Triaxial Permeability Tests Research Articles

Development of high-performance fiber-reinforced concrete for drilling wellbore walls in highly mineralized strata and its sulfate attack resistanceattack resistance

Metakaolin has been incorporated into high-performance fiber-reinforced concrete for wellbore wall drilling to enhance its durability in strata with highly mineralized water. This study established a benchmark, utilizing fly ash, slag powder, and metakaolin as the factors in an orthogonal test to assess the durability of concrete against sulfate attack. The range analysis and an integrated balance method were employed to optimize the mix proportion, the optimized mix proportion of high-performance concrete was determined as concrete: cement: fly ash: slag powder: metakaolin: pumping agents: gravel: sand: water: polyvinyl alcohol = 1: 0.2: 0.075: 0.05: 0.106: 2.767: 1.556: 0.371: 0.003. The apparent and microscopic morphologies before and after the erosion of both the benchmark group and optimized mix proportion group were investigated. The triaxial permeability tests were conducted on these groups under varying confining pressures to elucidate concrete permeability trends. Additionally, a damage constitutive model for concrete under a sulfate attack was formulated based on the durability tests. This study could provide valuable insights into the industrial utilization of concrete in deep shafts within highly mineralized water strata in Northwestern China.

Experimental Study on the Effect of Freeze-Thaw Cycles on the Mechanical and Permeability Characteristics of Coal

The safe and efficient mining of coal seams with low porosity, low permeability, and high heterogeneity under complex geological conditions is a major challenge, with the permeability of coal seams playing a crucial role in coal mine gas extraction. The development of coal seam permeability enhancement technology can help coal mines produce safely and efficiently, while the extracted coal bed methane can be utilized as green energy. To study the effect of freezing and thawing on the evolution of the mechanical and permeability properties of coal, triaxial permeability tests were conducted on low-permeability coal under two different confining pressures. Simultaneously, dry, saturated, and freeze-thaw coal samples were set up for comparison, and the effects of water and freeze-thaw were isolated from each other. The triaxial mechanics and percolation laws of dry, saturated, and freeze-thaw coal rocks were obtained; the results show that saturated coal has the lowest initial permeability, while freeze-thawed coal has the highest initial permeability. Through analyzing the effects produced by water, freezing and thawing on coal specimens, the mechanism of the influence of freeze-thaw on the permeability evolution of coal was revealed. The research results can provide theoretical guidance for the development of gas extraction technology for low-permeability coal seams.

Internal erosion and permeability of Na CMC-treated and PAM-treated geosynthetic clay liners

This paper investigates the occurrence of internal erosion and the variation of the permeability of geosynthetic clay liner (GCL) with respect to its geotextile component, polymer type and polymer amount added to its bentonite component, hydraulic head, and subgrade material by performing extensive triaxial permeability tests on the GCL specimens. To do this, Na carboxymethyl cellulose (CMC) and polyacrylamide (PAM) were treated with the bentonite component of the GCLs. Then, the GCLs were placed over poorly graded gravel (GP) and poorly graded sand (SP). Consequently, they were tested with two different woven geotextile components under the hydraulics heads of 0.3 m and 10 m. Results showed that only GCLs tested with woven geotextile, having lower tensile strength, lower mass/area and lower thickness over the gravel at a hydraulic head of 10 m, experienced internal erosion. Critically, Na CMC and PAM treatment caused 2–2.5 orders of magnitude decrease in the permeability. Actually, 2% Na CMC and 1% PAM treatment by dry mass were found to be the optimum polymer contents. Both the environmentally friendly biopolymer Na CMC and the synthetic polymer PAM can effectively be treated with Ca bentonite to enhance the hydraulic performance of the GCLs.

