Stratum Settlement during Depressurization of Horizontal Wells in Gas Hydrate Reservoirs

The shallow burial of gas hydrates results in stratum settlement owing to pore pressure reduction and hydrate dissociation during depressurization. To understand this behavior in hydrate-bearing stratum, a numerical model for determining the nonlinear mechanical constitutive relation of hydrate-bearing sediments was developed based on the physical and mechanical properties of hydrate-bearing sediments. The model was compared with an ideal elastic-plastic model on the core scale. Furthermore, a model for stratum settlement during depressurization in horizontal wells in hydrate-bearing strata was also established. Variance and range analyses were conducted using an orthogonal design to explore the kftd/iid influences of various factors on the stratum settlement. The results show that when compared with the ideal elastic-plastic model, the established numerical model presents a higher accuracy when characterizing the deformation behaviors of hydrate-bearing sediments at different stress states. During exploitation from vertical wells, the stratum settlement center corresponds to the wellhead. However, in the horizontal well design, the upper strata of horizontal well sections also settle apart from the zones around the wellhead. The wellhead settlement of horizontal wells is attributed to the load-bearing effect of the upper strata and the compaction effect of the middle strata. The load on the wellhead decisively influences the wellhead settlement, which is positively correlated with the drawdown pressure, production time, and build angle rate but shows no obvious correlation with the length of horizontal wells. The drawdown pressure exerts the greatest influence on the settlement of the upper strata in horizontal well sections, and the influences of the production time and the length of the horizontal wells successively follow this.

Automated summary

The numerical model developed for hydrate-bearing sediments accurately characterizes the deformation behaviors of sediments under different stress states compared to the ideal elastic-plastic model. Orthogonal design variance and range analyses show that the load-bearing effect of the upper strata and the compaction effect of the middle strata have decisive influences on wellhead settlement in horizontal wells. The drawdown pressure exerts the greatest influence on the settlement of the upper strata in horizontal well sections.