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Depleted oil and gas reservoirs are attractive sites for storing green energy carriers. During injection–production cycling in underground gas storage (UGS), variations in effective stress can cause repeated stress disturbances, which may trigger borehole sand production. Laboratory sand-production simulation tests were conducted to evaluate the effects of cyclic-loading stage, upper stress limit, and cycling frequency on borehole damage and sand-production behavior. The results show that sand production is stage-dependent. During the rapid-hardening and stable stages, the borehole remains largely intact and sand production is negligible. Once the failure and collapse stages are reached, borehole integrity deteriorates and sand production increases sharply, with fine particles becoming dominant. Cumulative sand production increases with the upper stress limit. Increasing the upper limit from 80% to 95% leads to a 2.53-fold increase in produced sand mass, together with a higher fine-sand fraction and a shift in the particle-size distribution (PSD) toward smaller sizes. The cycling frequency also plays an important role. When the frequency decreases, cumulative sand production increases and becomes 53.1% higher than the baseline at 0.001 Hz. Meanwhile, the median particle size (D50) decreases, indicating stronger particle breakage under low-frequency cycling. These findings provide guidance for designing injection–production schemes for UGS and for selecting appropriate sand-control completion strategies.
Large-scale energy storage in subsurface space has become a key approach to enhance energy-system flexibility and security. Depleted oil and gas reservoirs are widely regarded as suitable geological media for natural gas storage due to their large effective storage capacity and good sealing integrity, as well as existing well patterns and surface pipeline infrastructure. Gas-reservoir-type underground gas storage (UGS) typically follows a seasonal injection–withdrawal schedule characterized by high-rate injection and high-rate production. Such alternating injection–production operations induce persistent pore-pressure fluctuations, thereby causing repeated variations in formation effective stress and near-wellbore stress paths. These repeated stress perturbations facilitate cyclic damage accumulation, pore-structure evolution, and progressive strength degradation in both the reservoir and the surrounding rock. Consequently, the near-wellbore zone becomes more susceptible to sand production, accompanied by engineering hazards such as plugging and erosion. Compared with conventional oil and gas reservoirs, UGS systems undergo multiple injection–production cycles throughout their service life, resulting in stronger and more frequent stress disturbances, leading to higher sanding risk, more severe sand production, and more complex underlying mechanisms. Therefore, investigating the failure evolution of reservoir rock and the associated sand-production responses under cyclic (alternating) loading is of both theoretical and engineering significance. Such understanding supports efficient UGS development, safe high-rate operation, optimization of injection–production schemes, and the design of sand-control completions.
With the large-scale construction and commissioning of underground gas storage (UGS) facilities, sand production has been increasingly reported in UGS sites both in China and worldwide. It has become a prominent engineering problem that compromises operational safety and efficiency. For example, sanding occurred during the operation of the Wen 96 UGS in China. As a result, 2–3 choke beans (choke nozzles) had to be replaced within a single gas-withdrawal cycle, and erosion failure of the control-valve trim (valve plug) was observed in 12 wells. After sand production occurred in the Hutubi underground gas storage (UGS) in Xinjiang, the daily gas production rate declined from 8.0 × 10 5 m 3 to 4.0 × 10 5 m 3, indicating substantial deliverability loss. Similar risks have also been reported in overseas underground gas storage (UGS) facilities. For example, a large amount of sand was produced near the end of the withdrawal period in the Redfield & Vincent UGS (USA), which triggered three wellsite fires. Sand erosion further caused tubing perforation, markedly increasing operational safety hazards. In the Hungarian underground gas storage (UGS) facility developed from the Hajdúszoboszló gas reservoir, the reservoir sanding risk exceeds 70%, posing a potential threat to long-term stable operation. Overall, sand production in underground gas storage (UGS) is widespread and highly detrimental. Systematic studies on its controlling factors are urgently needed to support optimization of injection–production schemes and the development of sand-control strategies. To address this issue, previous studies have examined cyclic load-induced damage and sanding-risk assessment. It is generally accepted that, under “strong injection–strong production” alternating operation in gas-reservoir-type UGS, periodic fluctuations in pore pressure and effective stress can markedly modify the near-wellbore stress path and induce cyclic damage. Consequently, UGS reservoirs exhibit a higher sand-production risk and more complex mechanisms than conventional oil and gas reservoirs. Kang evaluated carbonate-reservoir core samples from the Xiangguosi underground gas storage (UGS) by conducting stress-sensitivity and velocity-sensitivity tests, together