Abstract

Geologic accumulations of natural gas hydrates hold vast organic carbon reserves, which have the potential of meeting global energy needs for decades. Estimates of vast amounts of global natural gas hydrate deposits make them an attractive unconventional energy resource. As with other unconventional energy resources, the challenge is to economically produce the natural gas fuel. The gas hydrate challenge is principally technical. Meeting that challenge will require innovation, but more importantly, scientific research to understand the resource and its characteristics in porous media. Producing natural gas from gas hydrate deposits requires releasing methane from solid gas hydrate. The conventional way to release methane is to dissociate the hydrate by changing the pressure and temperature conditions to those where the hydrate is unstable. The guest-molecule exchange technology releases methane by replacing it with a more thermodynamically stable molecule (e.g., CO2, N2). This technology has three advantageous:

  1. it sequesters greenhouse gas,

  2. it releases energy via an exothermic reaction, and

  3. it retains the hydraulic and mechanical stability of the hydrate reservoir.

Field testing of the guest-molecule exchange technology is currently in the planning stages for site within the Prudhoe Bay area, Alaska North Slope, where the average hydrate-bearing layer conditions are 6.9 MPa and 4.4°C. Previously, a numerical simulator was developed with capabilities for modeling gas hydrate production from geologic reservoirs using four technologies:

  1. depressurization,

  2. thermal stimulation,

  3. inhibitor injection and

  4. CO2-CH4 guest molecule exchange.

The original form of the simulator assumed equilibrium conditions between the mobile and hydrate concentrations of CO2 and CH4; where, the mobile components are those in the aqueous, gas, and liquid-CO2 phases. Additionally the simulator ignored dissolution of CH4 into the liquid-CO2 phase. Simulation results from this simulator predicted rapid pore plugging by the injected CO2, regardless of the form of injected CO2 (i.e., subcritical gas, liquid, supercritical gas, aqueous dissolved). In support of the technical planning for the arctic field demonstration, this paper re-examines the injectivity of liquid CO2 into hydrate bearing formations, using a new kinetic exchange model between the mobile and hydrate concentrations of CO2 and CH4. Simulation results using the kinetic implementation of the simulator demonstrate the importance of guest molecule exchange kinetics in maintaining formation injectivity.

Introduction

G as hydrates are clathrate compounds in which water molecules encapsulate a guest molecule within a lattice structure. The lattice structure of gas hydrates form under low-temperature, high-pressure conditions via hydrogen bonding between water molecules. Gas hydrates with methane (CH4) guest molecules occur as accumulations in sedimentary formations offshore and permafrost environments where sufficiently low temperatures and high pressures exist. From an energy resource perspective, these geologic accumulations of natural gas hydrates represent a significant component of the world's organic carbon. Assessments by the United States Geological Survey (USGS) have estimated that reserves of methane in hydrate form exceed the all other fossil fuel forms of organic carbon (Booth et al., 1996). Under geologic environmental conditions, the lattice structure of a gas hydrate depends primarily on the guest molecule (Englezos, 1993; and Sloan, 1998). Interestingly, the two most prevalent emitted greenhouse gases (U.S. EPA, 2006) carbon dioxide (CO2) and methane (CH4) both form sI hydrate structures under geologic temperature and pressure conditions. Whereas their clathrate structures are similar, CO2 hydrates form at higher temperatures and have a higher enthalpy of formation compared with CH4 hydrates (Sloan, 1998).

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