Experimental Study on Pore Structure, Mechanical Behavior and Permeability Characteristics of Weakly Cemented Sandstone

Abstract

To investigate the seepage and mechanical behavior of the overlying strata during solution mining in salt deposits, porous sandstones with different grain sizes were selected for study. First, a series of microscopic tests, including SEM, MIP, and NMR, was conducted to characterize the pore structure of the rocks. Subsequently, using a servo-controlled triaxial rock testing system, permeability tests covering the complete stress–strain process were performed under different confining pressures and seepage pressures based on the steady-state method, in order to analyze the seepage and mechanical characteristics of the sandstones during deformation and failure. The results indicate that the investigated aquifer sandstones are characterized by weak cementation, high porosity, large pore size, good pore connectivity, and relatively high permeability. High confining pressure enhances the mechanical strength of the sandstone while reducing its permeability, whereas increasing seepage pressure decreases mechanical strength and enhances permeability during triaxial compression under pore water pressure conditions. Throughout the complete stress–strain process, the evolution of permeability is jointly controlled by the intrinsic pore structure of the rock, the stress loading path, and the failure mode. Under high confining pressure, localized compaction bands may develop, and the formation of such localized structures suppresses any increase in permeability. Acoustic emission shows good correlations with both the stress–strain response and permeability evolution. This study provides new insights into the pore structure of loose, highly permeable sandstones and their hydromechanical coupling behavior throughout the complete stress–strain process.

1. Introduction

A certain inland basin in China, characterized by lacustrine sedimentary deposits, has abundant salt mineral resources and is classified as a large-scale deposit. According to regional 3D seismic surveys and mineral drilling data, the stratigraphy of the salt basin, from top to bottom, consists of the Quaternary Dongtai Formation (Qd), Neogene Yancheng Formation (Ny), Paleogene Sanduo Formation (Es), Paleogene Dainan Formation (Ed), Paleogene Funing Formation (Ef), and Paleogene Taizhou Formation (E1t). The salt deposit mainly exhibits a layered distribution. This salt deposit is characterized by considerable thickness and high grade, with large-scale brine extraction and a long history of salt well mining.

Mining activities alter the original stress state of the roof and floor strata in the extraction interval [1], thereby changing the coupled stress–seepage conditions of the surrounding rock mass. The continuous deterioration of permeability and mechanical integrity may induce common water-related hazards, such as roof water inrush [2,3], and thus adversely affect both the surface environment and mine safety. Because the permeability of roof and floor rocks with different lithologies varies significantly during deformation, such changes are difficult to observe directly under field conditions. It is therefore necessary to conduct laboratory-based rock mechanical permeability tests to further investigate the mechanical behavior and permeability evolution of sandstone under coupled stress–seepage conditions. As one of the most common sedimentary rocks in underground engineering, sandstone has long attracted extensive attention with respect to its mechanical properties.

Pores provide both the storage space for fluid occurrence and the pathways for seepage migration. The quantitative characterization of rock pore structures is therefore essential for evaluating permeability and understanding the mechanical behavior of rocks under stress–seepage coupling. At present, many methods are available for investigating rock pore structures, including scanning electron microscopy, cast thin sections, X-ray diffraction, conventional and constant-rate mercury intrusion porosimetry, nuclear magnetic resonance, CT scanning, and porosity–permeability testing. Zhu et al. [4] analyzed the microscopic pore structure characteristics of coal-bearing sandstones, including pore shape, size, and distribution, using cast thin sections, scanning electron microscopy, and mercury intrusion tests. SEM and CT images have also been widely used for the quantitative analysis of the pore structure and permeability characteristics of sandstones [5]. Combined with XRD, Rietveld refinement, Avizo, and COMSOL, these techniques have further been used to identify pore and pore–throat types, pore size distribution ranges, and connected porosity in sandstones overlying coal seams [6].