Experimental investigation on micro-fracture evolution and fracture permeability of oil shale heated by water vapor

The study investigated the relationship between the temperature field distribution and fracture connectivity of oil shale heated by water vapor using infrared thermal imaging experiment. High temperature triaxial permeability testing system was then used to study the permeability of oil shale containing single penetrating fracture under the condition of water vapor heating. Finally, micro CT imaging and hot stage microscope imaging experiments were conducted to study the microstructure characteristics of oil shale at different temperatures. It was observed that when the oil shale was heated by water vapor, fractures became the main flow channels, and the connectivity of fractures affected the heating area of oil shale. The magnitude of fracture permeability of oil shale decreased from 10−15 m2 at 25 °C to 10−17 m2 at 350 °C. When the temperature was higher than 350 °C, the permeability of oil shale increased rapidly and recovered to the order of 10−15 m2 after 500 °C. The micro-fractures had little contribution to permeability in the temperature range of 25 °C–350 °C. The decrease in the permeability in this temperature range was caused by the closure of the single penetrating fracture. When the temperature reached 400 °C, some new penetrating fractures appeared. This resulted in the increase in permeability.

A Model of Stress-Damage-Permeability Relationship of Weakly Cemented Rocks under Triaxial Compressive Conditions.

To unravel the permeability variation mechanism of weakly cemented rocks (WCR), the paper conducted triaxial permeability tests on weakly cemented sandstones (WCS) collected from the Jurassic formation in northwest China. The paper identified the correlation of WCS permeability versus porosity, cementation structure, and mineral composition, further developing a model to characterize the WCS stress-damage-permeability relationship. The research indicated that the WCS permeability was initially high due to the naturally high porosity, large pore diameter, and loose particle cementation, thus favoring a significant decline as pore convergence in the compaction stage. In the residual stage, kaolinite and montmorillonite minerals disintegrated into water and narrowed fractures, causing a slight permeability increase from the initial to the maximum and residual stages. The WCS matrix fracturing was phenomenologically accompanied by clay mineral disintegration. By assuming that the matrix can be compressed, jointed, and fractured, the paper defined a damage variable D and accordingly developed a stress-damage-permeability relationship model that incorporated matrix compression, jointing, and fracturing. The model can describe the WCS permeability regime regarding the high initial permeability and slight difference of the maximum and residual permeabilities versus the initial.

Sodium Carboxymethyl Cellulose-Enhanced GCLs in Freshwater Reservoirs

Biopolymers that are environmentally friendly organic products are increasingly used in geotechnical applications instead of synthetic polymers and other chemical additives. Cellulose is a biopolymer that is obtained from the cell walls of plants. In order to evaluate its effect on internal erosion, triaxial permeability tests were performed on a sodium carboxymethyl cellulose-enhanced geosynthetic clay liner (GCL) with Ca bentonite that was placed over a coarse gravel with an average grain diameter of 2.5 cm and permeated with tap water. Furthermore, unconfined compression tests were conducted on the cellulose-added bentonite component of the GCL in order to evaluate the mechanical effect of sodium carboxymethyl cellulose addition to the GCL on internal erosion. Test results indicated that 2% cellulose addition by dry weight maintained sufficient hydraulic and mechanical performances with almost 2.5 orders of magnitude decrease in permeability and 10 times increase in unconfined compressive strength. However, cellulose addition had only a slight enhancement against internal erosion. All the tested GCLs resisted against internal erosion up to a hydraulic head of 25 m. But 30 m hydraulic head resulted in internal erosion for the GCLs enhanced with up to a content of 2% cellulose.

Study on Pyrolysis-Mechanics-Seepage Behavior of Oil Shale in a Closed System Subject to Real-Time Temperature Variations.

In situ mining is a practical and feasible technology for extracting oil shale. However, the extracted oil shale is subject to formation stress. This study systematically investigates the pyrolysis–mechanics–seepage problems of oil shale exploitation, which are subject to thermomechanical coupling using a thermal simulation experimental device representing a closed system, high-temperature rock mechanics testing system, and high-temperature triaxial permeability testing device. The results reveal the following. (i) The yield of gaseous hydrocarbon in the closed system increases throughout the pyrolysis reaction. Due to secondary cracking, the production of light and heavy hydrocarbon components first increases, and then decreases during the pyrolysis reaction. The parallel first-order reaction kinetic model shows a good fit with the pyrolysis and hydrocarbon generation processes of oil shale. With increasing temperature, the hydrocarbon generation conversion rate gradually increases, and the uniaxial compressive strength of oil shale was found to initially decrease and then increase. The compressive strength was the lowest at 400 °C, and the conversion rate of hydrocarbon formation gradually increased. The transformation of kaolinite into metakaolinite at high temperatures is the primary reason for the increase in compressive strength of oil shale at 400–600 °C. (ii) When the temperature is between 20 and 400 °C, the magnitude of oil shale permeability under stress is small (~10−2 md). When the temperature exceeds 400 °C, the permeability of the oil shale is large, and it decreases approximately linearly with increasing pore pressure, which is attributed to the joint action of the gas slippage effect, adsorption effect, and effective stress. The results of this research provide a basis for high efficiency in situ exploitation of oil shale.

