This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper OTC 18087, "Environmentally Friendly CO2 Storage in Hydrate Reservoirs Benefits From Associated Spontaneous Methane Production," by A. Graue, SPE, and B. Kvamme, U. of Bergen; B.A. Baldwin, Green Country Petrophysics LLC; J. Stevens, SPE, and J. Howard, SPE, ConocoPhillips; E. Aspenes, SPE, G. Ersland, SPE, and J. Husebo, U. of Bergen; and D. Zornes, SPE, ConocoPhillips, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May.
Carbon dioxide (CO2) sequestration in natural-gas-hydrate reservoirs may offer stable long-term storage of a greenhouse gas with the additional benefit of methane (CH4) production. The sequestration process may thus offer large amounts of energy for the future while providing a more thermodynamically stable gas hydrate that is less sensitive to global temperature changes.
CO2 storage in natural-gas-hydrate reservoirs by replacing the CH4 in the hydrate with CO2 has attractive potential because it would produce free natural gas and establish a more thermodynamically stable hydrate accumulation. Natural-gas hydrates represent an enormous energy potential because the total energy corresponding to natural gas entrapped in hydrate reservoirs may be more than twice the energy of all known coal, oil, and gas energy sources. Natural-gas-hydrate accumulations rarely are completely stable in a strict thermodynamic sense. Typically, the hydrate is trapped between clay layers or other low-permeability structures. Gas hydrate exposed to the seafloor will dissociate because of the very-low hydrocarbon content in the surroundings. Though some of the released methane will be consumed by biological and chemical ecosystems, the methane reaching the surface represents an environmental concern because CH4 is a more aggressive (approximately 25 times) greenhouse gas than CO2.
However, a more serious concern is hydrate-formation stability. Changes in local temperature and pressure conditions or surrounding fluids or minerals change the system dynamics and can lead to catastrophic dissociation. The Storegga slide offshore Norway was created by several catastrophic hydrate dissociations. The largest of these is estimated to have occurred 7,000 years ago and is believed to have created a massive tsunami. The environmental disaster related to such enormous amounts of methane released into the atmosphere can have dramatic effects on the greenhouse scenario. The replacement of natural-gas hydrate with CO2 hydrate will increase the stability of hydrate formations. The hydrate content in the Storegga area is still significant and estimated to be on the order of 1012 m3 distributed over 4000 km2. Hydrocarbon exploitation in hydrate regions creates some additional concerns. Drilling operations produce heat and risk of local hydrate dissociation.