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The safety of aquifer gas storage (AGS) depends on the mechanical stability of the caprock to a great extent. The injection and production operation of AGS will cause the disturbance of the in-situ stress due to the change of pore pressure in the reservoir and caprock, which may lead to the failure of caprock. The con AGS is established based on geological structure and hydrogeological information. By using the finite element method, the factors such as boundary condition, properties of reservoir and caprock, and in-situ stress coefficient are considered, respectively, and the variation of formation pressure and disturbed stress in the process of injection and production are analyzed. The results show that the boundary conditions are of significant influence on the stress field. The shear failure risk of reservoir–caprock interface under closed boundary is the greatest. The in-situ stress coefficient has the greatest influence on the failure risk of the caprock, followed by the permeability of the reservoir and caprock. [ABSTRACT FROM AUTHOR]

The gas sealing capacity of caprock (SCC) is one of the key factors that determine whether aquifer trap can be constructed into underground gas storage (UGS). However, no standard protocol for evaluating SCC of candidate aquifers has been proposed. Based on the core observation, laboratory experiment, and well logging data, the sealing capacity of the target aquifer caprock of Permian mudstone in D5 block of Litan sag, China, is quantitatively evaluated. The important parameters of mineral brittleness, permeability, breakthrough pressure (BP), mechanical brittleness, thickness, and areal extent that affect the SCC are determined. The results of specific tests and data statistics show that the caprock of D5 block is a low permeability rock with a permeability of 10−4 mD, and the BP of undisturbed rock is greater than 38 MPa. Although the brittle mineral quartz is abundant with an average of 38.38%, the mechanical brittleness is not strong under formation conditions. The direct caprock has a thickness of greater than 50 m, and on the top of it is a high-quality indirect caprock that complements the physical closure. The results of a mathematical evaluation model show that except for the sealing index of sample 2, all the other samples have optimal sealing capacity. The field interference test shows that the optimal sealing capacity of the caprock meets the requirements of the construction of underground gas storage (UGS). The rationality of the comprehensive evaluation model can provide a reference for similar evaluation projects in the future. [ABSTRACT FROM AUTHOR]

The evaluation of caprocks' sealing capacity is exceedingly important for depleted gas reservoirs to be reconstructed into gas storage. In this paper, based on the physical sealing mechanism of caprock, four aspects of ten indexes of caprock quality evaluation were firstly selected, and the related classification standards were established. Secondly, based on the rock mechanical sealing mechanism, elastic and plastic indexes were selected to characterize the mechanical brittleness of caprock, and a brittleness evaluation method of caprock based on complete stress-strain curves was established. Then, a systematic comprehensive evaluation model (including 5 aspects and 12 evaluation indexes) for the sealing capacity of gas storage caprock was proposed, and the analytic hierarchy process (AHP) was used to determine the weight of the 12 indexes in the evaluation model, and the formula for calculating the suitability of the caprock sealing capacity was established. Finally, the geological data, laboratory, and field test data, including X-ray diffraction, poro-permeability test, displacement pressure, and tri-axial compression test, were used for the caprock sealing capacity evaluation of the X9 depleted gas reservoir, and the result from this model showed that the caprock quality is suitable for underground gas storage. [ABSTRACT FROM AUTHOR]

To deal with the pressure in the closed A-annular space caused by temperature and pressure fluctuations during injection and production of gas storage based on the heat-transfer theory, the elastic plane strain theory of tubular strings and the volume compatibility principle, a multi-unit pressure calculation model of annular A considering the non-linear properties of annulus protection fluid was established. Based on this subsection calculation method, the effect of isothermal compression coefficient and isobaslc expansion coefficient of the annular protection fluid, the linear expansion coefficient, elastic modulus and Poisson's ratio of tubing on the annulus pressure were analyzed. The results show that the accuracy of annular pressure prediction can be improved by using subsection computing method. To estimate the annulus pressure without considering the temperature distribution will cause the result to deviate from the actual value, and the higher the temperature, the greater the deviation. The nonlinear property of annulus protection fluid has a relatively great impact on annulus pressure, ignoring the change of its properties with temperature will lower the annulus pressure. The pressure caiculation model is in good agreement with the on-site monitoring value, which can provide reference for engineering application. [ABSTRACT FROM AUTHOR]

The rapidly increasing demand for the consumption of natural gas has attracted the interests to store natural gas in aquifer reservoir. However, natural gas injected into the aquifer reservoir, which could cause ground surface deformation and mechanical integrity destruction of caprock. Taking the aquifer gas storage of S trap as the research object, according to the geological structure and hydrogeological information, a coupling large-scale hydromechanical model is established to evaluate the damage risk of the gas reservoir in S aquifer. The proposed methodology is based on the development of fluid-solid coupling and application of FEM. The different failure mechanisms of S aquifer gas storage caprock can be evaluated on the basis of the tensile failure criterion and Mohr-Coulomb shear failure criterion. To analyze the change of caprock in gas injection and production process more clearly, a reference model is defined as an ideal calculation condition to discuss the mechanical response, pore pressure variation, and surface deformation characteristics of the caprock during injection and production. On this basis, the second scheme of sensitivity analysis is defined. The pressure injection rate, reservoir parameters, in situ stress, and other factors are considered, respectively, and the influence of different input parameters on mechanical stability and surface deformation of caprock is analyzed. Finally, the mechanical stability is analyzed and combined the above two criteria to predict the upper limit injection pressure of S. Simulation results show that the permeability and in situ stress have a significant influence on ground surface deformation and mechanical integrity of caprock, but Young's modulus and Poisson's ratio can be ignored; the upper limit pressure coefficient of S is 1.908. [ABSTRACT FROM AUTHOR]