Fractured Rock Samples Research Articles

Characterization of Stages of CO2-Enhanced Oil Recovery Process in Low-Permeability Oil Reservoirs Based on Core Flooding Experiments

Understanding the CO2 displacement mechanism in ultra-low-permeability reservoirs is essential for improving oil recovery. In this research, a series of displacement experiments were conducted on sandstone core samples from the Chang 6 reservoir in the Huaziping area using a multifunctional core displacement apparatus and Nuclear Magnetic Resonance (NMR) technology. The experiments were designed under conditions of constant pressure, variable pressure, and constant effective confining stress to simulate various reservoir scenarios. The results indicated that the distribution characteristics of the pore structure in the rock samples significantly influenced the CO2 displacement efficiency. Specifically, under identical conditions, rock cores with a higher macropore ratio exhibited a significantly enhanced recovery rate, reaching 68.21%, which represents a maximum increase of 31.97% compared to cores with a lower macropore ratio. Though fractures can facilitate CO2 flowing through pores, the confining pressure applied during displacement caused a partial closure of fractures, resulting in reduced rock permeability. Based on the oil-to-gas ratio and oil recovery in the outlet section of the fractured rock samples, the CO2 displacement process exhibited five stages of no gas, a small amount of gas, gas breakthrough, large gas channeling, and gas fluctuation. Although the displacement stage of different cores varies, the breakthrough stage consistently occurs within the range of 2 PV. These insights not only enhance our understanding of CO2 displacement mechanisms in low-permeability reservoirs but also provide actionable data to inform the development of more effective CO2-EOR strategies, significantly impacting industrial practices.

Experimental study on the macroscopic and mesoscopic mechanical characteristics of deep damaged and fractured rock

Most of the rock surrounding deep roadways is in a fractured state; fractured rock has significant rheological properties, and the time-dependent mechanical properties of fractured rock affect excavation construction, support design, and long-term stability of deep roadways. Triaxial compression and mercury intrusion tests are conducted on the bearing characteristics of severely damaged and fractured rock samples, indicating the strength degradation properties of these samples. The evolution of the internal pore structure in damaged and fractured rock samples is analyzed in relation to changes in unloading points (pre-peak stage, peak point, and post-peak stage), leading to the establishment of a quantitative evaluation index for rock damage based on the porosity evolution. Short-term rheological testing is performed on rock samples with varying degrees of damage and fracture, demonstrating the evolution of creep and stress relaxation characteristics. The findings contribute to a deeper theoretical understanding of the post-peak mechanical properties of coal and rock masses, which hold significant theoretical implications and can inform research on long-term stability in underground engineering applications, such as deeply buried roadways, tunnels, and chambers.

Image-based learning and experimental verification of crack propagation in random multi-fractures rock

Fractures and the rock matrix are fundamental components of rock masses, with their random distribution being a common characteristic. Previous studies often focus on regularly fractured rock samples to facilitate experimental and numerical analysis. However, the limited number of samples used in these studies hinders a comprehensive understanding of the mechanical properties and failure characteristics of fractured rock masses. In this paper, we implement batch numerical simulations using PFC (Particle Flow Code) with an original automatic control code, resulting in a dataset of 400 numerical simulation results. The crack propagation characteristics and failure parameters of rock mass with random multi-fractures have been studied by using GANs (Generative Adversarial Networks) and other neural network models. Randomly fractured granite samples were subjected to uniaxial compression loadings, and the evolution of the strain field was analyzed by applying digital image processing technology. The testing results were then compared with the training model results. After verifying the model’s accuracy, the obtained CNN (Convolutional Neural Networks) model can be used to predict the UCS (Uniaxial Compressive Strength) of the real experimental samples. Additionally, we analyzed and discussed the influence of various parameters of random fractured rock mass on its bearing capacity.

