Stability Analysis of Gas Hydrate Wellbore During Drilling Based on Thermal-Hydro-Mechanical-Chemical Coupling

ABSTRACT: During the drilling of hydrate-bearing formation, temperature and pressure differences between the reservoir and the wellbore cause the decomposition of hydrates around the wellbore. It reduces the mechanical properties of the reservoir and leads to wellbore instability. Most of the research primarily focuses on the stress disturbances around the well due to temperature and pressure changes, with relatively few studies addressing the stability of hydrate-bearing wellbores during the invasion of drilling fluids. This study investigates the mechanical response characteristics of hydrate-bearing formation during the drilling process, based on a dual-coupling platform for hydrate decomposition phase transition and geomechanics. The platform integrates the thermal–hydro– chemical module and the mechanical analysis module using TOUGH+HYDRATE and FLAC3D software, respectively. An interpolation algorithm is employed to transfer and call seepage field parameters and mechanical field parameters between the two software programs. This study considers changes in hydrate saturation, formation temperature, and pressure caused by drilling fluid invasion into the reservoir. Sensitivity analysis are conducted on the hydrate reservoir's response to drilling fluid density, temperature and formation permeability. Additionally, the wellbore collapse period for hydrate formations is predicted. The results show that invasion of drilling fluid into the hydrate formation causes a rapid rise in pore pressure around the wellbore. The decomposition of hydrate leads to a decrease in the mechanical properties of the formation, increasing the risk of wellbore collapse. With the increase of drilling fluid density and reservoir permeability, the decomposition of hydrate around the well accelerates. The period of wellbore collapse for the hydrate formation is 10.2 days under the basic case. 1. INTRODUCTION Natural gas hydrates can be considered as the future hydrocarbon energy (Kvenvolden, 1993; Wood and Jung, 2008; Sloan and Koh, 2007). China's sea areas hold about 80 billion tons of oil equivalent in natural gas hydrate resources, with 74.4 billion tons in the South China Sea. This makes it a key source of clean and efficient oil and gas replacement for the country. Currently, several countries are engaged in natural gas hydrate extraction, using methods such as depressurization, thermal stimulation, and chemical inhibition(Xu and Li, 2015; Moridis et al, 2011). However, safe and reliable drilling technology is essential for the successful exploration and development of these resources.