How methane release may affect carbon dioxide storage during replacement processes in natural gas hydrate reservoirs

Abstract

Methane replacement with carbon dioxide molecules in natural gas hydrate reservoirs mainly occurs in two different methods. The first is based on initial CH 4 hydrate dissociation and a following CO 2 hydrate formation, with all related problems for sediment deformation and so on. With the second method, the direct CO 2 /CH 4 exchange occurs in already present cages, thus avoiding environmental problems and ensuring an exchange ratio close to 1. Because of practical difficulties in perfectly applying the second method, the initial methane release may produce a binary gaseous phase over the hydrate sediment, whose concentration may affect hydrate formation, till favouring CH 4 hydrate re-formation rather than CO 2 hydrate formation. The present experimental work analysed such aspect, by showing how that concentration may modify the formation trend, mainly in terms of carbon dioxide storage, and also proving the importance of thermodynamic conditions in this sense. • Six hydrate formation experiments were made with a binary gaseous mixture containing methane and carbon dioxide. • Each test started with an initial CH 4 /CO 2 concentration: from 91.7/8.3 vol% to 29.6/70.4 vol%. • Results proved that gaseous phase composition has the same importance of thermodynamic conditions during replacement. • CH 4 concentration ranged from 29.6 to 91.7 vol% in the gaseous phase, while it was about 3.8 – 81.5 vol% into hydrates. • CO 2 concentration ranged from 8.3 to 70.4 vol% in the initial gaseous phase; it was equal to 11.7 – 50 vol % into hydrate. • The overall amount of CO 2 stored was about 44.7–82.3 % of the total amount involved in the process.