Experimental and numerical study on the damage evolution and acoustic emission multi-parameter responses of single flaw sandstone under uniaxial compression

Cracks are generally the weakest position in engineering rock mass. It is of great practical significance to study the influence of cracks on the fracture process of rock mass and the precursor criterion of failure for disaster prevention and mitigation projects. In this paper, particle flow program (PFC2D) is used to analyze the mechanical strength and damage evolution of fractured rock. The multi-parameter stage characteristics of acoustic emission (AE) and the difference of fracture precursor information of fractured rock mass under uniaxial compression (UC) are explored. The results are as follows: (1) The strength and modulus of elasticity of fractured rock samples are smaller than that of intact rock mass, and increase in quadratic function with the increase of fracture angle, and the strength of B-90 sample is closest to intact rock samples; (2) The PFC analysis shows that the cracks in the defective rock samples all originate from the tip of the prefabricated crack, and the crack initiation angle is basically in the range of 60°∼90°. The loading process mainly produces tensile damage, and the number of tensile cracks are far more than that of shear cracks. Compressive stress and tensile stress increase with the increase of load, but the force chain of failure surface is the smallest after complete failure of rock; (3) The acoustic emission ringing number, RA/AF value, b value and entropy value of sandstone have obvious stage characteristics, the main frequency signal is mainly low frequency signal, and large-scale cracks are mainly produced in the failure process. The sudden increase point of AE ringing times and RA/AF value accumulation curve can be used as critical failure point. The fluctuation of B value and entropy value reflects the instability of fracture development, and “cliff-like” descending point and ascending point can be regarded as the precursor of critical failure of rock; (4) The failure precursor of acoustic emission parameters of sandstone appears in the peak load period of 85 %–98 %. The order of response time of different parameters is: cumulative RA/AF value, cumulative event number, entropy value, B value. The response time of B-45 and B-15 is earlier, while that of B-60 and B-30 is later. The research results are expected to deepen the understanding of the relevant standards of damage evolution and failure precursors of defective rocks in engineering applications.

Study on multi-scale damage and failure mechanism of rock fracture penetration: experimental and numerical analysis

In natural rock masses, joints or fractures usually occur at different penetrations. To study the deformation and failure behavior of rock mass with different fracture penetrations, a series of triaxial compression experiments were conducted on rock specimens containing prefabricated fracture at penetrations of 0%, 25%, 50%, 75% and 100%. The results show that the greater the fracture penetration and the smaller the strength of the rock sample, the more obvious the plastic deformation section of the stress–strain curve. Confining pressure can greatly improve the compressive strength of rock samples. For intact rock samples, confining pressure will change the failure mode from tensile failure to shear failure. For fractured rock samples, the confining pressure will aggravate the degree of damage to the rock sample. PFC 3D simulation shows that the failure process of rock samples is accompanied by the transformation of strain energy to damping energy and particle sliding energy, which can quantitatively characterise the damage process of rock. The increasing trend of crack number is slow-steep-slow, which is the ‘S’ type. It began to grow rapidly after reaching peak strength and gradually stabilised after reaching residual strength. It is consistent with the trend of energy change.

Effect of internal fractures on mechanical properties and failure of sandstone under multi-physical fields

Under deep mining conditions, rocks are subjected to complex multi-physical fields and can contain numerous pores and fractures. To explore the influence and correlation of these factors on the physical and mechanical properties of fractured rock samples, this study conducted triaxial compression tests on sandstone specimens under various physical conditions using a rock full stress multi-field coupling triaxial tester. Additionally, a random fracture model for multi-field coupling numerical simulation was established. This allowed the study to obtain the mechanical parameters, failure mode, and internal fracture development of rocks under multi-physical field conditions. By analyzing the complete stress-strain curve, mechanical characteristic points, and permeability, a combination of laboratory tests and numerical simulations was used to examine how temperature, seepage, and stress fields affect the development of pores and fractures in rocks. It was found that the temperature field, under conventional geothermal conditions, generates tensile force through thermal expansion and the presence of fluid, thereby promoting fracture development within the rocks. This mechanism is similar to that of seepage. The confining pressure caused by deep geo stress uniformly inhibits the expansion of pores and fissures within the rocks.

