Applicability research of thermodynamic models of gas hydrate phase equilibrium based on different equations of state.

Choosing an appropriate equation of state and thermodynamic model is very important for predicting the phase equilibrium of a gas hydrate. This study is based on statistical thermodynamics, considering the changes in water activity caused by gas dissolution, and deriving and summarizing four thermodynamic models. Based on the 150 collected experimental data points, the accuracy of the four thermodynamic models in predicting the phase equilibrium of methane hydrate, ethane hydrate, and carbon dioxide hydrate were compared. In addition, the influence of five equations of state on each thermodynamic model's phase equilibrium prediction accuracy is compared. The analysis results show that in the temperature range of 273.40-290.15 K, the Chen-Guo model is better than other thermodynamic models in predicting the phase equilibrium of methane hydrate by using the Patel-Teja equation of state. However, in the temperature range of 290.15-303.48 K, the John-Holder model predicts that the phase equilibrium of methane hydrate will perform better. In the temperature range of 273.44-283.09 K, the John-Holder model uses the Peng-Robinson state to predict the phase equilibrium of carbon dioxide hydrate with the highest accuracy. In the temperature range of 273.68 K to 287.6 K, the Chen-Guo model is selected to predict the phase equilibrium of ethane hydrate with the highest accuracy. However, as the temperature increases, the predicted values of the vdW-P model and the Parrish-Prausnitz model deviate further from the experimental values.

Numerical Simulations of Decomposition of Hydrate Particles in Flowing Water Considering the Coupling of Intrinsic Kinetics with Mass and Heat Transfer Rates.

During the hydrate exploitation in a shallow marine layer by the mechanical crushing, the hydrate particle decomposition in a wellbore is one of the most concerning problems. In this research, a hydrate dynamic decomposition model coupling intrinsic kinetics with mass and heat transfer rates was established. The model can simulate the hydrate particle decomposition process in flowing water. By comparison, the model calculated results are in good agreement with the measured values. The numerical simulation results show that hydrate decomposition is a non-isothermal process. In the early stage, the hydrate decomposition rate mainly depends on the heat transfer rate. However, it is mainly affected by the hydrate intrinsic kinetics in the late stage. In contrast, the mass transfer rate has little effect on it during the whole decomposition process. By analyzing the influence of sensitivity parameters, it can be found that the activation energy has an important impact on the hydrate decomposition rate, and the hydrate decomposition rate constant decreases significantly at E/R > 9000 K. Increasing the water flowing rate is beneficial to the dissolution of hydrates. System temperature and pressure are two significant factors that directly affect the hydrate decomposition rate, and increasing the temperature or reducing the pressure can effectively increase the hydrate decomposition rate.

Sealing Failure Mechanism and Control Method for Cement Sheath during Hydraulic Fracturing.

This study focused on the sealing failure mechanism and control method for a cement sheath during hydraulic fracturing. Taking a shale gas well as an example, whole wellbore numerical models of the casing-cement sheath-formation assembly were established, failure modes of the cement sheath at different depths were clarified, and control methods were proposed based on the calculation results. The following conclusions were drawn. (1) The maximum radial/tangential stress of the cement sheath increased/decreased with an increase in the depth, and the cement sheath above the intermediate casing shoe posed the risk of tangential tensile failure, resulting in tensile cracks. The cement sheath below the intermediate casing shoe produced a micro-annulus under a cyclic casing pressure, and the tensile cracks and micro-annulus constituted passages for the sustained casing pressure. (2) The swelling stress of the expansion cement slurry could offset the circumferential tensile stress and increase the radial compressive stress. Because a cement sheath with a high Young's modulus usually exhibits high tensile and compressive strengths, it is recommended to use a high Young's modulus cement slurry system above the intermediate casing shoe and optimize the free expansion ratio. (3) In comparison with ordinary cement stone, low residual strain cement stone exhibited a larger elastic deformation interval. The cumulative residual strain caused by cyclic loading was smaller, and the Young's modulus demonstrated a lesser decrease. The results of an equivalent physical experiment demonstrated that an ordinary cement sheath lost its integrity after 13 loading cycles with a maximum casing pressure of 60 MPa. A low residual strain cement sheath could guarantee integrity after 30 loading cycles when the maximum casing pressure was 90 MPa, and sealing failure occurred after 11 loading cycles when the maximum casing pressure was 130 MPa. It is recommended to use a low residual strain cement slurry below the intermediate casing shoe to prevent a micro-annulus.