Challenges in Exploiting Deepwater G
来源: Exploration & Production: The Oil & Gas Review    发布时间: 2011-06-09 16:54   2437 次浏览   大小:  16px  14px  12px
Challenges in Exploiting Deepwater Gas Reserves

Introduction

Historically, deepwater exploration has been primarily targeting oil plays. In recent years, deepwater exploration for gas has gained greater focus. This is spurred by projected greater global demand for gas due to increased awareness of gas being a ‘green’ and ‘clean’ fuel of choice for the future. Advances in deepwater production technologies (subsea multiphase systems) have enabled the development of fields located over 100km from shore in water depths greater than 1,000m. The number of liquefied natural gas (LNG) plants is expected to increase worldwide as companies take advantage of increasing gas usage by consuming nations. High oil and gas prices have led oil and gas companies to search for gas in deeper water and harsh environmental conditions in potential hydrocarbon- bearing basins that would have been left undrilled 10 years ago. The traditional mature areas of the world are also attracting interest from smaller independents as they pursue opportunities that do not form part of the core portfolio of large multinationals.

Concern with global warming has been heightened by scientific data that supports the theory that hydrocarbon emissions lead to greenhouse gases which may potentially have a serious impact on the world weather patterns. This is leading to a shift towards increased use of ‘green’ and ‘clean’ natural gas, which generates lower greenhouse gases. Focus is shifting from the traditional hunting grounds of the Gulf of Mexico (GoM) and the North Sea to areas in deep waters off West Africa, South America and the Barents Sea. In the future, floating LNG plants may be used in remote locations for exploitation of deepwater gas accumulations that would otherwise be considered a ‘stranded resource’. Deepwater gas developments pose significant challenges, some of which are highlighted here.

Challenges

Development of deepwater gas fields presents significant technical and commercial challenges. Frequently, discoveries are made in remote locations, and in inhospitable environments, which increases the development capital expenditure. Offshore processing can be prohibitively expensive. In extremely remote locations it may be the only option but, depending on the distance to shore, a subsea development may be the least expensive solution for gas field developments. For subsea developments, the multi-phase production pipeline to shore may provide the optimum development scheme. However, hydrocarbons and water drop-out in the production pipe (export) line has to be considered. The pipeline has to follow the seabed terrain and there may be low sections in the pipeline where liquids may accumulate.

Field development capital costs improve the economics if the gas is ‘wet’ (gas with associated hydrocarbon liquids). Associated liquids (condensate) with the gas may increase the overall economic rate of return as liquids are high yield and provide premium prices. Liquids are also easier to transport to markets. However, it means perhaps having a processing facility located in a remote location with either tanker loading for sale to markets or a liquids pipeline to shore. The liquid content has to be sufficiently high to make a significant impact on the economics. New offshore floating LNG plants are being considered for exploitation of deepwater gas discoveries where distance to shore makes the development cost-prohibitive.

Flow Assurance

Flow assurance issues are very important. Sand production can be an issue particularly with unconsolidated sandstone formations (use open hole gravel packs, sand screens, etc.). Hydrate formation in the flow lines has to be considered. This may be inhibited by injection of chemical inhibitors such as methanol, ethylene glycol or triethylene glycol. If there are any associated liquids (water and condensate), this may drop out in the flow line, further complicating flow assurance. Flow regime in the pipe has to be considered (stratified, slug, mist, annular, dispersed, laminar and turbulent).