Research on stress sensitivity of fracture-hole carbonate reservoirs under drilling fluid immersion

Fracture-hole carbonate reservoirs feature the development of multi-scale fractures and dissolution holes. Invasion of drilling fluid into the reservoir and its prolonged interaction with the rock can readily induce a variety of damages, including stress-sensitive damage, consequently diminishing the productivity of oil and gas wells. Experiments were conducted on both dry/wet fractured rock samples and dry/wet fractured rock samples containing holes to investigate the mechanism of stress sensitivity in fracture-hole rocks subjected to fluid soaking. The results indicate that: (1) During the loading process, the highest damage rate to the permeability of fractured rock samples can reach up to 95%. The stress sensitivity coefficients for fracture-hole rock samples with half-hole depths of 2.5 mm, 5.0 mm, and 7.5 mm are 0.58, 0.41, and 0.25, respectively. The permeability damage rate of fracture-hole rock samples soaked with fluid is approximately 80%. (2) As the depth of the hole increases, the stress sensitivity coefficient decreases. The hole's depth is inversely proportional to the maximum stress, while being directly proportional to the maximum displacement of the rock sample. (3) Upon fluid soaking, pressure dissolution occurs within the fractured rock samples, shrinking the fracture width by 15.71–26.17%. (4) Following a 48-h immersion of the rock samples in an alkali solution, decalcification occurs and the pore structure is compromised, thus amplifying the degree of stress sensitivity.

Numerical Simulation of Turbulent Fluid Flow in Rough Rock Fracture: 3D Case

We investigate both laminar and turbulent flow regimes in a 3D rock fracture via numerical CFD simulations. We construct our realistic fracture model from 3D scan data of a fractured rock sample and implement changes in both shear displacement and contact ratio, examining their effect on fracture permeability and friction factor. While previous studies were investigating either fully viscous Darcy or inertial Forchheimer laminar flow regimes, we chose to cover the wide Reynolds number range of 0.1–106\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^6$$\\end{document}. We introduce CFD simulation of a turbulent flow for realistic 3D fractures, implementing the RANS approach to turbulence modeling. We focus on 3D fracture geometries and implement changes in both shear displacement and contact ratio, systematically examining their effect on fracture permeability and friction factor in a manner similar to the fundamental studies of the flow in rough-walled pipes. Growing Re leads to first stationary–non-stationary and then laminar–turbulent transitions. The presence of contact spots severely disrupts the flow pattern and adversely affects the overall permeability of the fracture. Regardless of shear displacement, ‘no contact’ 3D models can be reasonably approximated by the 2D profiles. Depending on the fracture geometry, Forchheimer β\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\beta$$\\end{document} fitting for both laminar and turbulent regimes can be performed either by a single or by two parameter values.

A novel calculation model, characteristics and applications of Archie's cementation exponent in dual porosity reservoirs with intersecting dual fractures

Fractured rock samples are difficult to use for ground conductive experiments, and there are few related simulation researches of Archie's cementation exponent in the reservoirs with intersecting dual fractures. In order to obtain the cementation index for saturation evaluation when no experiments and methods are available in such reservoirs, we focus on the researches of the cementation exponents in these reservoirs. Based on the Maxwell-Garnett mixing rule and the morphological characteristics of fractures in reservoirs, the method for obtaining the cementation exponents in reservoirs with parallel fractures at any inclination angle are first reviewed. secondly, the rock model of the dual porosity reservoirs with two intersecting fractures is constructed. The rock model is divided into four equal parts through equivalent processing, and the respective cementation exponents of the four parts can be calculated. Based on the series and parallel connection of Ohm's law, a novel calculation model of the overall cementation exponent of the rock model is established. Thirdly, using the novel model, the characteristics of cementation exponent of the rock model affected by different fracture dips, fracture porosities and included angles between the two fractures are calculated. In terms of detail and overall, the trend curves of these characteristics exhibit different nonlinear characteristics. The combined effect of the dip angles of the dual fractures, the included angle between them, the porosities of them and the porosity of matrix makes the cementation exponent characteristics more complex than that of single fracture or parallel fractures. Fourthly, the novel calculation model is used to evaluate the cementation exponent and fluid saturation of the two wells in the Chang 8 Formation in the southwest Ordos Basin, China, and the application results can demonstrate the reliability and availability of the novel model of cementation exponent in the reservoirs with intersecting dual fractures. At last, the application effect, influencing factors and applicable conditions of the novel calculation model of cementation exponent are discussed in detail. These applications and analyses show that it can provide an alternative solution for cementation exponent estimations in such fractured reservoirs, especially when experiments are not available.