Improving Mudstone Materials in Badland in Southwestern Taiwan by Increasing Density and Low-Cement Amount

The southwestern foothills in Taiwan contain abundant mudstone, which softens or slakes easily when contacting water. Thereafter, this condition causes high rates of erosion during heavy rain, eventually leading to the formation of badlands. To increase land use value in Taiwan, this study aimed to offer a new solution with the use of compaction techniques and also adding a small amount of cement as soil amendment. We examined the feasibility of this concept by performing a series of tests, including the basic physical property test, compaction test, unconfined compression test, static triaxial CU and UU tests, consolidation test, California bearing ratio (CBR) test, and triaxial permeability test. The specimens were created based on the controlled moisture at the wet side along the compaction curve. Three clods, three preparation methods, and two types of cement content were used for specimen preparation. The test results revealed that clod will affect the results of unconfined compression strength and soil permeability. For mudstone with added cement, its unconfined compression strength increased 7- to 10-fold and its shear strength increased 2- to 3-fold. The results of consolidation and the CBR test showed that improvement is possible by using low amounts of cement. Overall, the present method not only adheres to low-carbon and environmental protection requirements, but also verifies the feasibility of using compacted mudstone as an engineering material.

Evaluating the Hydraulic Conductivity of Dense Nonaqueous Phase Liquid in a Single Fracture of Rock-like Material

To investigate the seepage characteristics of dense nonaqueous phase liquids (DNAPLs) in rock fractures, two types of NAPLs (paint and creosote) were used in triaxial permeability tests conducted on single-fracture samples. The hydraulic conductivity of rock fractures with different apertures, confining pressures, and fluid properties was measured, and the influence of various physical factors on transmissivity was explored. The results demonstrated the following: (1) Fracture aperture and fluid viscosity are the main factors influencing transmissivity; (2) The widely used cubic law fails to effectively predict the transmissivity of high-viscosity liquids in a fracture, and the influence of liquid viscosity is considerably higher than that predicted by the cubic law; and (3) This study proposed a transmissivity prediction model of DNAPLs in a rock fracture based on multivariate regression analysis. The proposed model provides more accurate prediction results than those predicted by the cubic law, and is applicable to fracture apertures ranging from 5 × 10−4 to 2.5 × 10−3 m as well as to every kind of fluid used in this study.

Influence of Multistage Target Temperature and Cyclic Loading/Unloading on the Permeability of Polypropylene Fiber Concrete

Due to the combined effect of temperature and cyclic loading and unloading, the gas permeability of polypropylene fiber reinforced concrete structures changes during service. However, the current gas permeability test of polypropylene fiber reinforced concrete is based on a single influencing factor or a single test condition (monotonic loading), and the test conditions are quite different from the actual working conditions of the structure. To explore the permeability of polypropylene fiber reinforced concrete under cyclic loading and unloading under the influence of temperature, based on the stress principle that the specimen does not have structural damage and according to the steady‐state equation of Darcy’s law, the Cembureau method is adopted. The gas permeability of polypropylene fiber reinforced concrete under single loading and unloading and multistage cyclic loading and unloading at eight target temperatures is tested by the triaxial permeability test system. The results showed that (1) when the target temperature was 120°C < T ≤ 200°C and 200°C < T ≤ 280°C, the fiber experienced two stages of “softening, melting‐cooling recovery” and “melting and absorption,” which caused damage to the matrix pore structure. The gas permeability at 200°C and 280°C was 246 times and 350 times that at 22°C, respectively. (2) The damage degree of the matrix strength structure increases during cyclic loading and unloading, and the permeability loss rate during cyclic loading and unloading is 1.24∼1.57 times that of single loading and unloading. (3) The high target temperature leads to pore structure damage of the matrix, which not only affects the permeability of the matrix but also affects the strength structure of the matrix. When the stress ratio R ≥ 0.37, the pore structure damage and the strength structure damage of the specimen are superimposed, resulting in the antipermeability effect of the specimen developing in the unfavorable direction. The test simulated the actual working conditions of polypropylene fiber reinforced concrete, providing a reference for building fire protection, seismic design or postdisaster evaluation.

