Abstract
As we all know, development and utilization of clean energy is the only way for society to achieve its sustainable development. Although natural gas hydrates is a new type of clean energy, uncontrollable hydrate dissociation and accompanying methane leakage in drilling operation threaten drilling safety, as well as marine environment. However, the dissociation range of natural gas hydrates around wellbore cannot be reasonably determined in previous investigations, which may lead to the inaccurate estimation of borehole collapse and methane leakage. Then, the marine environment will be greatly damaged or affected. The purpose of the present work is to experimentally explore the dissociation characteristics of gas hydrates around wellbore in drilling operation and analyze the influence law and mechanism of various factors (such as hydrate saturation) on hydrate dissociation. It is expected to provide reference for exploring effective engineering measures to avoid the uncontrolled hydrate dissociation, borehole collapse and accompanying methane leakage. The experimental results demonstrate that acoustic velocity of hydrate-bearing sediment can be accurately expressed as quadratic polynomial of hydrate saturation, which is the theoretical basis for determination of hydrate saturation in subsequent experiments. Owing to the fact that hydrate dissociation is an endothermic reaction, hydrate dissociation gradually slows down in experiment. Throughout the experiment, the maximum dissociation rate at the beginning of the experiment is 8.69 times that at the end of the experiment. In addition, sensitivity analysis found that the increase in the stabilizer concentration in drilling fluid can inhibit hydrate dissociation more effectively than the increase in the hydrate saturation. Hydrate dissociation was completely inhibited when the concentration of soybean lecithin exceeds 0.60wt%, but hydrate dissociation definitely occurs in the near-wellbore region no matter what hydrate saturation is. In this way, based on the requirements of drilling safety and/or environment protection, hydrate dissociation and accompanying methane leakage can be controlled by designing and adjusting the stabilizer concentration in drilling fluid.
Highlights
Natural gas hydrates are ice-like cage crystals composed of host water molecules and guest natural gas molecules under low temperature and high pressure conditions (Sloan 2003; Ye et al 2018; Liu et al 2021)
An experimental apparatus used for determination of hydrate saturation by ultrasonic measurement was designed and assembled, and the influence of various factors on hydrate dissociation around wellbore during drilling in hydrate reservoir was investigated
In order to reduce drilling risk and marine environmental pollution, engineering recommendations to prevent uncontrollable hydrate dissociation and methane leakage are given according to the experimental results
Summary
Natural gas hydrates are ice-like cage crystals composed of host water molecules and guest natural gas molecules (methane commonly consists more than 95%) under low temperature and high pressure conditions (Sloan 2003; Ye et al 2018; Liu et al 2021). Gas hydrates are mainly buried in offshore sediments, and diffused in a small amount in permafrost, the total global reserves amount to 2.1×1016 m3 (Zhang et al 2021; Gambelli 2021). In addition to huge reserves, utilization of natural gas hydrates is environmentally friendly, the combustion products are almost only carbon dioxide (CO2) and water (H2O) (Misyura 2020). Own to the above-mentioned two aspects, natural gas hydrates have been attracting significant global attention, a series of offshore trial production activities have been performed in recent years (Zhu et al 2021; Sahu, Kumar and Sangwai, 2021). It is believed that with the gradual improvement of exploitation techniques, natural gas hydrates are likely to become a potential alternative energy source for oil and gas in the near future (Zhao et al 2019b; Wang et al 2021b)
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