Characteristics of Energy Dissipation in T-Shaped Fractured Rocks under Different Loading Rates

T-shaped fractured rocks in the engineering rock mass with different inclination angles, quantities, and cross patterns will cause slope landslides, cavern collapse, roof fall, and other disasters under the action of external forces. Deformation evolution of the T-shaped fractured rock is also significant for monitoring the stability of rock engineering structures. In this paper, the compression test of T-shaped fracture specimens was carried out under different loading rates. By modulating both the fracture inclination angle and the loading rate, the attributes pertaining to energy dissipation in the T-shaped fractured specimen were scrupulously scrutinized and subsequently expounded upon. The difference in the energy characteristics between fractured rock and intact rock was investigated to understand the deformation evolution of T-shaped fractured rock samples. The results show that when the fracture angle is 45° and 90°, the elastic strain energy and dissipated energy decrease as the secondary fracture angle increases. At the peak point, as the secondary fracture angle increases from 0°, the total absorbed energy, elastic strain energy, and dissipated energy of the T-shaped fractured rock increase, the ratio Ue/U of elastic strain energy to total energy increases, and the ratio Ud/U of dissipated energy to total energy decreases. The increase in loading rate leads to an increase in Ue/U and a decrease in Ud/U at the peak point of the T-shaped fractured rock specimen. The increase in loading rate leads to an increase in the total absorbed energy and elastic energy at the peak point of the T-shaped fractured rock, while the dissipated energy decreases. Investigative endeavors into the mechanics and energetic attributes of T-shaped fractured rocks bestow pragmatic and directive significance upon the safety assessment and stability prognostication of sundry geological undertakings.

Unloading-induced permeability recovery in rock fractures

Underground space creation and energy extraction, which induce unloading on rock fractures, commonly occur in various rock engineering projects, and rock engineering projects are subjected to high temperatures with increasing depth. Fluid flow behavior of rock fractures is a critical issue in many subsurface rock engineering projects. Previous studies have extensively considered permeability evolution in rock fractures under loading phase, whereas changes in fracture permeability under unloading phase have not been fully understood. To examine the unloading-induced changes in fracture permeability under different temperatures, we performed water flow-through tests on fractured rock samples subjected to decreasing confining pressures and different temperatures. The experimental results show that the permeability of fracture increases with unloading of confining pressure but decreases with loading-unloading cycles. Temperature may affect fracture permeability when it is higher than a certain threshold. An empirical model of fracture hydraulic aperture including two material parameters of initial normal stiffness and maximum normal closure can well describe the permeability changes in rough rock fracture subjected to loading-unloading cycles and heating. A coupled thermo-mechanical model considering asperity damage is finally used to understand the influences of stress paths and temperatures on fracture permeability.

Experimental study on the nonlinear flow characteristics of fractured granite after high-temperature cycling

To understand the influence of temperature on the flow characteristics of fractured granite, high-temperature cyclic thermal treatment and flow tests on the fractured rock sample with different joint roughness coefficients and intact rock samples were conducted. The larger confining pressure and larger joint roughness coefficient will increase the resistance of fluid flow and affect the flow characteristics of the fluid. With the temperature increasing, the aperture of the fractures, the number of micro-fractures, and micropores increase which forms a large number of new connected hydraulic channels in the matrix. Forchheimer's law and Izbash equation can well describe the nonlinear flow characteristics, and the fitting coefficients are greater than 0.99. As the increasing temperature, the slope of the curve between the volumetric flow rate and pressure gradient gradually decreases, and the coefficients in Forchheimer's law and the Izbash equation decrease. The transmissivity decrease with the increasing Reynolds number and the change range of that increase with the increasing temperature. When the temperature is at the lower level (T = 200 ~ 600 °C), the contribution of split fracture to the permeability is greater than that of the matrix. When the temperature continuously increases to 800 °C, the contribution of the matrix to the permeability gradually rises and then exceeds that of split fracture. The results indicate that 400 °C is the critical temperature, after which the flow characteristics of fractured granite after high-temperature cycling change more obviously.