Many scholars have investigated the influence of pore water pressure on rock deformation, strength, and failure modes through laboratory tests such as triaxial compression. Zong et al. analyzed deformation and strength characteristics under different stress states and found that peak strength, elastic modulus, and other deformation and strength parameters increase linearly with confining pressure. The failure mode also changes from brittle failure under low confinement to ductile failure under high confinement, accompanied by pronounced volumetric expansion characteristics [7,8]. When confining pressure remains constant, increases in pore water pressure gradually reduce the peak strength, elastic modulus, and stiffness of the rock [9,10]. Deep rock masses are subjected over long periods to the coupled action of the in situ stress field and seepage field, making their mechanical response and permeability behavior highly complex. Disturbance caused by excavation induces dynamic fracture development as a result of rock failure under stress concentration, and the permeability of the overlying strata correspondingly evolves during stress redistribution. Numerous studies have experimentally investigated permeability evolution during rock deformation and failure from different perspectives, yielding abundant results. However, the permeability evolution patterns of rocks vary considerably with lithology. Under triaxial stress, impurity-bearing rock salt exhibits a two-stage permeability response [11], characterized by a rapid increase associated with early damage and microcrack initiation, followed by permeability saturation once the dilatant volumetric strain exceeds approximately 1~2%. Liu et al. [12] investigated the deformation behavior and damage-induced permeability evolution of sandy mudstone under triaxial stress and found that the decrease in permeability was related to the restricted flow of clay minerals within seepage channels during the yield stage and after the formation of large fractures post-peak. Zhao et al. [13] examined the mechanical behavior, permeability, and failure mode of limestone under stress–seepage coupling and demonstrated the relationship between permeability and effective confining pressure; the stress–strain evolution experienced the crack compaction and closure stage, linear elastic deformation stage, stable crack propagation stage, unstable crack propagation stage, and post-peak stage. Rong et al. [14] quantitatively analyzed the relationship between damage, volumetric strain, and permeability evolution in sandstone under compression and proposed a new model for investigating deformation, damage, permeability behavior, and their coupling effects. Under triaxial cyclic loading, the stress–strain behavior, crack development, and permeability of granite evolve through different stages, including crack closure, initiation, propagation, and coalescence [15]. Under short-term triaxial compression, the dynamic evolution of permeability during sandstone deformation and failure can be linked to the complete stress–strain process. This evolution is jointly controlled by multiple factors, including pore structure, confining pressure, seepage pressure, stress loading path, and failure mode [16,17,18,19]. In laboratory studies on permeability evolution under different stress paths, acoustic emission (AE) monitoring not only reveals more clearly the initiation and propagation of cracks during rock deformation and failure [9,20] but also shows strong correlations with both stress–strain behavior and permeability evolution [7,21,22]. However, these studies mainly focus on dense sandstones, mudstones, and granites that are relatively insensitive to changes in the hydraulic environment, easy to prepare, and associated with high testing success rates, whereas weakly cemented, high-porosity, and highly permeable sandstones have received much less attention. Unlike dense brittle rocks, such sandstones generally possess large primary pores, good pore connectivity, and relatively loose grain skeletons. Under the combined effects of confining pressure and pore water pressure, their pore structure characteristics and deformation response control the storage and seepage capacities of the strata [23,24] and may even lead to the development of localized compaction bands.

Because salt deposits are extracted by solution mining, the geomechanical responses induced by improper or incompletely controlled mining differ from sudden water inrush hazards, such as roof or floor water inrush, commonly observed in coal mining involving aquifers [25]. Instead, increasingly more anthropogenic aquifers are gradually formed in the overlying strata. The distinctive lithological assemblage of the overlying Dainan Formation, characterized by alternating aquifers and aquitards, provides space for mining-induced deformation and fracture development [26]. While leaked fluid migrates upward along mining-induced fractures or damaged wellbores, it can also be stored within the sandstone strata and accumulate pressure in a timely manner. The progressive formation of such anthropogenic aquifers from an initially non-water-bearing state changes the stress–strain state of the overburden and consequently induces surface uplift through poroelastic effects [27]. Frequent borehole water inflow has recently been reported in the exploration wells of a salt mine in China, and it is currently suspected that mining-induced fractures have propagated into the overlying sandstone. After the closure of neighboring well groups with abnormal injection-to-production ratios, the inflow rate did not decrease rapidly or stabilize within a short period. Together with the fact that the exploration area is as far as 2 km from the mining zone, this phenomenon indicates that the sandstone has strong permeability. Therefore, porous sandstones with different grain sizes were selected in this study. A combination of X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and low-field nuclear magnetic resonance (NMR) was employed to qualitatively and quantitatively analyze the microstructure, mineral composition, and pore structural characteristics of the sandstones, including pore size distribution, pore connectivity, and permeability.

During salt mining, the stress field of the overlying strata is dynamically adjusted, while the original seepage conditions of the rock mass are also altered. Accordingly, triaxial compression tests under pore water pressure and permeability measurements throughout the complete stress–strain process were carried out to systematically analyze the strength, deformation, and seepage responses of sandstones with different pore structures during the entire loading process. This study mainly focuses on two aspects:

(1)

The pore structure of porous sandstones with different grain sizes and its relationship with high porosity, good connectivity, and high permeability.

