Sand–rubber mixtures (SRMs) consisting of stiff sand particles and soft rubber particles are typical binary mixture materials that possess a variety of complicated properties. The complexity of the properties of sand–rubber mixtures is increased when complex stress path is involved. This study investigates the mechanical behavior of sand–rubber mixtures under generalized loading conditions using the discrete element method. A series of numerical true triaxial shear tests were conducted on pure sand and sand–rubber mixtures. The effect of rubber content and loading path on both of the macroscopic and microscopic performances of sand–rubber mixtures was investigated, and the associated microscale mechanism was also discussed. Numerical simulations show that the relationship between the peak friction angle ${\varphi }_p$ and the intermediate principal stress ratio $b$ is influenced by the addition of rubber particles, and a suggested explanation of this phenomenon is that the rubber particles mainly affect the inherent stability of the strong network. Particle-scale observations, including the coordinate number, the proportion of strong contacts, and the fabric anisotropy, are also presented in this study. Microscopic results confirm the explanation above, and explore the force transmission characteristics of sand–rubber mixtures under generalized loading conditions. This research can provide a reference for the constitutive model development of sand–rubber mixtures.

The good sealing caprocks are significant for the integrity of underground gas storage (UGS) in depleted natural gas reservoirs. The screening of parameters, weight assignment, and evaluation method are important for evaluating the sealing performance of caprocks. Many factors can affect the sealing performance of caprocks, including caprock thickness, lithology, brittleness, porosity and permeability, breakthrough pressure, etc. In this paper, the dominant factors in the sealing performance of caprocks in UGSs are systematically analyzed, and the weights of these factors are analyzed by the analytic hierarchy process (AHP). The fuzzy comprehensive evaluation method (FCEM) is applied in the sealing evaluation of caprocks in three typical underground gas reservoirs (i.e., Zhujiadun, Xu-2, and Xing-9) in China. The sandstone reservoir in the Zhujiadun gas field is only about 20 m, and the thickness of the overlying mudstone is about 600 m. The caprock of the Xu-2 gas reservoir in Zhongba gas field is well distributed and developed, and the breakthrough pressure is relatively large. The caprock of Xing-9 gas field is mudstone with a thickness of over 400 m. The results show that the breakthrough pressure and permeability are the key parameters affecting the sealing ability of caprocks, with weights of 0.4291 and 0.2157, respectively. Among these three examples of gas fields, the sealing performance of caprocks in Zhujiadun gas storage is the best. The evaluation procedure and methods proposed in this paper are valuable for the evaluation of the tightness of caprocks in depleted gas reservoirs.

The underground gas storage (UGS) in depleted sandstone reservoirs forms the largest proportion of the UGS market in China. Multiple cycles of natural gas injection and production in the sandstone cause the rapid increase and drawdown of pore pressure, which may induce damage to the rock skeleton structure, and cause complex fluid flow paths in the sandstone reservoir. In this paper, transverse relaxation time (T₂), nuclear magnetism resonance imaging, and high-pressure mercury intrusion analysis are combined to evaluate the variation in pore structure of medium-grained sandstone. The results show that cyclic injection and production of fluid leads to a slight increase in total pore volume, indicating that weak damage to rocks occurs. The T₂ spectrum at the low pore pressure (10 MPa) and high pore pressure (25 MPa) both show that the shrinkage of the medium-size pores occurs after multiple cycles of injection and production. The pore volume of large-size pores was not highly correlated with the number of cycles. With the increase in pore pressure, the pore volume ratio under high pore pressure increased with the number of cycles, while it fluctuated strongly under low pore pressure.

