Phase field theory modeling of CH4/CO2 gas hydrates in gravity fields

INTERNATIONAL JOURNAL OF GEOLOGY
Issue 2, Volume 5, 2011

Phase field theory modeling of CH4/CO2 gas hydrates in gravity fields

M. Qasim, B. Kvamme1, and K. Baig

http://www.naun.org/main/NAUN/geology/20-084.pdf

Abstract—Natural gas hydrates in reservoirs are thermodynamically unstable due to the interactions with surrounding fluids (aqueous, gas) and mineral surfaces. Depending on the local flow hydrate will dissociate as well as reform. If the dissociation rate is faster than the capacity of the surrounding fluids to dissolve the released gas, the gas will form bubbles. Depending on the rate of released gas and possible fracture patterns this may lead to venting of gas. The proper implementation of hydrodynamics will provide a deeper insight of the hydrate kinetics involved during dissociation
and formation processes which involve hydrate former phase as smaller or larger bubbles or even continuous gas phase. In this work the phase field theory coupled with hydrodynamics model is
implemented with variable density using the relative composition, phase field parameter and flow, which is an extension of our previous work which considers a constant density.

Keywords—Phase field theory, Natural gas hydrate, Hydrodynamics, Dissociation, Hydrate.
I. INTRODUCTION
as hydrates are ice-like substances of water molecules encaging gas molecules (mostly methane) that form under specific pressure and temperature conditions within the upper hundred meters of the sub-seabed sediments. They occur worldwide and are a potential energy resource for the near future [1].
Natural gas hydrates are widely distributed in sediments along continental margins, and harbor enormous amounts of energy. Massive hydrates that outcrop the sea floor have been reported in the Gulf of Mexico [2]. Hydrate accumulations have also been found in the upper sediment layers of Hydrate ridge, off the coast of Oregon and a fishing trawler off Vancouver Island recently recovered a bulk of hydrate of approximately 1000kg [3]. Håkon Mosby Mud Volcano of Bear Island in the Barents Sea with hydrates are openly exposed at the sea bottom [4]. These are only few examples of
the worldwide evidences of unstable hydrate occurrences that

Paper submitted October 29, 2010: Revised version submitted March 5, 2011. This work was supported financially by Norwegian research council under INJECT project and PETROMAKS project Gas hydrates on the Norwegian Research Council Sea-Svallbard margin (GANS, Project number. 175969/S30).

M. Qasim, is with the University of Bergen, Post box 7803, 5020 Bergen,
Allegt. 55 Norway. (e-mail: Muhammad.Qasim@ift.uib.no).
B. kvamme1, is with the University of Bergen, Post box 7803, 5020
Bergen, Allegt. 55 Norway (phone: +47-555-83310; e-mail: Bjorn.Kvamme@
ift.uib.no).
K. Baig, is with the University of Bergen, Post box 7803, 5020 Bergen,
Allegt. 55 Norway. (e-mail: kba062@ift.uib.no).

leaks methane to the oceans and eventually may be a source of methane increase in the atmosphere.
Hydrates of methane are not thermodynamically stable at mineral surfaces. From a thermodynamic point of view the reason is simply that water structure on hydrate surfaces are not able to obtain optimal interactions with surfaces of calcite, quarts and other reservoir minerals. The impact of this is that hydrates are separated from the mineral surfaces by fluid channels. The sizes of these fluid channels are not known and are basically not even unique in the sense that it depends on the local fluxes of all fluids in addition to the surface thermodynamics. Stability of natural gas hydrate reservoirs therefore depends on sealing or trapping mechanisms similar to ordinary oil and gas reservoirs. Many hydrate reservoirs are in a dynamic state where hydrate is leaking from top by
contact with groundwater/seawater which is under saturated with respect to methane. Dissociating hydrate degasses as bubbles if dissociation rate is faster than dilution in surrounding fluids and/or surrounding fluid is supersaturated.

The kinetic rate depends on mass transport dynamics as well as thermodynamic driving force. Phase field theory will be a power full tool to quantify this balance and provide basis for development of simplified models for reservoir modeling tools.

The primary focus in this work is to incorporate the density of all phases based on relative compositions and calculation of free gas exist in form of bubbles which can escape through
empty channels and hence will be useful in calculation of an accurate natural gas flux.

II. PHASE FIELD THEORY
Phase field model follows the formulation of Wheeler et al.
[5], which historically has been mostly applied to descriptions of the isothermal phase transition between ideal binary-alloy liquid and solid phases of limited density differences. The hydrodynamics effects were incorporated in a three components phase field theory by Kvamme et al. [6] through
implicit integration of Navier stokes equation. The phase field is an order parameter describing the phase of the system as a function of spatial and time coordinates. The field is allowed to vary continuously on the range from solid to liquid.

The solid state is represented by the hydrate and the liquid state represents fluid and aqueous phase. The solidification of hydrate is described in terms of the scalar phase field , , where , , represents the molar fractions of CH4, CO2 and H2O respectively with obvious constraint on Phase field theory modeling of CH4/CO2 gas hydrates in gravity fields M. Qasim, B. Kvamme1, and K. Baig G
INTERNATIONAL JOURNAL OF GEOLOGY Issue 2, Volume 5, 2011