with core-flow (core-flooding) experiments under UGS operating conditions. The results showed that pressure fluctuations are a key trigger for migration of fine sand in UGS reservoirs. Dong noted that long-period, high-rate alternating injection–production in UGS causes cyclic variations in stress, flow direction, and the gas–water interface. This makes sand-production prediction and mitigation more challenging. Sui performed alternating-load sanding tests and modified a sanding index by incorporating the damage level, number of loading cycles, and loading frequency. This enabled sanding-risk assessment and sanding-onset time prediction, providing support for sand-control completion and production-scheme design. Ma et al. conducted high-pressure sanding experiments and identified two critical thresholds, namely, an equivalent initial sanding pressure and the collapse pressure, which can be predicted by a thick-walled cylinder model. Staged changes in sand production as well as particle size were reported when the confining pressure exceeded the corresponding thresholds, providing an experimental basis for quantifying hole-failure-controlled sanding. McLellan et al. systematically examined sand production from the perspectives of completion design and risk assessment, with particular attention to hole/perforation collapse, reservoir-pressure variation, and cyclic-loading-induced strength reduction. In addition, the adverse impact of overly conservative sand-control designs on injection–production capacity was highlighted. Zhang et al. employed China’s first sand-production simulation apparatus for underground gas storage (UGS) to conduct alternating-load sanding tests on an offshore UGS developed in unconsolidated sandstone. The experiments indicated that alternating loading, together with flow rate, constitutes a key factor governing sanding in unconsolidated sandstone reservoirs. Hou and Liao evaluated operating conditions, including formation pressure, water content (water cut), and flow rate/pressure drawdown. The analysis indicated that low formation pressure and elevated water content aggravate sand production, whereas critical ranges and plugging effects occur with respect to flow rate and effective stress. Owing to limitations of the experimental setups, the influence of cyclic alternating operating schemes in UGS was not incorporated. Song et al. performed laboratory mechanical tests to assess the effects of water-content variation and cyclic loading on sandstone properties. Pronounced strength degradation was observed with increasing water content and an increasing number of cycles. Based on these observations, damage evolution associated with water content and cyclic loading was parameterized, providing key mechanical inputs for calculating and predicting the critical drawdown pressure (CDP) for sanding. Tian et al. investigated alternating loading through fatigue-damage experiments combined with discrete element method (DEM) simulations. The results suggested that appreciable damage emerges only after the rock enters the plastic regime. The effects of stress upper/lower limits and loading rate were further ranked, while a hole-scale effect was identified, offering insights for predicting the critical pressure drawdown for sanding under alternating loading. Collectively, prior work indicates that sanding mechanisms under UGS operating conditions differ from those under conventional production. Nevertheless, mechanistic understanding remains incomplete. Existing studies predominantly emphasize final sand yield or CDP, whereas systematic investigation of hole-failure morphology evolution across different stages of cyclic loading, together with the periods of abrupt sanding escalation, remains limited. Fine sand represents a major operational risk because fine sand readily plugs screens and accelerates tubular erosion; however, the current understanding of the proportion of fine sand and its particle-size refinement, as well as its relationship with geometric features of hole damage, is still insufficient. Moreover, the influence of cycling frequency has often been addressed via empirical corrections or statistical correlations, leaving the role of frequency in governing grain crushing and particle-size distribution evolution under cyclic loading insufficiently explored. Previous studies have indicated that spatial variability in grain-size distributions, together with permeability heterogeneity, can significantly influence fine-sand generation and particle mobilization in sand-prone formations. Grain-size variability inferred from gamma ray measurements and permeability-based assessments has been used to capture spatial differences in particle-size distributions and fine-sand susceptibility in outcrops. These observations highlight the importance of considering inherent grain-scale heterogeneity when interpreting sand-production behavior, particularly under cyclic stress conditions. To address these gaps, the present study conducted sanding experiments under UGS-type cyclic loading using outcrop red sandstone collected from the Xinjiang M UGS area. The effects of loading stage, stress amplitude, and cycling frequency on sanding behavior were investigated. Multi-scale characterization—including sieving, laser particle-size analysis, and quantitative CT-image analysis—was further applied to elucidate the associated sanding mechanisms, thereby providing more direct support for optimizing UGS injection–production schemes and designing sand-control completions.
