Research methods and devices for hydrate characteristics during oil and gas transportation: A review

Abstract

Due to the high-pressure and low-temperature exploitation environment, the characteristics of hydrates are directly related to the safety of pipeline transportation, which is an important research topic for deep-sea flow assurance. In this review, six kinds of extensively used experimental equipment and three types of hot computer simulation methods, which are employed to explore the hydrate characteristics under deep-sea conditions, are comprehensively summarized, covering micro to macro research scales. The experimental equipment includes rotational rheometer, flow loop, high-pressure reactor, differential scanning calorimeter (DSC), micromechanical force (MMF) testing apparatus and microscopic morphology observation (MMO) device. The computer simulation methods involve numerical simulation, molecular dynamics (MD) simulation, Monte Carlo (MC) simulation and first-principles calculation. Their advantages and disadvantages are compared in detail, and their basic principles, main applications and the latest research progress are introduced. Some suggestions for future research methods are also provided. This work aims to help readers quickly grasp the characteristics of the most used research methods, choose suitable methods for their study and further expand these methods, so as to advance the development in hydrate research area.

1. Introduction

Hydrogen-bonded water molecules can form various types of cages (512, 51262, 51264, 51268, 435663, etc.), and guest molecules (such as methane, propane, cyclopentane, etc.) with appropriate size may be captured in these cages. In this way, the combination of specific numbers and types of cages forms natural gas hydrates, which can be mainly divided into three types: structure I, II and H [1]. Hydrates are ice-like compounds, and thus have ice-like properties: once formed in pipelines, they can develop into 'ice blocks' and ultimately lead to pipe plugging [2]. Partial or full obstructions in pipelines can induce pipeline vibrations, abnormal pressures and mechanical damages [3]. Turner [4] proposed the mechanism of hydrate plugging. Specifically, water is dispersed in the oil phase under the shear action of turbulent flow, pipe walls, pumps, etc. Hydrates nucleate and grow in high-pressure and low-temperature pipeline transportation environments. Then, the growing particles adhere together due to liquid or solid bridge forces during collision, resulting in hydrate aggregation. Subsequently, the hydrate aggregates gradually deposit at the bottom and top of the pipeline under the influence of gravity and wall adhesive force. Ultimately, they develop into a partial pipe blockage or total pipe plugging.

In order to prevent hydrate plugging, many physical and chemical methods have been adopted, each with its own advantages and disadvantages. Since the hydrate cold flow technology is put forward, the development of low-cost and environment-friendly low-dose hydrate inhibitors (LDHIs) has become a research hotspot. Therefore, getting knowledge of the characteristics of hydrate slurry is helpful to design and evaluate better hydrate inhibitors, control risks, ensure safe transportation and support its development as a future energy source, separation technology [5] and cold storage technology [6].

Currently, researchers and engineers in the field of hydrate flow assurance have conducted extensive research on a laboratory scale. Heretofore, the reported review articles on hydrates mainly focus on the research process of one kind of hydrates (e.g., CO2 hydrates [7]), hydrate kinetics [8,9], thermodynamics [8,9], rheology [10], interfacial behavior [2,11], application [6,9], hydrate inhibitors [12e15], and a certain research method [16]. However, there is still no comprehensive review that integrates the experimental and computational research methods specifically. In view of this, a comprehensive summary on the basic principles, pros and cons, and applications of six widely used laboratory scale experimental equipment and three types of computer simulation technology is conducted in this work. It will facilitate readers to quickly read and understand the state-of-the-art methods in hydrate research.