(2)

The stage-dependent evolution of permeability throughout the complete stress–strain process and its coupling with pore compaction, crack propagation, and the development of secondary structures. The present work is expected to improve the understanding of the hydromechanical coupling behavior of weakly cemented, high-porosity sandstones and to provide an experimental basis for evaluating the mechanical and seepage responses of similar aquifer sandstones.

2. Methodology

2.1. Sample Preparation and Experimental Apparatus

Core samples from the two boreholes were processed into multiple specimen types, including: thin sections with surface dimensions < 1 × 1 cm (with undisturbed test surfaces); powders ground to 45 μm, small blocks (<1 × 1 × 1 cm); and standard cylindrical specimens with dimensions of 25 × 100 mm and 50 × 100 mm (Figure 1a). A comprehensive characterization program, including porosity–permeability testing, scanning electron microscopy (SEM), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and nuclear magnetic resonance (NMR), was employed to qualitatively and quantitatively investigate the microstructure, mineral composition, and pore structure of the sandstones, including pore size distribution, pore connectivity, and permeability. The measurements of porosity and permeability were conducted in accordance with ASTM standards (using the rock sample integrated porosity–permeability tester). SEM was used to observe the microstructure and morphology of the samples (Instrument model: Quanta250; FEI, Hillsboro, Oregon, USA). Mineral phases were identified by XRD through comparison between the diffraction patterns of the samples and standard reference patterns (Instrument model: Smartlab; Rigaku, Tokyo, Japan). MIP was applied to analyze pore structure and pore size distribution by forcing mercury into the material under external pressure (Instrument model: AutoPore V; micromeritics, Norcross, GA, USA). Low-field NMR was used to obtain pore size distribution from the fluid relaxation time (T2) spectrum (Instrument model: MicroMR12-040V; NIUMAG, Suzhou, China), while pore connectivity was further evaluated based on long-relaxation signals; centrifugation tests were carried out using a benchtop high-speed refrigerated centrifuge (Instrument model: CSC-12; BIORIDGE, Shanghai, China). Porosity is determined by the total volume of fluid that can be injected into the sample, whereas permeability depends primarily on the connectivity between pores. Therefore, pore connectivity in the porous medium was comprehensively evaluated by integrating NMR, MIP, and the other aforementioned techniques.

In accordance with the testing procedures recommended by the International Society for Rock Mechanics (ISRM), the specimens were prepared as standard cylinders with dimensions of 50 mm in diameter and 100 mm in height, and both ends were ground to satisfy the required parallelism tolerance (≤0.05 mm). The loading system consisted of an MTS815 testing system equipped with three independently closed-loop servo control subsystems for axial loading, confining pressure, and pore water pressure. The system includes a high-precision pore water pressure loading unit, a data acquisition unit, and an electro-hydraulic servo control unit. Its main technical specifications are as follows: a maximum axial load of 4600 kN, a maximum confining pressure of 140 MPa, and a maximum pore water pressure of 140 MPa.

Acoustic emission (AE) refers to the phenomenon in which strain energy is released in the form of elastic waves when a material undergoes deformation or fracture under external or internal forces. The parameters commonly used to characterize AE behavior include ring-down count rate, energy count rate, and hit count rate. AE signals were monitored using an AE21C dual-channel acoustic emission detection system, which is capable of fully automatically high-speed sampling and recording AE information and can directly provide AE indices such as ring-down count rate, event count rate, and energy count rate per unit time. After the modification of the testing system, the AE sensors were directly coupled to the specimen, thereby avoiding signal attenuation and noise interference caused by the pressure vessel and the hydraulic oil inside the chamber.

In this study, all experiments were carried out under room temperature (25 ± 2) °C, and a total of 80 specimens were used in the aforementioned tests. Among them, 12 specimens were subjected to porosity–permeability tests, 9 specimens were examined by scanning electron microscopy (SEM), 20 specimens were analyzed by X-ray diffraction (XRD), 8 specimens were measured by mercury intrusion porosimetry (MIP), 18 specimens were tested by nuclear magnetic resonance (NMR), and 13 specimens were used for triaxial compression–permeability tests. The lithology and basic physical parameters of some specimens are presented in

Table 1. Information on some specimens.