The experiments employed red sandstone samples from the reservoir of the M underground gas storage (UGS). The porosity ranged from 21.6% to 22.8%, and the permeability ranged from 210.8 to 305.6 mD, indicating a medium-porosity and medium-permeability sandstone. Mineralogical analysis indicated that the sandstone contained 57% quartz, 5.2% K-feldspar, 20.9% plagioclase, 6.2% calcite, 2.1% ankerite (ferroan dolomite), and 8.6% clay minerals, with quartz and feldspar as the dominant constituents. Petrographic thin-section photomicrographs indicate that intergranular pores are the primary pore type in the M UGS reservoir. Quartz commonly exhibits secondary overgrowth, while feldspar generally shows dissolution features.
The rock blocks were machined into standard cylindrical specimens using a TX-SHM200C programmable double-end surface grinder. The specimens were 50 mm in diameter and 100 mm in length, and the machining tolerance met the requirements of international rock mechanics testing standards. To minimize the influence of specimen heterogeneity on the experimental results, the P-wave (longitudinal wave) velocity was measured with an RSMSY5 ultrasonic tester. Only specimens with no visible defects and a P-wave velocity variation within 5% were selected for subsequent experiments.
The thick-walled cylinder sand-production test was designed to replicate downhole perforation-tunnel conditions. Therefore, a mechanical hole was drilled at the center of each core sample to create an artificial perforation. Dry drilling on a lathe was adopted to keep the hole centered and to avoid hydration effects on the rock properties. After drilling, residual chips were removed using compressed air. Based on the geometric similarity criterion with an outer-diameter-to-hole-diameter ratio of 3:1, the hole diameter was set to 16 mm and the drilling depth was 60 mm, leaving a 40 mm intact section at the bottom.
A laboratory-scale thick-walled cylinder sand-production experiment was designed to investigate the controlling factors of sand production in underground gas storage (UGS) under cyclic loading. The experiments were conducted using a GCTS-RTR-1000 rock mechanics testing system. To enable direct observation of hole damage during loading, a visualized sand-production apparatus was developed. A miniature camera with an integrated light source, together with a signal acquisition system, was installed on the loading head of the core sample. The morphological evolution of the hole could be continuously recorded after the test had started. Based on the real-time camera observations, the confining pressure was increased until the core sample fractured and sand production occurred. This pressure (75 MPa) was defined as the critical confining pressure for sand production. Subsequent cyclic-loading sand-production tests were then performed by applying cyclic loading at prescribed ratios relative to the critical confining pressure to examine sand-production behavior under different cyclic parameters.
