Numerical simulations of CO2 –assisted gas production from hydrate reservoirs

Abstract:

A series of experimental studies over the last decade have reviewed the feasibility of using CO₂ or CO₂+N₂ gas mixtures to recover CH₄ gas from hydrates deposits. That technique would serve the dual purpose of CO₂ sequestration and production of CH₄ while maintaining the geo-mechanical stability of the reservoir. In order to analyze CH₄ production process by means of CO₂ or CO₂+N₂ injection into gas hydrate reservoirs, a new simulation tool, Mix3HydrateResSim (Mix3HRS)^[1], was previously developed to account for the complex thermodynamics of multi-component hydrate phase and to predict the process of CH₄ substitution by CO₂ (and N₂) in the hydrate lattice.

In this work, Mix3HRS is used to simulate the CO₂ injection into a Class 2 hydrate accumulation characterized by a mobile aqueous phase underneath a hydrate bearing sediment. That type of hydrate reservoir is broadly confirmed in permafrost and along seashore. The production technique implies a two-stage approach using a two-well design, one for an injector and one for a producer. First, the CO₂ is injected into the mobile aqueous phase to convert it into immobile CO₂ hydrate and to initiate CH₄ release from gas hydrate across the hydrate-water boundary (generally designating the onset of a hydrate stability zone). Second, CH₄ hydrate decomposition is induced by the depressurization method at a producer to estimate gas production potential over 30 years. The conversion of the free water phase into the CO₂ hydrate significantly reduces competitive water production in the second stage, thereby improving the methane gas production. A base case using only the depressurization stage is conducted to compare with enhanced gas production predicted by the CO₂-assisted technique. The approach also offers a possibility to permanently store carbon dioxide in the underground formation to greater extent comparing to a direct injection of CO₂ into gas hydrate sediment. Numerical models are based on the hydrate formations at the Prudhoe Bay L-Pad region on the Alaska North Slope.

References

[1] N.Garapati, "Reservoir Simulation for Production of CH₄ from Gas Hydrate Reservoirs Using CO₂/CO₂+N₂ by HydrateResSim", Ph.D. thesis, West Virginia University, 2013.