Rock Sample Number	Lithology	Diameter (cm)	Length (cm)	Mass (g)
5-3-23-1	Siltstone	4.993	10.009	417.30 g
5-3-23-2	Siltstone	4.931	10.004	408.84 g
5-7-23-1	Siltstone	5.005	9.9890	409.23 g
5-8-23-1	Siltstone	5.004	9.9990	418.84 g
5-9-23-1	Siltstone	4.955	9.9660	385.34 g
8-46-51-1	Siltstone	4.996	9.999	438.49 g
8-46-51-2	Siltstone	4.994	9.996	433.59 g
15-11-73-1	Fine Sandstone	4.958	9.953	383.52 g
15-12-73-1	Fine Sandstone	5.005	10.025	417.28 g
15-12-73-2	Fine Sandstone	5.049	10.012	422.56 g
15-12-73-3	Fine Sandstone	5.100	10.001	445.28 g
7-23-56-1	Medium-Fine Sandstone	5.005	10.002	417.01 g
7-23-56-2	Medium-Fine Sandstone	5.007	10.008	424.21 g

The sample nomenclature. Taking “5-3-23-1” as an example, “5” denotes the drilling run (the 15th run); ”3” denotes the segment number of the core recovered in the current run (the 3rd segment); ”23” denotes the total number of segments in the core from the current run (23 segments in total); ”1” denotes the test specimen number obtained from that core segment (Specimen No. 1).

.

2.2. Test Scheme

The MTS815.04 electro-hydraulic servo-controlled rock mechanic testing system is functionally comprehensive. In addition to conventional uniaxial and triaxial tests, it can also be used to conduct pore water pressure loading and water permeability tests (Figure 1a). The system can be operated in manual, module control, and digital control modes. During the tests, axial stress was applied under displacement control at a loading rate of 0.00167 mm/s, whereas both confining pressure and pore water pressure were applied under stress control at a loading rate of 0.0167 MPa/s. In addition, for the safety of the testing equipment, the pore water pressure was maintained lower than the confining pressure (P < σ₂ = σ₃) throughout the saturated triaxial compression tests. Under constant confining pressure and pore water pressure, axial loading was then applied in displacement control mode at a deformation rate of 0.002 mm/s until specimen failure, while acoustic emission parameters were synchronously recorded (Figure 1b).

When permeability testing was carried out using the MTS815 triaxial testing system, a constant axial displacement mode was adopted. In this testing mode, the axial displacement of the specimen was kept unchanged during permeability measurement. Under such conditions, the rock specimen exhibited stress relaxation; that is, the axial load gradually decreased, while the axial displacement remained constant. On the stress–strain curve, this behavior is reflected by a segment approximately parallel to the stress axis. Triaxial compression–permeability tests were performed on siltstone, fine sandstone, and medium-coarse sandstone under different pore pressures and confining pressures. Six permeability measurement points were defined for each specimen. In addition to the initial permeability measured under hydrostatic pressure conditions, the other permeability points corresponded to the five deformation stages during the complete stress–strain process, namely crack compaction, elastic deformation, yield failure, post-peak strain softening, and residual failure. Based on the acquired data, permeability–stress–strain curves were established, and the elastic modulus and Poisson’s ratio were calculated. The elastic modulus was determined using the tangent modulus method (adopting the tangent of the linear elastic stage of the stress–strain curve), and Poisson’s ratio was defined as the ratio of lateral strain to axial strain in the linear elastic stage, from which the seepage behavior of each specimen under different stress–strain states was analyzed.

Because the tested rock belongs to loose and highly permeable sandstone, the steady-state method was adopted to determine permeability. In the laboratory tests, a stable pressure difference was applied across the two ends of the specimen, and permeability was obtained by measuring the seepage flow rate. The corresponding principle and procedure of the seepage test are shown in Figure 1. A stable upstream–downstream pressure gradient was established by first applying a certain axial stress, confining pressure, and pore pressure to the specimen and then reducing the pore pressure at one end of the specimen to generate a seepage pressure difference across the sample, thereby driving water flow through the rock specimen (Figure 1c). Once the hydraulic head difference and the inlet flow rate reached stable values, steady flow was established within the sandstone. The theoretical basis of the steady-state method is Darcy’s law, and the permeability was calculated accordingly:

$$K = {\displaystyle \frac{Q\cdot L\cdot {\mathsf{\mu }}_{\mathrm{f}}}{A\cdot \mathsf{\Delta }P}}$$

(1)

where K is rock permeability (10⁻¹⁵ m²), Q is the flow rate of fluid through the rock specimen (mL/min), and ΔP is the pressure difference between the upstream and downstream ends of the core (kPa). L is the length of the core specimen (cm), A is the cross-sectional area of the specimen (cm²), and μ_f is the dynamic viscosity of the fluid, which is taken as 1.005 × 10⁻³ Pa·s.