First, two monotonic confining-pressure ramp tests were performed under an axial stress of 2 MPa. Confining pressure was continuously increased until sand production occurred at the borehole; the corresponding confining pressure, approximately 75 MPa, was defined as the reference value for subsequent tests. Subsequently, under the same axial stress, one set of confining-pressure cyclic loading–unloading tests was conducted to simulate the cyclic loading process in underground gas storage, and the test was continued until fatigue failure of the rock occurred. After collapse induced by cyclic loading, the radial strain remained in a compressive state (positive values), and the confining pressure–radial strain curve exhibited a distinct stress hysteresis loop. The confining pressure–radial strain response can be divided into four stages: rapid hardening, stable, failure, and collapse. The four stages were coded as 1, 2, 3, and 4, respectively. The tests were terminated when the cyclic response entered the specified stage interval, so as to compare differences in sand-production behavior under different injection–production cycle durations. For the cyclic upper limit, four stress levels (95%, 90%, 85%, and 80%) were adopted to simulate conditions below the critical sand-production pressure and to evaluate the effect of different cyclic upper-pressure limits on sand production in UGS. Considering the influence of injection–production frequency during UGS operation, while keeping the loading path unchanged, the cyclic loading frequencies were set to 0.033 Hz, 0.017 Hz, 0.0033 Hz, and 0.001 Hz. Because the actual injection–production frequency in field operations cannot be fully replicated in the laboratory, these frequencies were varied to investigate the time-dependent effect of injection–production cycling on sand production.
A triangular stress waveform was adopted to represent the cyclic pressure variations associated with injection–production operations in underground gas storage. Field operation data indicate that the injection–production pressure typically varies from 18 to 38 MPa, corresponding to a fluctuation range (amplitude) of 20 MPa. Accordingly, the cyclic stress amplitude in the laboratory tests was set to 20 MPa, consistent with the pressure fluctuation range observed under actual operating conditions. The reference value for the upper confining-pressure limit was taken as the critical sand-production pressure. In total, 10 test groups were designed to investigate the effects of injection–production cycle duration, the cyclic upper stress level, and cyclic loading frequency on sand production in UGS.
The main experimental procedures were as follows: (1) The thick-walled cylinder core was placed on a 50 mm-diameter base. The upper platen, core, and lower platen were wrapped with a double layer of heat-shrink tubing and heated until fully shrunk and sealed, in order to prevent external silicone oil from entering the core’s interior. (2) The upper loading head of the GCTS RTR-1000 system was slowly lowered onto the top surface of the core, and an axial stress of 2 MPa was applied. The confining-pressure cell was then lowered to the bottom sealing ring, and silicone oil was injected into the cell to provide the required confining pressure. (3) Nitrogen was injected through the top inlet using a Dustec dual-piston displacement pump to establish a steady flow. Based on field data (effective reservoir thickness of 31.8 m, daily gas injection rate of 21 × 10 4 m 3/d per well, and a 7-inch wellbore diameter), the laboratory injection rate was determined following the calculation method proposed by Liao Wei, and 2 L/min was selected as the gas injection rate. (4) Confining-pressure cyclic loading tests were carried out according to the experimental design. After each test, the loose sand that had accumulated in the hole was collected, weighed using an electronic balance, and recorded as the produced sand mass. (5) After the tests, the particle-size distribution of the produced sand was measured using a Malvern laser particle size analyzer with a measurement range of 0.02–2000 µm. In addition, CT scanning was performed to reconstruct the hole’s inner wall and to characterize the hole damage and failure features.
The sand-production process and the evolution of cumulative produced sand mass for core samples with different IDs at various stages under cyclic loading is shown. For Cores #1 and #2, the hole geometry remained stable during the rapid-hardening and stable stages, and no noticeable damage or sand production was observed. For Core #3, the hole remained relatively intact in the early stages. In the failure stage, slab-like spalling appeared on the hole wall. Localized damage then propagated in the axial direction. Sand production started at this stage. Core #4 experienced a full progression from rapid hardening and stability to failure and ultimate collapse. With continued cycling, hole integrity progressively deteriorated, and symmetric compressive-failure zones formed along the hole axis. In the collapse stage, sand production from the failed zones intensified, and the damage evolution shifted from predominantly axial propagation to extension into the surrounding rock, with abundant fine loose particles detaching and accumulating in the hole. The cumulative sand-mass histogram indicates a sharp increase in sand production after entering the failure stage, whereas negligible amounts of sand production occurred during the early cycling period.