Crack propagation and scale effect of random fractured rock under compression-shear loading

Fractures and rock blocks are the essential components of rock mass, random fractured rock is the general manifestation of fractured rock mass, which is particularly significant in scale effect. To facilitate the tests, most of the previous studies on fractured rock mass were conducted on regular fractured rocks, which is not conducive to fully revealing the mechanical properties and failure characteristics of fractured rock mass. In this paper, the random fractured rock samples were produced with rock-like materials to investigate the fracture propagation and scale effect under compression-shear condition. Specifically, the samples were loaded by controlling the compression-shear angle to study the mechanical responses. In the test, speckle changes on the samples surface were recorded in real time, and the evolution of strain field was analyzed by digital image processing technology. Then, the internal relations among compression-shear angle, fracture distribution and crack propagation were discussed. Meanwhile, the failure modes of the random fractured samples with different compression-shear angles were summarized to reveal the influence of compression-shear angle on failure mode. Combined with PFC2D, the random fractured rock models of different scales were established to explore the mechanical characteristic variation, and the representative elementary volume of shear modulus was finally determined.

Experimental study on mechanical properties of single fracture-hole red sandstone

Various fractures and holes in the natural rock mass affected the mechanical properties of the rock mass and the safety construction of engineering. In this study, we investigated the mechanical properties of a single fracture-hole rock specimen using particle flow code 2D (PFC2D) numerical simulation software and through laboratory tests. We analysed the failure behaviours and mechanical properties of the rock specimen with a single fracture-hole specimen under different fracture angles. The failure modes of single fractured rock samples with different fracture angles were revealed. The fracture propagation and stress evolution of the rock specimen with a single fracture-hole under different fracture angles were investigated. The experimental results shown that the peak strength, peak strain, elastic modulus, initial fracture stress, and damage stress of the single fracture-hole rock specimen with different fracture angles were significantly less than those of the intact rock specimen. Moreover, fracture hole defects accelerated the generation of fractures and promote the failure of the rock specimen. The failure modes were divided into Y, inverted Y, and V types. Before the rock specimen fractures, the stress concentration area was mainly distributed at both ends of the fracture. The stress concentration area at both ends of the fracture gradually decreased, and the stress concentration area near the hole gradually increased as the fracture angle increased. By experiments, the acoustic emission of the model had gone through three stages: initial, steady growth, and rapid decline. The size of the inclination angle affected the number of acoustic emission hits and the generation of acoustic emission signals. Failure behaviours of the rock specimen with a single fracture-hole were systematically investigated, which could promoted the development of fracture rock mechanics and improved the understanding of instability failure mechanism in rock engineering, such as nuclear wasted treatment engineering and deep underground engineering.

Mechanical properties and failure law of composite rock containing two coplanar fractures

Composite rocks comprise the rock structures that are commonly used in geotechnical engineering. The fracture configuration has a substantial influence on the mechanical behavior, failure mode, and crack propagation of composite rocks. In this study, we considered a composite rock with two prefabricated coplanar fractures. Through laboratory uniaxial compression tests and using a digital image acquisition system, we systematically studied the effects of different fracture lengths and inclination angles on the mechanical properties and failure characteristics of the rocks. We obtained the following results: 1) during the loading deformation of the rock sample, the peak stress and elastic modulus increased with an increase in the fracture inclination angle and decreased with an increase in the fracture length. The deterioration coefficient k (the ratio of the difference between the peak strength of intact and fractured rock sample to that of intact rock sample) decreased with an increase in the fracture inclination angle and increased with an increase in the fracture length. 2) The failure type of the rock samples was primarily controlled by the fracture inclination angle and material of the two rock types, and the fragmentation degree was primarily controlled by the fracture length. With an increase in the fracture inclination angle, the failure mode of rock sample exhibited the following order of changes leading to failure: a double-Y type (trwo wing and one antiwing cracks appeared on each prefabricated fracture) → double-Z type (two wing cracks appeared on each prefabricated fracture) → Z type (one wing crack appeared on each prefabricated fracture). 3) The type of coalescence of the rock bridge was controlled by the fracture inclination angle and structural plane. The crack positions were primarily affected by the fracture length. 4) At a low fracture inclination angle (α ≤ 30°), the propagation of the microcracks showed aggregated band formation. Above moderate fracture inclination angles (α > 30°), the microcrack aggregation band gradually weakened and expanded in the direction of dispersion.

