An Overhead Receiver for Capture, Re-formation, Storage and Transportation of Gas Hydrates Produced from Deepwater Sediments
Authors: Zhang Hong-Quan , Brill James P. , Sarica Cem 
Published: July 2009

Gas hydrates bind immense amounts of methane in marine sediments. If produced cost effectively, they can serve as a stable energy supply. No viable technologies for extracting gas hydrates from deep ocean deposits have been developed to date. Due to the shallow thickness, low hydrate concentration, low permeability of the gas hydrate stability zone, lack of driving pressure and the slow melting process, low productivity is anticipated for gas production from gas hydrates in marine sediments. Therefore, high production rate and economical outcome can only be achieved with a large number of wells and low cost production technologies, which must be very different from the current high cost offshore oil and gas production technologies. The method of harvesting natural gas from deepwater hydrates presented in this paper is a combination of several production stages with a key equipment - overhead receiver. It includes using the overhead receiver to capture the produced gas, allowing gas to form hydrates again in the overhead receiver and accumulate, and transporting hydrates with the overhead receiver from the sea floor to surface or to a distant receiving terminal with a floating vessel. This approach uses gas hydrates as the medium for production, storage and transportation in order to exploit the nature of hydrates and the subsea pressure and temperature profiles. Consequently, it leads to a safe and economical production system which does not require subsea pipelines and risers. Analyses and calculations on the feasibility, and heat transfers in hydrate re-formation and dissociation are presented.

Introduction

Gas hydrates are ice-like crystalline solids formed from a mixture of water and natural gas, usually methane. They can occur in marine sediments as disseminated cements, nodules, veins and massive layers (Collett et al., 2000). It is common knowledge that the amount of methane bound in gas hydrates in marine sediments is enormous, although the estimates are speculative (Kvenvolden and Lorenson). If produced cost effectively, they can serve as a stable energy supply. There are three major challenges faced by future hydrate production from marine sediments. They include hydrate dissociation, flow assurance and cost efficiency. In order to produce gas from hydrates in marine sediments, gas must first be released and made flowable. Thermal energy is required for the dissociation of the hydrates. Three methods have been proposed in the past for hydrate dissociation, including thermal injection, depressurization and hydrate inhibitor injection. However, their applications are extremely difficult in the deep water environment. No viable technologies for extracting gas from deep ocean hydrates have been developed to date.

The second challenge is the flow assurance of gas produced from hydrates. The high pressure, low ambient temperature and existence of water on the sea floor are ideal conditions for possible hydrate re-formation and blockage of gas flow. Proper insulation, heating and/or inhibitor injection will be needed to avoid hydrate re-formation. These measures are either technically challenging or cost prohibitive.

The third and perhaps most important challenge is the cost efficiency of the overall production process. Due to the shallow thickness of the gas hydrate stability zone (GHSZ), low concentration of hydrates (normally several volumetric percentages), requirement of melting of the solid hydrates, and lack of driving pressure, low productivity is anticipated for gas production from gas hydrates in marine sediments. Current offshore oil and gas production technologies and facilities have been developed for high production rates from large oil or gas reservoirs. These methods will not be cost effective for gas production from hydrates in marine sediments.