After the sand-production tests, the produced sand and the residual material remaining in the hole were collected. A sieve analysis was performed to characterize the produced sand, and particles were classified into coarse and fine fractions using a 1180 μm sieve. The particle-size distribution of the produced sand and the cumulative produced-sand mass varied markedly across the cyclic-loading stages. As the specimens evolved from the rapid-hardening stage to the collapse stage, the cumulative produced sand mass increased continuously, and the mass contributions of both coarse and fine fractions increased to different extents. In the early stage of sand production, the produced sand was dominated by coarse agglomerates, accounting for 79.17% of the total produced sand mass. After the specimens entered the collapse stage, the produced sand mass rapidly increased to 9.25 g, which is 3.24 times that in the failure stage, while the proportion of coarse sand decreased significantly. Combined with the visual observations, these results confirm that fine particles became the primary source of produced sand in the later stage. Comparisons between the two test stages indicate that the mass of coarse produced sand changed only slightly, whereas the overall variation in sand production was mainly governed by the fine-sand fraction.
In sieve analysis for formation-sand particle-size characterization, the largest sieve opening commonly used is 1180 μm. Particles larger than 1180 μm are typically not further subdivided into finer size classes. Therefore, in this study, 1180 μm was adopted as the threshold to distinguish fine and coarse fractions in the produced sand. In contrast, fine sand poses a higher risk of equipment plugging and erosional wear due to particle impingement, leading to more severe impacts on production performance and sand-control design. Accordingly, the production characteristics of fine sand during cyclic loading warrant particular attention. In the experiments, the separated fine-sand fraction was analyzed for particle-size distribution using a Malvern laser particle-size analyzer. The median particle size (D50) was 48.4 µm for Group #3 and 50.0 µm for Group #4. From the differential (frequency) distribution curves, the peak particle size of Group #4 is smaller than that of Group #3. From the cumulative volume distribution curves, the cumulative curve of Group #3 lies overall below that of Group #4, indicating a lower cumulative volume fraction for Group #3 at the same particle size. Moreover, Group #4 reaches 100% cumulative volume fraction more rapidly, suggesting that the cumulative contribution of smaller particles increases faster in Group #4 and that relatively finer particles account for a higher proportion of the produced sand. In addition, the cumulative curve of Group #3 reaches 100% only after extending into the coarser size range, implying that the produced sand from Group #3 contains a higher proportion of coarse particles compared with Group #4.
To investigate the influence of different cyclic-loading stages on hole fragmentation during sand production, CT scanning was performed on the core samples after the tests to capture the internal and external damage around the hole. The hole characteristics of the specimens after being subjected to different stages of cyclic loading is illustrated. The newly formed fractured area was quantified using ImageJ. Upon entering the failure stage, the newly fractured area reached 92.09 mm 2. At this stage, a V-shaped dog-ear failure pattern developed, with symmetric fractured zones formed along the axial direction; the damage was mainly concentrated in the middle-to-lower part of the hole depth, and the damaged area of the hole began to increase. In the collapse stage, the newly fractured area further increased to 173.60 mm 2. Extensive spalling occurred with substantial sand production, and the hole exhibited large areas of damage with pronounced axial extension; the fractured area expanded from the middle-to-lower section in the failure stage to nearly the top and bottom ends. In terms of the increase factor, the newly fractured area in the collapse stage was approximately 1.89 times that in the failure stage. Overall, as cyclic loading progressed from the rapid-hardening and stable stages to the failure and collapse stages, hole damage continuously intensified. Correspondingly, sand production evolved from negligible to limited and then to substantial, while the hole morphology transitioned from relatively intact to extensive damage accompanied by axial propagation.