Sorption and Spatial Distribution of 137Cs, 90Sr and 241Am on Mineral Phases of Fractured Rocks of Nizhnekansky Granitoid Massif

The sorption behavior and spatial microdistribution of Cs-137, Sr-90 and Am-241 onto the surface of a fractured rock sample from the R-11 borehole of the exocontact zone of the Nizhnekansky granitoid massif were studied. The sorption efficiency of the fractured sample increases in the row of Sr < Cs < Am, where americium is the most retained radionuclide. Based on the image processing of radiograms and scanning electron microscopy data, the mineral relative sorption efficiency (RSE) values were determined to quantify the contribution of the mineral phases of the fractured sample to radionuclide retention. It was established that cesium and strontium are predominantly retained in cracks filled with secondary mineral chlorite. Zeolite is a less effective sorbent with respect to cesium and strontium. Americium sorption is uniform over the whole surface of the fractured sample, with close RSE values for all mineral phases (RSE ~1). The behavior of cesium in heterogeneous and monomineral systems of crushed mineral phases of quartz, biotite and zeolite NaA imitating minerals of the fractured rock sample R-11 was determined. It was shown that the fraction of the sorbed cesium in a heterogeneous system of two mineral phases—biotite and quartz—was larger than the sum of sorption values for the same separated mineral phases. Based on the models of radionuclide sorption on illite–smectite minerals, it was able to estimate the depth of the penetration of solution into the fractured rock sample R-11. The variations of penetration depths for solutions of specific radionuclide into the fractured rock sample were established.

Study on the stress field concentration at the tip of elliptical cracks

Abstract The holes and defects in rock materials have a great impact on the mechanical properties and failure mechanism of rock, cracks often appear in the form of flat ellipses in natural rock mass, and the current research is still insufficient. For this purpose, based on the Particle Flow Code (PFC) of discrete element particle flow program, the numerical prefabricated fractured rock samples with the ratios of the long and short axes of the elliptical fractures being 5, 7.5, 10, 12.5, and 15 were simulated in order to obtain the strength and failure characteristics, stress concentration characteristics at tips of numerical rock samples in uniaxial compression test. The results of the numerical test simulation show that: (1) When the rock models with prefabricated elliptical crack were damaged, the initial cracks occurred at the end of the short axis of the elliptical crack, and penetrated up and down from the surface of the elliptical crack, the wing cracks occurred at both ends of the long axis, gradually formed a macro-crack, and the secondary cracks extended near the wing crack. (2) With the increase of the ratios a:b of long and short axes of the elliptical fracture, the strength and elastic modulus of the numerical rock samples gradually decreased, Poisson’s ratio gradually increased, and the total number of micro-cracks in the rock models decreased. (3) The numerical solutions of stress concentration factor k obtained by numerical simulations at the tip of the elliptical crack increased with the increase in the ratio of a:b; it was highly consistent with the variation law of the analytical solution of the stress concentration factor calculated by the theory of flat ellipse. The stress concentration is an important reason for failure of rock with elliptical cracks. Study on the crack tip will be very useful and significant.

Experimental evaluation of enhanced oil recovery in unconventional reservoirs using cyclic hydrocarbon gas injection

