly to understanding the role that fluids play in the formation of unconformity-related U deposits in Canada and Australia and also in the exploration for these deposits. Kurt's exploration research was collaborative with industry, and arguably the most productive of the many industry-collaborative relationships Kurt developed was with Uravan Minerals, a junior exploration company. Ten years of collaborative U-deposit and exploration geochemical research with Uravan, including development of innovative multi-media surface sampling and analytical methods, culminated with the Stewardson Lake uranium exploration project in the Proterozoic Athabasca Basin, Canada. Soil clay separates, pine and spruce tree cores and vegetation, and glacially transported sandstone boulders collected at Stewardson Lake record 207Pb/206Pb ratios as low as 0.18, indicative of radioactive decay of U in a Proterozoic-aged U deposit, and elevated pathfinder elements such as Ni and Co in two areas of the Stewardson Lake property named Areas A and B. Four diamond drill holes in Areas A and B tested targets consisting of anomalous surface-media geochemistry coincident with conductive host rocks determined using geophysical methods. The favorable targeting characteristics were not explained by two of the drill holes in Area A, where only narrow intervals of elevated U, radiogenic Pb, and pathfinder elements or structural disruption that can accompany Athabasca U deposits were intersected. Two drill holes in Area B intersected a broad zone of characteristic chlorite + hematite + illite alteration and U concentrations >1 ppm (aqua regia) in the basal host sandstone. One drill hole in Area B intersected a ∼30-meter-thick hydrothermal alteration zone consisting of silicification, chlorite + kaolinite clay alteration, smoky quartz, Ni-Fe sulfides, and disseminated uraninite accompanied by elevated gamma count rates. Fractures in the host sandstone in Area B record the highest pathfinder concentrations and the most radiogenic Pb isotope ratios and were likely a conduit for secondary dispersion of alteration-related components to the surface. The results validate the Uravan/QFIR exploration model developed over a decade of collaborative applied research and confirm that the Stewardson Lake area is highly prospective to host unconformity-related U mineralization. Lastly, clay alteration mineralogy in the Stewardson Lake area is contrasted with that in the eastern Athabasca Basin, and the value of the industry–academic relationship that benefited both Uravan Minerals and Kurt's Queen's Facility for Isotope Research laboratory is discussed.

Full-Sample X-ray Microcomputed Tomography Analysis of Supercritical CO2 Fracturing in Tight Sandstone: Effect of Stress on Fracture Dynamics

The commercial production from tight oil and gas reservoirs has been facilitated by the multistage hydraulic fracturing of horizontal wells. This process typically requires the pumping of large amo...

Research on the Characteristics of Microscopic Damage to Different Pore Types by Slippery Water Fracturing Fluid in the Chang Member Tight Sandstone Reservoir, Ordos Basin, NW China

This paper examines the damage caused by slickwater fracturing fluid to the microscopic pore structure of tight sandstone in the Chang 7 member in the Ordos Basin. A submicron CT in situ displacement system was used to analyze and graphically represent changes in pores in core samples following fracturing fluid damage. The results show the following: (1) the damage caused by slickwater fracturing fluid to tight sandstone fractures mainly occurs in the early stage of fluid incursion. The damage is characterized by a decrease in the effective pore volume, increase in the number of pores, and insignificant subsequent damage. The main causes of pore damage by slickwater fracturing fluid are retention of slickwater in the liquid phase and hydration swelling of clay minerals in the pores. (2) After the high pressure intrusion of slickwater fluid, the pore size of large-size intergranular pores increases, and there is no obvious damage after water flooding. However, fractures and small dissolution pores in the cores are the main areas of fluid retention after fracturing fluid invasion due to their small flowing radius and complex structure. These are the locations where damage mostly occurs.

Experimental Study on Seepage Properties of Postpeak Fractured Rocks under Cyclic Loading-Unloading Confining Stress and Axial Stress

Excavation in rock masses always encounters safety problems from rock fracture seepage in water-rich areas, which needs to be paid much attention, especially for fractured rocks under complicated stress state. For this reason, the permeability of fractured sandstone and granite is experimentally investigated under cyclic loading-unloading confining stress and axial stress. The variation of permeability coefficient and seepage flow with increasing and decreasing the confining stress and axial stress are comprehensively analyzed. Results show that the changing patterns of permeability with loading-unloading cycles of confining stress for both fractured sandstone and granite are similar. The permeability is most sensitive to the initial loading-unloading stages. After several loading-unloading cycles, the confining stress has little effect on permeability. The seepage flow decreases as the confining stress is unloaded to the same level in the loading process, indicating a hysteresis effect on the recovery of seepage capacity. The seepage properties under cyclic loading-unloading the axial stress are quite different from those under the confining stress. The permeability of fractured sandstone is most sensitive to the first cycle of loading-unloading of axial stress. The irrecoverable shear slide between fractures under the axial stress causes dilatancy or contraction, which makes the permeability coefficient to consecutively decrease at the subsequent cycles. The permeability of granite first decreases during the first loading of axial stress, while this trend is disordered at the subsequent stages no matter loading or unloading the axial stress. This is because of the accumulation of breakage fragments between fractures, which further disturbs the seepage flow. These findings may be useful for further understanding the seepage properties of fractured granite and sandstone under complex loading-unloading history.

Evolution of anisotropic permeability of fractured sandstones subjected to true-triaxial stresses during reservoir depletion

Reservoir rock masses contain numerous fractures caused by regional tectonic movements or hydraulic fracturing operations. An accurate understanding of the evolution of permeability of fractured reservoir rocks during depletion is thus crucial for evaluating the long-term performance of producing wells. Although extensive study on this issue has been conducted, the anisotropic permeability of fractured reservoir rock under in-situ true-triaxial stress conditions remains unclear. In this work, the anisotropic permeability behaviour of fractured sandstones in three principal stress directions is experimentally investigated. The tight gas reservoir sandstone is sampled from Tarim oilfield. An artificial fracture with different inclination angles is introduced into the cubic specimen. The true-triaxial stress path simulating the depletion process is first calculated using poroelastic theory and the initial geological stress data. During the true-triaxial testing, the permeability in three principal directions is simultaneously measured at each loading step. The intact sandstone specimen is also tested for comparison purpose. Experimental results show that directional permeability exponentially decreases with the increase of the effective stress and exhibits remarkable anisotropy along the principal stress axes. Moreover, the directional permeability for the fractured specimens is about one order of magnitude higher than that of intact rock at the same stress state, and the permeability variation obviously relies on the fracture geometry. Furthermore, the permeability along the fracture plane shows fluctuations as the effective stress increases, which is probably induced by the variation of the fracture aperture due to micro-slipping and the cracking of the matrix skeleton enclosing the fracture surface.