Pressure-dependent flow characteristics of proppant pack systems during well shut-in and the impact of fines invasion

Near-wellbore screenouts during the shut-in stage due to the development of compact proppant clusters is an issue reported in several fracturing treatments for Coal Seam Gas (CSG) reservoirs. The usual solutions to counter this issue implemented in deep rocks such as Shale and Siltstone do not solve for coal reservoirs due to its geological characteristics. Therefore, the development of proppant packs is an expected outcome during CSG extraction, and the permeability in the proppant pack could decide the overall gas productivity. This study investigates the permeability of proppant packs prepared using the Under-compaction method to simulate compact proppant clusters experienced in near-wellbore screenouts. In this regard, two commonly used proppant types are selected; sand and ceramic, and a series of tri-axial permeability tests were conducted under various effective stresses relevant to CSG environments. According to the experimental outcomes, interestingly, both proppant packs exhibited non-Darcy and Darcy flow regimes, while the non-Darcy effect was more profound at low confining pressures due to its high packing porosity. Results showed that the permeability through the compact proppant pack depends on both the reservoir pressure and the type of proppants selected. The sand proppant pack (SPP) showed 1–2 orders higher than the permeability values of the ceramic proppant pack (CPP) until a threshold pressure is reached (10 MPa), after which the ceramic proppants produced higher permeability values. We also investigate the potential impact of the invasion of coal fines into the proppant packs and the corresponding pore blockings. The impact of fines invasion on CPP permeability was more potent compared to the fines induced SPP permeability. The shape of the ceramic particles, the polar effect of the α-Al2O3 atoms, and the attraction of the oxygen functional groups of the coal fines to these polar sites increase the likelihood of fines clogging in a ceramic proppant pack.

Study on the Permeability Evolution Model of Mining-Disturbed Coal

Coal permeability plays an important role in the simultaneous exploitation of coal and coal-bed methane (CBM). The stress of mining-disturbed coal changes significantly during coal mining activities, causing damage and destruction of the coal mass, ultimately resulting in a sharp increase in permeability. Conventional triaxial compression and permeability tests were conducted on a triaxial creep-seepage-adsorption and desorption experimental device to investigate the permeability evolution of mining-disturbed coal. The permeability evolution models considering the influence of the stress state and stress path on the fracture propagation characteristics were established based on the permeability difference in the deformation stages of the coal mass. The stress-strain curve of the coal was divided into an elastic stage, yield stage, and plastic flow stage. As the axial stress increased, the permeability decreased and then increased, and the curve’s inflection point corresponded to the yield point. The permeability models exhibited a good agreement with the experimental data and accurately reflected the overall trends of the test results. The results of this study provide a theoretical basis for coal mine disaster prevention and the simultaneous exploitation of coal and CBM.

Sustainable reuse of coarse aggregates in clay-based impervious core: Compactability and permeability

The reuse of coarse aggregate (CA) on clay-based materials is a sustainable and cost effective solution to minimize the extraction of natural farmland resources and reinforce the mechanical behaviour of impervious materials. However, limited knowledge is available regarding the compactability and permeability of CA-clay mixtures and their intrinsic connections. This study reports the results of a series of modified dynamic compaction tests and triaxial permeability tests performed on impervious clay materials reinforced with various contents of CA. Results demonstrate that compaction behaviour of CA-clay mixtures changes significantly with coarse aggregate content (Cg). A peak value of global maximum dry density is achieved at Cg = 70%, while the fine clay matrix is compacted to its densest state when Cg<30%. The permeability, as a good indication of meso-structure in CA-clay mixtures, is significantly dominated by coarse aggregate content as well as the clay matrix void ratio. From the perspective of preferential flow path concept, permeability reaches a lowest value at Cg = 30%. Due to the clay bridge effect induced by the heterogeneous density field, the larger the coarse particle size is, the smaller the permeability coefficient will become.