3. Results

3.1. Characteristics of Sandstone Aquifers

The integrated laboratory characterization of field-retrieved borehole samples using MIP, NMR, and XRD indicates that the target sandstone aquifer is a weakly cemented, high-permeability, high-porosity reservoir with a well-connected pore network. The pore structure is well developed and is dominated by relatively large pores. To some extent, the mercury intrusion distribution can reflect the connectivity of the pore system.

As shown by the logarithmic differential mercury intrusion curves and pore size distribution curves for the siltstone, medium-fine sandstone, and fine sandstone (Figure 2a), the pore diameters corresponding to the peak values of the curves, namely the most probable pore diameters, are 24.17 μm, 30.21 μm, and 17.26 μm, respectively. The parameters obtained from the mercury intrusion porosimetry (MIP) tests on rock blocks are listed in

Table 2. Rock block parameters (from MIP).

	1	2	3
Porosity	23.7335%	12.53%	19.4339%
Permeability	623.87 md	130.05 md	181.15 md
Fractal dimension	2.962	2.851	2.966
Tortuosity	2.2732	2.6022	2.4243

Calculation method of fractal dimension: $\mathrm{D}=3-{\displaystyle \frac{\mathrm{d}\left(\mathrm{l}\mathrm{o}\mathrm{g}{\mathrm{S}}_{\mathrm{H}\mathrm{g}}\right)}{\mathrm{d}\left(\mathrm{l}\mathrm{o}\mathrm{g}{\mathrm{P}}_{\mathrm{c}}\right)}}$. (P_c: mercury intrusion pressure; S_Hg: mercury intrusion saturation). Calculation method of tortuosity: $\mathsf{\tau }=\sqrt{{\displaystyle \frac{\mathsf{\varphi }{\overline{\mathrm{r}}}^2}{8\mathrm{k}}}}$. (k: permeability; Φ: porosity; r: average pore–throat radius).

. Based on the measured parameters, this further confirms its well-developed pore system and favorable water storage capacity.

In addition, cylindrical specimens with dimensions of 25 × 50 mm and medium-to-high permeability were prepared and tested using the steady-state method. After helium flow through the rock pores had reached steady state, the measured permeabilities were 64.14 md, 12.36 md, and 19.76 md, while the porosities determined based on Boyle’s law were 15.15%, 5.14%, and 10.62%, respectively.

NMR measurements performed on a 25 × 50 mm fine sandstone core show that the movable water saturation reaches 67.47%, suggesting that most in situ water occurs as free fluid or weakly bound fluid, which is a key indicator of a highly permeable aquifer. Only one prominent T₂ main peak is observed, and the T₂ cutoff value is 18.04 ms (Figure 2b(2)). The relatively long T₂ cutoff further implies that a considerable proportion of the pore volume is hosted in relatively large pores and/or well-connected pore clusters, where surface relaxation is weaker (i.e., lower S/V), thereby allowing for free-fluid behavior. In addition, under a dehydration pressure of 100 psi, the main T₂ peak is drained, indicating that the principal storage space resides in pressure-sensitive, relatively large, and well-connected pores that directly control permeability. The high saddle point between the main peak and the two minor peaks in the T₂ spectrum (Figure 2b(1)) suggests substantial intermodal connectivity, i.e., coupling between large and medium pores, rather than isolated multimodal pore domains. Collectively, these observations indicate that the pores controlling fluid storage are essentially the same as those governing fluid flow.

Both NMR and MIP effectively characterize the pore structure of porous media, and the two methods show good consistency. A mercury intrusion analysis of core fragments revealed a relatively high volumetric porosity (23.73%) and a large most probable pore diameter (24.17 μm), indicating that the pore system is dominated by mesopores to macropores (Figure 2b). The relatively low tortuosity (2.27) further suggests limited flow-path sinuosity and reduced hydraulic energy loss, which is consistent with efficient through-flow. The criteria derived from MIP and NMR for pore size and connectivity are mutually consistent, indicating that this formation is an aquifer with strong storage capacity and high transmissivity.