The upper stress limit is an important parameter in cyclic loading. To clarify the sand-production behavior at different upper limits, sand-production tests were carried out in the collapse stage, where sand production was the most evident. The upper limits were set to 80%, 85%, 90%, and 95%. The cumulative sand mass increased clearly as the upper limit rose from 80% to 95%. At an upper limit of 95%, the cumulative sand mass increased sharply and was about 2.53 times that at 80%. In addition, the produced sand was still dominated by fine particles. With a higher upper limit, the fraction of fine sand increased, while the fraction of coarse sand decreased. The fine-sand fraction was ~59.20% at 80% and increased to 70.52% at 95%. These results indicate that a higher upper stress limit not only increases the amount of produced sand but also shifts the particle-size distribution toward finer particles.
The fine-sand fractions produced under different cyclic upper stress limits were analyzed by laser diffraction particle-size measurement. In the frequency distributions, the curves shift toward smaller particle sizes as the cyclic upper stress limit increases, indicating progressive particle-size refinement. At upper stress limits of 90% and 95%, the modal size range remains unchanged, whereas the peak magnitude differs; relative to the 90% group, the 95% group exhibits higher volume fractions over most of the finer-size domain. Overall, the produced sand becomes finer with increasing cyclic upper stress limits. In the cumulative volume distributions, a higher upper stress limit leads to a steeper rise in cumulative volume percentage. Consistently, the median particle size (D50) decreases from 127.6 μm at 80% to 113.6 μm at 85%, 92.4 μm at 90%, and approximately 87.8 μm at 95%, confirming a monotonic reduction in D50 with increasing cyclic upper stress limit.
CT scanning was employed to compare borehole fracture features under different cyclic upper stress limits. As the upper stress limit increased, the V-shaped (“dog-ear”) damage around the borehole expanded progressively. At an upper limit of 80%, the newly damaged area was 159.81 mm 2; this value increased slightly to 161.05 mm 2 at 85% and further to 173.61 mm 2 at 90%. A pronounced increase was observed at 95%, where the newly damaged area rose sharply to 264.26 mm 2, demonstrating a clear positive relationship between borehole damage area and the upper stress limit. Moreover, the radial fracture zones on both sides propagated deeper into the specimen with increasing stress level, indicating enhanced damage penetration and expansion of the damaged zone. The most substantial damage development, including the largest increment in the damaged area and the highest overall damage severity, occurred at the 95% upper stress limit.
In addition to the loading stage and cyclic upper stress limit, cycling frequency can also influence sand production under cyclic loading. In this study, an injection–production frequency of 0.033 Hz was adopted as the reference condition, while three lower frequencies (0.017 Hz, 0.0033 Hz, and 0.001 Hz) were applied to quantify the frequency effect. A clear trend is observed: the cumulative sand mass increases as the cycling frequency decreases. At 0.001 Hz, the cumulative sand mass is 53.1% higher than that under the reference condition. Fine sand dominates at all tested frequencies, implying that cyclic loading preferentially mobilizes small particles from weakly cemented zones or intergranular boundary regions. With decreasing frequency (i.e., longer loading time per cycle), the fine-sand fraction increases initially and then decreases. Notably, at 0.0033 Hz and lower frequencies, the fine-sand fraction becomes relatively stable, with markedly reduced fluctuations. This behavior suggests that when the loading time scale is sufficiently long, the granular framework tends to evolve toward a new quasi-stable failure state, thereby reducing the sensitivity of particle-size partitioning to further frequency changes.
