Linlin Wang

Summary: With this study, we present experimental and simulation investigations into microannulus generation at the casing/cement interface, a primary cause of integrity loss in the casing/cement/formation system. For the experiments, we used a full-scale apparatus, utilizing fiber Bragg grating (FBG) sensors to measure cement sheath deformation and monitoring leakage at the interface. The results demonstrate that under cyclic loading, plastic strain in the cement sheath accumulates and leakage of interface initiates during the unloading phase of a specific cycle (beyond the first one). Leakage correlates strongly with this accumulated plastic strain. We developed a finite element method (FEM) analysis, incorporating a nonlinear kinematic hardening plastic model to capture the cement sheath’s cyclic plasticity and the interface damage, to simulate the mechanical response and damage during cyclic loading. The analysis reveals that the formation and propagation of microannulus results from the combined effects of cyclic plasticity of cement sheath and interface damage. Plastic deformation in the cement sheath induces tensile radial stress during unloading, and this tensile stress intensifies with loading cycles, progressively leading to the damage of the interface. Additionally, we assess the threshold width of the microannulus for the onset of leakage and discuss its influencing factors.

The mass and energy transfer between the burden layers and the hydrate layer will directly affect the gas production and reservoir mechanical response during the depressurization. At present, numerous studies have focused on the impact of the hydrate layer itself. To explore the effect of burden layers on hydrate exploitation, this paper predicts the gas production and seabed settlement using a coupled thermal-hydrological-mechanical-chemical simulator. The simulation results indicate that the hydrate decomposition edge reaches the burden layers (breakthrough), leading to a significant decrease in the gas production rate. This is primarily contributed to the fact that more fluid in the burden layers flows toward the wellbore, inhibiting the gas hydrate decomposition. The horizontal well should be located in the lower middle of the hydrate layer, which can effectively utilize the formation pressure without producing a breakthrough. However, the burden layers are subject to a pressure drop whether a breakthrough occurs or not, resulting in the burden layers' settlement. And, the burden layer settlement increases significantly with the decrease in the permeability. As a result, employing artificial impermeable barriers within burden layers to improve gas production may raise safety issues. In conclusion, this paper elucidates the mechanism of burden layers' influence on gas hydrate exploitation, which is of significance for the design and optimization of plans.

Summary: In the full life cycle of a well, thermal and mechanical loads may yield serious issues for the cement sheath integrity. However, the information for the integrity assessment, such as temperature and strain, is difficult to acquire underground. In this study, a full-scale experimental facility is used, allowing us to mimic the casing-cement sheath-formation (CCSF) system of a well. The system is monitored by fiber Bragg grating (FBG), enabling a real-time, high-accuracy, nondestructive measurement of temperature and strain inside the cement sheath in the sequence of setting and completion stage. Our observation reveals that the temperature of the cement sample cured in the mold is 22.3°C higher than the curing temperature; however, this temperature difference is not observed in the cement sheath cured in the CCSF system. This implies that the data obtained from the cement sample may overestimate the early-age performance of the cement sheath. Besides, the FBG measures a free strain of the tested cement during the hydration to be −370 με. This shrinkage can yield an internal stress in the CCSF system, which leads the cement sheath to swell circumferentially during the setting stage. During the completion stage, when the cement sheath is subjected to cyclic loading at three casing pressure levels, (i i.e., 10, 20, and 50 MPa), the maximum increment of circumferential strain reaches 160, 270, and 850 με, respectively. A plastic strain is observed for the 50 MPa pressure level, but not for the two other pressure levels (10 and 20 MPa). Unlike the observations in cyclic loading tests on cement samples, the plastic strain in the CCSF system accumulates linearly in the first 10 cycles and then increases slowly afterward. This difference is suggested to be attributed to the redistribution of internal stress along with the accumulation of plastic strain. Finally, the strains measured by the FBG are validated by the simulation, demonstrating the promising applicability of the FBG technology for monitoring the integrity of cement sheath.