The structure of caprocks is often greatly altered by different scales of faults or fissures in long-term geological tectonic evolution, and the sealing performance may be deteriorated. In this paper, a simplified geological model characterized as multilayered sandstone and fault-bearing caprocks extracted from the Shuang 6 underground gas storage located in the Liaohe oilfield was established. Different fault geometry (e.g., fault length, fault dip angle, and fault type) and seepage attributes (porosity and permeability) were considered to illustrate their impacts on natural gas migration during the cyclic high rate of injection and production of natural gas. The results showed that the seepage anisotropy and the natural gas front are strongly affected by the formation properties and, especially, are hindered by the low permeability sandstone layers. The difference in the lateral migration distance of natural gas in different layers can reach 110 m at the end of the injection period, with an annual injection volume of 10⁸ m³. The migration of natural gas along the fault zone is mainly controlled by the permeability of faults, followed by fault scale, fault dip angle, and fault type. The sealing failure of caprocks in the fault zone does not occur based on the simulated gas migration distribution, showing that a very limited amount of natural gas migrates into the caprocks.

Recycled concrete aggregate (RCA) is a typical construction and demolition (C&D) material generated in civil engineering activities and has been widely used as the coarse-grained filler added to sand for roadbed fillings. The effect of RCA content on the mechanical behavior of sand–RCA mixtures is complicated and still not fully understood. To explore the effect of RCA content on the macroscale and microscopic behavior of the sand–RCA mixtures with various RCA contents, laboratory direct shear tests and numerical simulations using the 3D discrete element method were performed. Experimental direct shear tests on sand–RCA mixtures with different contents of RCA were first carried out. Numerical direct shear models were then established to represent the experimental results. The particle shape effect was also considered using a new realistic shape modeling method to model the RCA particles. Good agreement was observed between the DEM simulation and experimental results, verifying the ability of the numerical direct shear models to represent the direct shear behavior of sand–RCA mixtures. The macroscopic responses of both experimental and numerical tests showed that all samples presented an initial hardening followed by a post-peak strain softening. The peak-state friction angles increased with the RCA content for samples under the same vertical stress. The effect of RCA content on the microscopic behavior based on DEM simulation was also found. The microscopic properties of RCA–sand mixtures, such as coordination numbers, PDFs and contact force transformation features, were analyzed and related to the macroscopic results.

The Daniudi gas field is located in the Ordos Basin’s northern section of the Yishan slope. The intertidal–subtidal depositional environment dominates the lithology of the Ma55 sub-member, resulting in a stable, thick-layered dark gray–gray–black limestone and lime dolomite. The stratum is stable laterally as well as dolomite, with an average thickness of 26.8 m. Fractures, dissolution expansion pores, and inter-crystalline dissolved pores are the primary reservoir space kinds, with a minor number of karst caves and fractures generated as well. The main distribution ranges for porosity and permeability are 1–8 percent and 0.01–1 mD, respectively. Low porosity, tightness, and ultra-low permeability are common characteristics, and a single well typically has no natural productivity. Production stimulation technologies like pre-fluid acid fracturing, compound sand addition acid fracturing, and multi-stage injection + temporary plugging volumetric acid fracturing have been gradually optimized using the horizontal well development method, and breakthroughs in the development of tight and low-permeability carbonate rock reservoirs have been made. However, the conditions of different types of reservoirs are quite different, and the acid fracturing process is not matched and imperfect, resulting in large differences in the productivity of different horizontal wells after fracturing, as well as a high proportion of low-yield wells, which cannot meet the needs of cost-effective and effective development of this type of gas reservoir. In light of the aforementioned issues, a series of laboratory tests have been carried out to explore the stimulation effects of acid fracturing on different types of reservoirs and to optimize the acid fracturing process in the Daniudi gas field. The results show that the rock mechanical performances and the acid etching conductivities of the rock specimens are related to the types of reservoirs. The rock mechanical properties can be deteriorated after acidizing, but different types of reservoirs have different degrees of deterioration. According to the results of acid etching conductivity of different types of reservoirs, conductivities obtained by high and low viscosity and cross-link-gelled acid (two stage injection) processes are higher than those of high viscosity systems. The experimental results of process suitability suggest adopting high and low viscidity acid systems for pore type and fracture-dissolved pore type reservoirs, and cross-linked acid systems for fracture-pore type reservoirs. The findings of this study can help form a better understanding of the performance of different types of reservoirs under the various acidified conditions that can be used for the optimization of acid fracturing processes in carbonate formations.