Effect of Confining Pressure and Time on the Permeability of Saturated Remodeled Loess

The permeability of saturated remodeled loess is closely related to slope engineering and digging-filling engineering. In order to explore the permeability of saturated remodeled loess, a list of triaxial permeability test is conducted on compacted loess under different confining pressures. The results show that the permeability coefficient of saturated remodeled loess decreases with the increase of permeability time, and finally stabilizes. The variation of confining pressure in the range of low and high confining pressures greatly affects the permeability coefficient of loess. A model considering the relationship between permeability coefficient and confining pressure is given, and applied to predict the permeability coefficient of loess under different confining pressures.

Permeability and Microstructure of a Saline Intact Loess after Dry-Wet Cycles

Influenced by both dry-wet cycles and salt weathering, the loess will exhibit significant changes in microstructure and permeability, which threatens the stability of loess slopes. Triaxial permeability tests and industrial computed tomography (CT) scans were carried out on saline intact loess with sodium sulfate. The relationship between permeability and pore structure of the loess after dry-wet cycles was discussed. Results show that the permeability coefficient of loess increases after dry-wet cycles, with the increment declining. After specified dry-wet cycles, the permeability coefficient increases approximately linearly with sodium sulfate content. However, the permeability coefficient significantly declines at higher confining pressures, while its attenuation rate decreases. An empirical relationship based on log 10 1 + e − log 10 k was proposed to estimate the permeability coefficient of saline intact loess considering dry-wet cycles and salt content. Comparisons of measured and calculated results proved its rationality. CT scan images imply the damage to soil microstructure induced by dry-wet cycles and salt weathering, corresponding to the decline of the mean CT value (ME) and the increase of both crack ratio and fractal dimension of crack network.

Influence of freeze-thaw cycles on microstructure and hydraulic conductivity of saline intact loess

In this paper, influence of freeze-thaw (F-T) cycles on the microstructure and hydraulic conductivity were investigated on saline intact loess specimens by infiltrating sodium sulfate. X-Ray computed micro tomography (Micro-CT), and triaxial permeability tests were carried out to understand the relationship between the soil microstructure and hydraulic conductivity. The CT scan images suggest tiny holes in the soil structure expand to form cracks due to F-T cycles; these cracks were more pronounced in specimens with higher salt contents. The soil morphology was reconstructed using three-dimensional (3D) reconstruction technology, which highlights the significant influence of salt weathering on the microstructure of saline intact loess. Triaxial permeability test results suggest that the void ratio and hydraulic conductivity of saline intact loess both increase after F-T cycles or at higher sodium sulfate contents, which are consistent with the changes of the porosity of the 3D reconstructed model. Finally, an empirical relationship between hydraulic conductivity and void ratio was proposed incorporating the influence of F-T cycles and salt content. There is a good comparison between the calculated and measured data that establishes the rationality and validity of the proposed equation.

Triaxial Permeability Experimental Study on Deformation and Failure Processes of Single-Fractured Rock Specimens

A series of rock-like specimens with specific sizes and fracture inclinations was created in the laboratory. The different effects of seepage pressure on the deformation and failure characteristics between a conventional triaxial compression test and a triaxial permeability test were studied using a servo-controlled testing machine. Furthermore, the change in the permeability of single-fractured specimens was explored based on a triaxial permeability test. The results were as follows. Compared with those observed in the conventional triaxial compression test, the peak stress and corresponding axial strain decrease under seepage pressure in the triaxial permeability test, while the deformation modulus increased. With the increase of fracture length, the peak stress of specimen decreases due to the seepage pressure and the specimen showed tensile failure horizontally. The failure mode of the single-fractured specimens was changed by the seepage pressure. A closed relationship was observed between the failure modes and the permeability-stress curves. A shear failure along the crack surface will occur when the permeability abruptly changed later than the peak stress point. The dramatic change in the permeability indicated that the permeability channel was extended or new seepage paths were created. These conclusions can provide a valuable theoretical reference for the numerical simulation of excavation and design in stability analysis of jointed rock masses.

