Search

On the basis of "Carbon Peak and Carbon Neutral" goals, carbon sequestration projects are increasing in China. The integrity of cement sheaths, as an important factor affecting carbon sequestration projects, has also received more attention and research. When CO2 is injected into the subsurface from sequestration wells, the cement sheath may mechanically fail due to the pressure accumulated inside the casing, which leads to the sealing of the cement sheath failing. The elasticity and strength parameters of the cement sheath are considered in this paper. The critical bottom-hole injection pressures of inclined well sections under anisotropic formation stresses at different depths were calculated for actual carbon-sealing wells in the X block—the CO2 sequestration target block. The sensitivity factors of the critical bottom-hole injection pressure were also analyzed. It was found that the cement sheath damage criterion was tensile damage. The Young's modulus and tensile strength of the cement sheath are the main factors affecting the mechanical failure of the cement sheath, with Poisson's ratio having the second highest influence. An increase in the Young's modulus, Poisson's ratio, and tensile strength of the cement sheath can help to improve the mechanical stability of cement sheaths in CO2 sequestration wells. This model can be used for the design and evaluation of cement in carbon sequestration wells. [ABSTRACT FROM AUTHOR]

In order to avoid the occurrence of caprock integrity damage and gas escape due to injection pressure overrun in CO2 sequestration, an optimized back propagation (BP) neural network model based on Monte Carlo simulation (MCS) and an improved lion swarm optimization (ILSO) algorithm is proposed for the maximum sustainable injection pressure prediction. Firstly, a hydromechanical model is constructed to simulate the damage changes of the reservoir caprock during injection by ABAQUS. Secondly, in view of the uncertainties of formation parameters that could lead to deviations between the model calculation results and actual geological condition, the MCS method is used, and then, the probability distribution interval of the maximum injection pressure with high probability of caprock failure under different formation parameters is obtained by MATLAB. This effectively reduces the uncertainty and improves the calculation accuracy. Finally, based on the numerical simulation results, the maximum injection pressure prediction model is constructed. Aiming at the problems of the sensitivity of the BP neural network to initial weights and its poor convergence, tent chaotic mapping and difference mechanism are introduced to improve the LSO algorithm. Following this, the neural network is optimized by ILSO algorithm whose superiority is verified through 8 benchmark functions, and the maximum injection pressure is effectively predicted according to porosity, permeability, and other parameters. Experimental results show that, compared to the other three optimization methods, the ILSO_BP model has a faster convergence speed, higher prediction accuracy, and stability, which can provide a powerful guide for the safe injection of CO2 and efficient sequestration management. [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 creep behavior is one of the most important topics on the stability analysis of rock mass in underground engineering. In order to characterize the creep deformation of clayey rock more accurately, this study proposes a nonlinear elasto‐viscoplastic (EVP) model based on a new yield surface and a modified Mohr‐Coulomb (MC) flow potential. First, the finite element (FE) formulation for the constitutive creep behavior is derived and a user subroutine to define the material's mechanical behavior (UMAT) is implemented in the commercial finite element package ABAQUS. The numerical simulation is validated against the triaxial creep tests of a cylinder, and the results show that the proposed model can effectively predict the creep responses of clayey rock and thus compensate for the deficiency of the creep model in ABAQUS. Then, the proposed model is used to study the long‐term stability of clayey rock formation surrounding an underground experimental site. Based on the results from triaxial creep tests and in situ monitoring of the lining deformation, the time‐dependent material creep parameters are obtained for rock by using the back analysis method. The predicted lining deformation is in good agreement with the field data with most of the relative errors less than 3.5%. The creep zone in the surrounding rock demonstrates that the horizontal scope is larger than the vertical one. The proposed model is proven to provide accurate prediction on the creep characteristics of clayey rock, showing a potential to assess the long‐term stability and reliability of large‐scale underground engineering structures. [ABSTRACT FROM AUTHOR]