Laser diffraction particle-size measurements were performed on the produced sand obtained under different cycling frequencies, and the particle-size frequency distribution and cumulative volume distribution curves are shown to evaluate the effect of cyclic frequency on particle-size distribution. The particle-size frequency distributions exhibit a systematic shift toward finer sizes as the cycling frequency decreases, indicating that low-frequency cyclic loading facilitates particle breakage and the generation of finer fragments. Among the four cases, the 0.033 Hz group shows the rightmost peak, corresponding to comparatively coarser particles. In contrast, the 0.001 Hz group displays a pronounced shift in the peak toward smaller sizes, accompanied by generally higher volume fractions in the fine-size region, which suggests a stronger breakage intensity. The cumulative volume curves further corroborate this trend: decreasing frequency results in a steeper rise in cumulative percentage, implying that a larger proportion of particles is concentrated in the fine-size range. Consistently, the median particle size (D50) decreases from 92.4 μm at 0.033 Hz to 89.9 μm at 0.017 Hz, 76.3 μm at 0.0033 Hz, and 72.5 μm at 0.001 Hz. Overall, D50 decreases monotonically with decreasing cycling frequency, confirming that low-frequency cyclic loading more readily induces sand-particle breakage and produces finer particles. These observations demonstrate that cycling frequency exerts a clear control on the breakage characteristics of produced sand. This enhanced breakage under low-frequency loading is likely associated with longer loading–unloading durations per cycle, which provides more time for grain rearrangement, intergranular sliding/crushing, and progressive damage accumulation within the granular framework.
CT imaging was used to characterize the evolution of borehole-related cracking under different cycling frequencies. Overall, cycling frequency exerts clear control on the extent of borehole damage, with a markedly larger crack-damaged zone developing under low-frequency loading. As the frequency decreases from 0.033 Hz to 0.001 Hz, the newly developed crack-damaged area around the borehole increases from 173.61 mm 2 to 293.49 mm 2, representing an increase of approximately 69%. This trend suggests that lower frequencies, which correspond to longer loading–unloading durations per cycle, provide more time for crack initiation and stable propagation, thereby enhancing cumulative damage.
Regarding damage morphology, at higher frequencies (0.033 Hz and 0.017 Hz), cracking remains largely confined to the near-borehole region and exhibits a relatively compact V-shaped (“dog-ear”) pattern. When the frequency decreases to 0.0033 Hz and 0.001 Hz, cracks propagate radially along both sides of the borehole, leading to pronounced expansion of the crack-damaged zone. In addition, the damage becomes more asymmetric and extends deeper into the specimen. These observations indicate that low-frequency cycling promotes more pronounced crack opening–closing and sustained stress concentration around the borehole, making local weak planes more susceptible to progressive failure under repeated tensile–shear loading. In summary, reducing cycling frequency increases the characteristic time scale of cyclic loading, which governs the rate of crack accumulation and the development path of the damage zone. Under low-frequency cycling, the longer time per cycle facilitates time-dependent deformation and progressive microstructural degradation, ultimately resulting in larger-scale borehole damage.
This study focuses on sand production induced by cyclic stress disturbances during injection–production cycling in underground gas storage (UGS). Laboratory sand-production tests under cyclic loading were conducted to systematically evaluate the effects of loading stage, upper stress limit, and cycling frequency on hole damage and sand-production behavior.
Depleted oil and gas reservoirs are attractive sites for storing green energy carriers. During injection–production cycling in underground gas storage (UGS), variations in effective stress can cause repeated stress disturbances, which may trigger borehole sand production. Laboratory sand-production simulation tests were conducted to evaluate the effects of cyclic-loading stage, upper stress limit, and cycling frequency on borehole damage and sand-production behavior. The results show that sand production is stage-dependent. During the rapid-hardening and stable stages, the borehole remains largely intact and sand production is negligible. Once the failure and collapse stages are reached, borehole integrity deteriorates and sand production increases sharply, with fine particles becoming dominant. Cumulative sand production increases with the upper stress limit. Increasing the upper limit from 80% to 95% leads to a 2.53-fold increase in produced sand mass, together with a higher fine-sand fraction and a shift in the particle-size distribution (PSD) toward smaller sizes. The cycling frequency also plays an important role. When the frequency decreases, cumulative sand production increases and becomes 53.1% higher than the baseline at 0.001 Hz. Meanwhile, the median particle size (D50) decreases, indicating stronger particle breakage under low-frequency cycling. These findings provide guidance for designing injection–production schemes for UGS and for selecting appropriate sand-control completion strategies.
