Study on Hydrate Risk in the Water Drainage Pipeline for Offshore Natural Gas Hydrate Pilot Production

Yu Guimin,Kong Qingwen,Jin Hao

doi:10.3389/feart.2021.816873

Abstract

Hydrate plug in the drainage line is a serious flow assurance problem for the pilot production of offshore natural gas hydrates. Current research focuses on hydrate deposition in the annular flow and the oil-dominated system. The multiphase flow system in the drainage line is a water-dominated system which is normally a bubbly flow. In this work, a new model is developed to study the temperature and pressure field in the drainage line considering that the flow pattern is bubbly flow. Combining with the methane hydrate phase equilibrium curve, the hydrate formation region in the drainage line can be established. The hydrate formation region is enlarged with the ESP pressure increasing and the water production rate decreasing, since the ESP can supply extra pressures in the drainage line and the heat transfer phenomenon is enhanced between the drainage line and environment under the low water production rate condition. The model pointed out that the risk of hydrate formation rises up as the hydrate concentration increases beyond 6%. This study can lay a theoretical foundation for the efficient prevention of gas hydrates in the drainage line during offshore natural gas hydrate pilot or long-term production.

Highlights

Natural gas hydrate is a kind of new energy resource which compensates the lacks of onshore crude oil and natural gas
The hydrate formation region and the hydrate risk in the drainage line are evaluated by the developed model
When the hydrate concentration is beyond 6%, the outflow pressures are decreasing dramatically for all water production rate conditions

Summary

Introduction

Natural gas hydrate is a kind of new energy resource which compensates the lacks of onshore crude oil and natural gas. In South China Sea, natural gas hydrates are located in the region where water depth is about 1000 m and local temperature is close to 4°C (Fu et al, 2020a; Sun et al, 2020). China has conducted two successful pilot productions of natural gas hydrate in the Shenhu area of South China Sea. The production rates of natural gas hydrates for two pilot production have reached 5000 m3/d and 2.87 × 104 m3/d respectively (Xu et al, 2017). Since decomposition of natural gas hydrates results in large water production rate in the well, the down hole separation technology is utilized in the natural gas hydrate well. Since the down hole separation cannot complete separate gas and liquid, some parts of gas may be entrained into the drainage line, as shown Figure 1

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