During the preparation of porosity and permeability specimens, powder from the entire cross-section was collected for compositional analysis. Because the sampled powder was obtained from the whole cross-sectional surface, the measured composition is statistically representative. As shown in Figure 3, the mineral composition of this interval is dominated by quartz (47.3%), K-feldspar (2.6%), plagioclase (35.2%), calcite (1.7%), halite (1.5%), analcime (1.6%), and clay minerals (9.15%). Quantitative X-ray diffraction analysis further reveals the relative contents of different clay minerals, including smectite-group minerals (62%), illite (30%), and chlorite (8%). The overall clay mineral content is low, indicating that most pores in the loose sediments have not been filled by cementing materials. The sandstone is therefore insufficiently cemented and retains a relatively well-developed pore structure. At present, its diagenesis is inferred to have been controlled primarily by compaction.

3.2. The Strength and Deformation Characteristics

A series of triaxial tests was conducted to investigate the mechanical behavior of the sandstone under different hydraulic conditions. Triaxial tests with pore water pressure were performed under displacement-controlled loading. The results show that the mechanical behavior of the siltstone differs markedly from that of low-porosity rocks. During the water-saturated triaxial compression process, the pore spaces within the solid skeleton and between mineral particles of the siltstone were subjected to the combined effects of axial stress, confining pressure, and pore pressure.

The experimental results indicate a pronounced pore pressure effect on the strength characteristics of the siltstone under a given confining pressure:

(1)

The peak strength decreases with increasing hydraulic pressure gradient, and this effect becomes more significant at lower confining pressure. As shown in Figure 4a, under a confining pressure of 3 MPa, when the initial hydraulic pressure difference increased from 1.0 to 2.0 MPa, the peak strength decreased from 28.57 to 20.18 MPa, corresponding to a reduction of 8.39 MPa or 29.4%. With increasing confining pressure, however, the rate of strength variation with pore water pressure gradually diminished, indicating that confining pressure weakens the influence of pore water pressure on the peak strength. For example, under a confining pressure of 10 MPa, when the initial hydraulic pressure difference increased from 0 to 7.0 MPa, the peak strength decreased from 51.69 to 42.25 MPa, representing a reduction of 18.3%. This can be attributed to the fact that higher confining pressure promotes the tighter closure of internal fissures, thereby reducing the inflow of pore water into the internal flow channels of the rock. As a result, part of the water pressure acts only as an external load on the specimen ends, increasing the effective stress and thus weakening the effect of hydraulic pressure difference on rock strength.

(2)

Poisson’s ratio showed an overall increasing trend with increasing initial hydraulic pressure difference. Under a confining pressure of 3 MPa, when the initial hydraulic pressure difference increased from 1 to 2 MPa, Poisson’s ratio increased from 0.24 to 0.33, representing an increase of 37.5%. Under a confining pressure of 10 MPa, when the initial hydraulic pressure difference increased from 0 to 7.0 MPa, Poisson’s ratio increased from 0.14 to 0.32, representing an increase of 128.6%. This is because, under triaxial compression and constant confining pressure, increasing pore water pressure reduces the compressive stress between particles, thereby enhancing the lateral deformation of the rock.

(3)

The elastic modulus generally showed a decreasing trend, with an increasing initial hydraulic pressure difference. For instance, under a confining pressure of 3 MPa, when the initial hydraulic pressure difference increased from 1 to 2 MPa, the elastic modulus decreased from 5.65 to 4.63 GPa, corresponding to a reduction of 1.02 GPa or 18.1%. Under a confining pressure of 10 MPa, when the initial hydraulic pressure difference increased from 0 to 7 MPa, the elastic modulus decreased from 6.96 to 4.60 GPa, corresponding to a reduction of 33.9%.

Although the elastic modulus decreases with increasing pore water pressure, the peak failure strength, σ_1max, increases when the effective confining pressure, σ₃′, is maintained constant, while both confining pressure and pore water pressure increase simultaneously (Figure 4b). Specifically, for the three siltstone specimens tested at confining pressures of 6, 10, and 15 MPa, with corresponding pore water pressures of 3, 7, and 10 MPa, the measured peak failure strengths, σ_1max, were 24.43, 32.09, and 37.00 MPa, respectively.

3.3. Permeability Characteristics Under the Complete Stress–Strain Process

A steady-state method was employed on the MTS815 rock mechanic testing system to conduct permeability tests under conventional triaxial compression. The complete stress–strain curves and the corresponding permeability evolution curves of the sandstones overlying the salt mine were obtained under different confining pressures and pore water pressures. Based on these experimental results, the seepage behavior, strength characteristics, and deformation and failure mechanisms of porous sandstones with different grain sizes throughout the complete stress–strain process were analyzed.