Permeability of Oil Shale Under In Situ Conditions: Fushun Oil Shale (China) Experimental Case Study

Oil shale is a critical strategic energy source with very large reserves, and it is acknowledged as an alternative energy source to crude oil. Oil shale in situ retorting is the only technologically feasible method to achieve large-scale industrial development. In situ permeability is one of the most critical technical parameters for successful implementation of oil shale in situ retorting. In this study, the permeability of Fushun oil shale at different temperatures was investigated using a triaxial permeability testing machine under in situ conditions. The results show that the permeability of the oil shale under three-dimensional stress can be divided clearly into three stages from 20 °C to 600 °C. First, the oil shale remains almost impermeable from 20 °C to 200 °C. Second, in the temperature range from 200 °C to 350 °C, the permeability of the oil shale first increased and then decreased to almost zero at 350 °C owing to the dual constraints of the surrounding stress and additional expansion stress under in situ conditions. Third, from 350 °C to 600 °C, the permeability of the oil shale increased sharply and reached a maximum of 100 × 10−5 μm2 at 600 °C. Thermal cracking and solid organic matter pyrolysis were the main factors controlling the change in oil shale permeability in the test temperature range. The results suggest that the threshold temperature for oil shale permeability change was 350 °C under in situ conditions. In addition, under in situ conditions, the oil shale permeability decreased with increasing pore pressure at different temperatures. This work has direct applications for oil shale in situ large-scale exploitation.

Analysis of Factors Affecting Permeability of Cement-bentonite Mud Impervious Wall

The permeability coefficient of the cement-bentonite mud anti-seepage wall is measured by triaxial permeability test (100kPa confining pressure 80kPa osmotic pressure), variable head permeability test and one-dimensional consolidation test (100kPa consolidation pressure). Under the conditions of different indoor mixing proportions, the effects of water-cement ratio, water bentonite ratio, curing age, and a small amount of silty clay soil on the permeability of the impervious wall were studied. The test results show that: (1) The average value of the ratio of the permeability coefficient of the variable head test and the triaxial permeability test is 16.75 and 16.73 at the age of 28d and 60d, respectively, and the coefficient of variation is 0.27 and 0.35, respectively; The average value of the ratio of the permeability coefficient of the one-dimensional consolidation test and the triaxial permeability test was 1.20 and 1.21 at the age of 28d and 60d, respectively, and the coefficients of variation were 0.24 and 0.17, respectively. (2) When the water-cement ratio is between 5.88 and 3.85 and the water-bentonite ratio is between 7.14 and 3.85, the permeability coefficient decreases significantly with the increase of age. The ratio of the permeability coefficients at 60d and 28d in age is not significant depending on the test method. (3) When the water-cement ratio and water-bentonite ratio in the mud are certain, the silty clay is blended, and the water-soil ratio decreases from 50 to 10, and the permeability coefficient decreases linearly.

Experimental Study on the Saturated Compacted Loess Permeability under K0 Consolidation

This paper investigates the permeability characteristics of compacted loess by focusing on the anisotropy parallel and perpendicular to the compaction. Three tests are conducted on compacted loess: triaxial permeability test under confining pressure consolidation, triaxial permeability test under K0 consolidation, and SEM test. Samples are maintained and tested at different dry densities under saturated conditions. The test results show that the saturated permeability coefficient of compacted loess is exponentially related to the initial dry density under both confining pressure consolidation and K0 consolidation. The fitting equation can estimate the saturated permeability coefficient of compacted loess at different depths. The horizontal saturated permeability coefficient of compacted loess is larger than that in the vertical direction, showing obvious anisotropy. The saturated permeability anisotropy ratio is linearly related to the initial dry density. Comparing and analysing the saturated permeability coefficient, the saturated permeability coefficient of compacted loess under the K0 consolidation condition is smaller than that under the confining pressure consolidation condition. Under the condition of K0 consolidation, the connectivity of vertical and horizontal pores of compacted loess is weakened, the tortuosity is strengthened, and the void ratio is decreased. K0 consolidation makes the flake‐, plate‐, and needle‐like particles in compacted loess rotate continuously parallel to the compaction surface, which enhances the orientation of particles and leads to the saturated permeability anisotropy increase. The research results provide the basis for water field analysis in loess filling engineering.