Large-scale energy storage in subsurface space has become a key approach to enhance energy-system flexibility and security. Depleted oil and gas reservoirs are widely regarded as suitable geological media for natural gas storage due to their large effective storage capacity and good sealing integrity, as well as existing well patterns and surface pipeline infrastructure. Gas-reservoir-type underground gas storage (UGS) typically follows a seasonal injection–withdrawal schedule characterized by high-rate injection and high-rate production. Such alternating injection–production operations induce persistent pore-pressure fluctuations, thereby causing repeated variations in formation effective stress and near-wellbore stress paths. These repeated stress perturbations facilitate cyclic damage accumulation, pore-structure evolution, and progressive strength degradation in both the reservoir and the surrounding rock. Consequently, the near-wellbore zone becomes more susceptible to sand production, accompanied by engineering hazards such as plugging and erosion. Compared with conventional oil and gas reservoirs, UGS systems undergo multiple injection–production cycles throughout their service life, resulting in stronger and more frequent stress disturbances, leading to higher sanding risk, more severe sand production, and more complex underlying mechanisms. Therefore, investigating the failure evolution of reservoir rock and the associated sand-production responses under cyclic (alternating) loading is of both theoretical and engineering significance. Such understanding supports efficient UGS development, safe high-rate operation, optimization of injection–production schemes, and the design of sand-control completions.
With the large-scale construction and commissioning of underground gas storage (UGS) facilities, sand production has been increasingly reported in UGS sites both in China and worldwide. It has become a prominent engineering problem that compromises operational safety and efficiency. For example, sanding occurred during the operation of the Wen 96 UGS in China. As a result, 2–3 choke beans (choke nozzles) had to be replaced within a single gas-withdrawal cycle, and erosion failure of the control-valve trim (valve plug) was observed in 12 wells. After sand production occurred in the Hutubi underground gas storage (UGS) in Xinjiang, the daily gas production rate declined from 8.0 × 10 5 m 3 to 4.0 × 10 5 m 3, indicating substantial deliverability loss. Similar risks have also been reported in overseas underground gas storage (UGS) facilities. For example, a large amount of sand was produced near the end of the withdrawal period in the Redfield & Vincent UGS (USA), which triggered three wellsite fires. Sand erosion further caused tubing perforation, markedly increasing operational safety hazards. In the Hungarian underground gas storage (UGS) facility developed from the Hajdúszoboszló gas reservoir, the reservoir sanding risk exceeds 70%, posing a potential threat to long-term stable operation. Overall, sand production in underground gas storage (UGS) is widespread and highly detrimental. Systematic studies on its controlling factors are urgently needed to support optimization of injection–production schemes and the development of sand-control strategies. To address this issue, previous studies have examined cyclic load-induced damage and sanding-risk assessment. It is generally accepted that, under “strong injection–strong production” alternating operation in gas-reservoir-type UGS, periodic fluctuations in pore pressure and effective stress can markedly modify the near-wellbore stress path and induce cyclic damage. Consequently, UGS reservoirs exhibit a higher sand-production risk and more complex mechanisms than conventional oil and gas reservoirs. Kang evaluated carbonate-reservoir core samples from the Xiangguosi underground gas storage (UGS) by conducting stress-sensitivity and velocity-sensitivity tests, together with core-flow (core-flooding) experiments under UGS operating conditions. The results showed that pressure fluctuations are a key trigger for migration of fine sand in UGS reservoirs. Dong noted that long-period, high-rate alternating injection–production in UGS causes cyclic variations in stress, flow direction, and the gas–water interface. This makes sand-production prediction and mitigation more challenging. Sui performed alternating-load sanding tests and modified a sanding index by incorporating the damage level, number of loading cycles, and loading frequency. This enabled sanding-risk assessment and sanding-onset time prediction, providing support for sand-control completion and production-scheme design. Ma et al. conducted high-pressure sanding experiments and identified two critical thresholds, namely, an equivalent initial sanding pressure and the collapse pressure, which can be predicted by a thick-walled cylinder model. Staged changes in sand production as well as particle size were reported when the confining pressure exceeded the
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