The mechanical properties of sandstone are governed not only by external factors, such as in situ stress, temperature, and water, but also by its intrinsic material composition. Sandstones with different grain sizes therefore exhibit markedly different mechanical behaviors. SEM observations at a magnification of 70× show that the surfaces of the sandstone specimens with different grain sizes are relatively rough, with particle sizes ranging from 10 to 500 μm. The grains are comparatively uniform in size, well sorted, distinctly angular, and loosely arranged, with blocky surface protrusions, clear interfaces between adjacent particles, and relatively large intergranular pores (Figure 5). Combined with the results of XRD, MIP, NMR, and porosity–permeability measurements, these observations indicate that the sandstone is characterized by weak cementation, relatively large pore sizes, and good pore connectivity, resulting in a porosity of up to 15.15% and a permeability ranging from 12.36 to 64.14 md.

The complete stress–strain behavior of sandstone under triaxial compression provides the basis for understanding its deformation, strength, and seepage characteristics, each of which carries distinct physical significance at different loading stages. As shown by the stress–strain relationships in Figure 6, the three sandstone types exhibit broadly similar curve patterns, all undergoing five successive stages: fissure compaction, elastic deformation, yield hardening, post-peak strain softening, and residual failure [28]. Substantial volumetric deformation persists until the residual failure stage. A comparison of the permeability values of the three sandstone types shows that permeability decreases with decreasing grain size. For the siltstone tested under a confining pressure of 5 MPa and a pore water pressure of 3 MPa, the specimen is relatively compact, and failure in the post-peak strain-softening stage occurs rather abruptly (Figure 6(a-1)). In contrast, the fine sandstone tested under a confining pressure of 8 MPa and a pore water pressure of 5 MPa exhibits the most pronounced ductile failure characteristics, together with the highest peak strength and elastic modulus (Figure 6(b-1)). For the medium-fine sandstone tested under a confining pressure of 3 MPa and a pore water pressure of 1 MPa, the relatively low hydraulic loading produces a failure pattern that is commonly observed in loose, highly permeable, soft rocks (Figure 6(c-1)).

Under different stress and strain conditions, the hydraulic conductivity or permeability of rock is not a constant but varies as a function of both the stress state and the strain history. The stress state refers to whether confining pressure is applied, i.e., uniaxial versus triaxial loading, as well as whether pore pressure is present and the magnitude of the pore pressure. The strain history refers to the five stages in the complete stress–strain process of the rock, namely microcrack and pore compaction, elastic deformation, yield hardening, post-peak softening, and residual strength. Throughout the triaxial compression process, the permeability of the siltstone ranges from 0.56 to 1.11 md, that of the fine sandstone from 0.37 to 4.76 md, and that of the medium-fine sandstone from 2.92 to 6.88 md (Figure 6).

During the fissure compaction stage, permeability shows a slight increase. Owing to the relatively high initial permeability, and because the hydraulic pressure applied at both the upstream and downstream ends exceeded 3 MPa, the rock skeleton of the porous sandstone expanded. As a result, even during the compaction stage, permeability increased with increasing stress. Specifically, the permeability of the siltstone increased from 0.94 to 1.11 md (Figure 6(a-2)), whereas that of the fine sandstone increased from 4.63 to 4.76 md (Figure 6(b-2)).

During the elastic deformation and yield-hardening stages, permeability began to decrease because increasing stress led to the compaction of primary pores and closure of pre-existing fractures. At the peak stress stage, permeability further decreased, although the rate of decline became smaller (Figure 6(b-1)).

In the post-peak softening and residual failure stages, although crack coalescence and the generation of new fractures would normally be expected to enhance permeability, the opposite trend was observed. In highly porous and loosely structured rocks, the closure of open pores and natural fractures, together with the production of sand particles within the shear band during volumetric expansion, can reduce permeability [29]. Consequently, the permeability of all three sandstone types continuously decreased from deformation to failure, except for the compaction-stage increase caused by hydraulic pressure. The permeability of the fine sandstone decreased from an initial value of 4.63 md to only 0.36 md at the residual failure stage, whereas that of the medium-fine sandstone decreased from 6.88 to 2.92 md.

In the residual failure stage, with a further increase in strain, crushed particles in the loose and highly permeable sandstone progressively blocked the seepage pathways, which is considered the principal reason for the continued decrease relative to the initial permeability. Nevertheless, in the residual stress stage, permeability may also increase with further strain once secondary fractures become sufficiently well connected (Figure 6(a-2)). Under such conditions, the through-going fracture along the failure plane of the siltstone exerts a stronger influence than other factors controlling permeability. As also shown in Figure 6(a-1), this rock is relatively compact and highly brittle and fails abruptly, in contrast to the more ductile failure characteristics of the fine sandstone and medium-fine sandstone.