The significant amount of gas produced from unconventional reservoirs in the US in recent years has reduced market prices for hydrocarbon gas and created an opportunity for using low-cost gas in enhanced oil recovery (EOR) projects. In addition to its availability in large quantities and at low cost, injecting produced gas back into the reservoir prevents flaring the gas, reducing environmental concerns associated with greenhouse gas emissions to the atmosphere. Furthermore, while CO2 has shown tremendous success as an EOR injectant in huff-and-puff operations in tight liquid-rich shales, the lack of sufficient pipeline infrastructure supplying low-cost CO2 as well as operational issues (e.g., corrosion) has forced field operators to consider other gas options, such as produced gas, to substitute for or mix with CO2.While limited, experimental and numerical studies conducted on produced gas huff-and-puff in unconventional resources have shown promising results and highlighted the great potential of this technique for enhanced oil recovery from ultra-tight shale resources. The objective of the current work is to investigate the feasibility of produced gas huff-and-puff operations in one of the major oil-producing shale formations in North America further by performing several flow experiments using various mixtures of hydrocarbon gases. Three reservoir-condition core-flooding experiments are carefully designed and executed on shale core plugs in the absence and presence of proppant-filled fractures. After establishing initial two-phase brine-oil conditions in the core samples, hydrocarbon gas is injected into the medium through a huff-and-puff approach. The cycle of injection-soaking-production is repeated multiple times in each experiment. The results show that the use of hydrocarbon gas in huff-and-puff operations has great potential similar to CO2 for enhanced oil recovery from tight shale rocks and can lead to remarkable oil recovery. The hydrocarbon gas mixture in the fracture penetrates the matrix due to pressure and concentration gradients during the injection and soaking periods and forces the oil to leave the matrix during the depressurization process. It is observed that the rate of oil production is high in the first cycle and decreases in subsequent cycles as the rate of mass transfer and the amount of oil remaining in the matrix decrease. The average oil production due to the cyclic gas injection process in an intact and fractured composite rock sample was more than 48 and 21%, respectively. The results show that even though the ultratight formations could be heavily fractured, a proper cyclic gas injection EOR scheme could potentially improve oil recovery from the shale oil reservoirs, significantly.

Shear performance and reinforcement mechanism of MICP-treated single fractured sandstone

There are a large number of fractured rock masses in the Three Gorges Reservoir area. Traditional reinforcement methods have disadvantages such as large engineering investment, high material consumption, and poor ecological environmental protection. Therefore, it is necessary to develop new environmentally friendly materials and methods to strengthen and control them. The microbial-induced carbonate precipitation (MICP) technology that has emerged in recent years has the characteristics of low carbon and environmental protection and has great prospects in the restoration and reinforcement of rock and soil materials. Therefore, Bacillus cereus extracted in situ from the Three Gorges Reservoir area is proposed to be used for MICP reinforcement of single-fractured sandstone, and its reinforcement mechanism is revealed by studying the macroscopic impermeability and shear performance improvement of the fractured rock sample after reinforcement, and the microstructure changes. The results show that after 10 cycles of grouting reinforcement, the fracture surface of the rock sample is well sealed, the permeability coefficient is reduced by two orders of magnitude, the shear stress is increased by 26%–40%, and the shear stiffness is increased by 70%. The shear stress–shear displacement curve shows the peak shear strength, and the residual shear strength also increases to a certain extent. The MICP process improves the mechanical properties of fractured rock samples from three aspects, namely, the cementation between sand grains and the fracture surface, the cementation effect between sand grains, and the filling effect of fractured rock samples. The shear failure surface of the samples after reinforcement is the recheck interface between the cementation body and the cementation interface. The relevant research results can provide references for the MICP reinforcement technology of fractured rock mass.

Pore structure evolution in andesite rocks induced by freeze–thaw cycles examined by non-destructive methods

In this paper, we compare the values of petrophysical properties before and after 100 freeze–thaw (F–T) cycles, as well as recorded length change behaviour and temperature development on a vacuum-saturated fractured andesite rock sample taken from the Babina Quarry in Slovakia using a specially-constructed thermodilatometer, VLAP 04, equipped with two HIRT-LVDT sensors. We also used non-destructive visualization of the rock pore network by µCT imaging in order to study the development of the pore structure and fracture network in pyroxene andesites during the freeze–thaw process. The results show that the andesite rock samples, due to good fabric cohesion, low porosity, and low pore interconnection, showed good resistance against frost-induced damage. However, it must be stated that the main process causing disintegration of this type of rock is fracture opening, which is caused by internal stresses induced by water–ice phase transition. The overall residual strain recorded after 100 F–T cycles was not significant, however, the increase of 31 pp in volume of the fracture showed us that repeated freezing and thawing can lead to long term deterioration in terms of subcritical crack growth in brittle-elastic solids like pyroxene-andesite rocks.