Whereas the stress–strain curve reflects the macroscopic deformation response of sandstone during loading, acoustic emission (AE) records the processes of damage evolution, crack propagation, and unstable fracture among internal units in the form of elastic waves. The AE results are generally consistent with the overall permeability evolution of the medium-fine sandstone (Figure 7a). During the 0–350 s interval, the specimen was in the stage of applying upstream and downstream hydraulic pressures. Because the pore water pressure reached as high as 12 MPa, the weakly cemented and highly porous skeleton was highly sensitive to pore pressure disturbance, and skeleton expansion became dominant. The rapid establishment and diffusion of pore pressure induced a pronounced transient AE response, characterized by high energy peaks and dense, high-amplitude AE events. Thereafter, as loading continued, the overall AE activity weakened and then gradually intensified during the fissure compaction, elastic deformation, and yield-hardening stages, accompanied by a continuous decrease in permeability, reflecting the progressive development of primary pore compaction, fracture closure, and particle rearrangement. After entering the post-peak softening and residual failure stages, the AE response became further enhanced, whereas permeability continued to decrease, indicating that the weakening effect of crushed particles and fine debris clogging the seepage channels remained dominant. It should be noted that locatable AE events did not occur extensively before the peak stress but were recorded only gradually after the specimen entered the residual failure stage following peak stress. A total of 77 AE events were obtained by the end of the test (Figure 7b), indicating that the later-stage damage activity was mainly concentrated in the residual failure stage.

4. Discussion

During the triaxial compression–permeability tests in this study, an uncommon phenomenon was observed. After the compaction stage, permeability decreases continuously throughout the subsequent deformation stages. Notably, the permeability of the sandstone does not increase at the peak stress stage but instead continues to decline. Experimental observations show that permeability decreases progressively with increasing deformation (Figure 6(c-1,c-2)), with a final reduction of more than one order of magnitude.

We speculate that this behavior may be associated with compaction localization, and the reduction becomes even more pronounced once localized compaction structures are formed. A compaction band is a localized deformation mode characterized by volumetric compaction with negligible shear deformation [30]. Permeability generally decreases with increasing axial strain and may drop abruptly during the formation of the first localized structure. In addition, the local rock skeleton is crushed to a certain extent, and the fragmented framework grains, including fine debris and argillaceous particles generated during failure, are sufficiently small to clog interconnected seepage pathways such as macroscopic fractures [31]. Th formation of localized structures in loose, high-porosity sandstone under high confining pressure is shown in Figure 8.

5. Conclusions

This study first employed microscopic experiments to characterize the pore structure of loose, highly permeable sandstone. On this basis, triaxial compression coupled with seepage tests was conducted on highly porous rocks to systematically investigate deformation, strength, and permeability characteristics during the triaxial compression and failure process of sandstone under pore water pressure. The main conclusions are as follows:

(1)

Integrated SEM-XRD-MIP-NMR characterization demonstrates that the target aquifer consists of weakly cemented sandstone with high porosity and strong hydraulic conductivity, as evidenced by large pore sizes, good pore connectivity, and high permeability.

(2)

During the deformation and failure of sandstone with different pore sizes, permeability exhibits a staged variation along the stress–strain curve. Rocks with high porosity and loose structure possess relatively high initial permeability. In the compaction stage, permeability rises briefly compared with the hydrostatic pressure stage, which is attributed to the expansion of the rock skeleton in loose, high-permeability sandstone under downstream seepage pressure. Subsequently, permeability decreases continuously with increasing stress. In the residual failure stage, permeability further declines with increasing strain, as crushed particles block pore channels.

(3)

Under the same confining pressure, the peak strength of the rock decreases with increasing pore water pressure, and this pore pressure effect becomes more pronounced at lower confining pressure. With increasing pore water pressure, the elastic modulus shows a decreasing trend, whereas Poisson’s ratio increases.

(4)

At relatively high confining pressure, once localized compaction bands develop in highly porous rocks, permeability generally decreases continuously with increasing axial strain. The initiation and propagation of microcracks promote permeability enhancement, whereas the compaction of primary pores and pore–throats, closure of pre-existing fractures, crushing of framework grains, and formation of compaction bands inhibit permeability increase. The overall evolution of permeability is therefore governed by the combined effects of these competing mechanisms.

This research will help deepen the understanding of the hydromechanical coupling behavior of highly porous, weakly cemented sandstone and provide an experimental basis for evaluating the mechanical and seepage responses of similar aquifer sandstones.