Summary
Gas hydrates are a potentially vast untapped source of natural gas. Recent
numerical and field studies suggest the Mallik gas-hydrate field in Canada's
Mackenzie Delta may represent a technically producible and potentially
economically viable reservoir of natural gas. Our initial reservoir simulations
using a kinetic reaction approach indicate that gas evolution and transport
within porous geologic reservoirs have a significant effect on fluid production
characteristics, while field and laboratory data suggest that significant
amounts of evolved gas can be trapped for some time within the reservoir,
depending on the field operation.
In this work, we invoke modelling concepts extensively employed in
quantifying gas ex-solution from viscous oils to further assess the kinetic
behaviour of gas-hydrate ex-solution through depressurization. Here, the gas
bubbles can be categorized into three groups with explicit transport behaviour:
small bubbles (water phase), large bubbles (immobile) and connected bubbles (or
free gas). These concepts allow the development of a new set of kinetic
reactions for hydrate dissociation: one representing the (possibly delayed)
conversion of hydrate into water and dispersed gas bubble phases, and one
representing the evolution from dispersed bubbles to connected bubbles. These
reactions can effectively capture the nonequilibrium fluid-flow behaviour
observed in field production tests.
For modelling of the transport phenomenon, we assumed two explicit mobility
formulations: (1) trapped bubbles (no mobility) and a flowing water phase and
(2) large connected gas bubbles and flowing water (with relative mobility).
Relative mobility can be estimated by using traditional gridblock-relative
permeability curves. We then develop a simple mechanistic gas bubble trapping
tool as a function of the capillary number, which can easily be incorporated
into our numerical simulator. This entrapment of the nonwetting gas-phase
results in higher values of critical gas saturation.
Two case studies based on alternative representations of a Mallik-like
gas-hydrate reservoir demonstrate that significant errors can result in
reservoir modelling if these fluid transport phenomena are not adequately
represented in numerical simulations. Aspects of the model developed here have
been applied to history matching and prediction of natural gas recovery from a
clastic, sand-dominated reservoir at the Mallik site.
© 2010. Society of Petroleum Engineers
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History
- Original manuscript received:
17 August 2010
- Meeting paper published:
19 October 2010
- Revised manuscript received:
5 November 2010
- Manuscript approved:
8 November 2010
- Published online:
1 January 2011
- Version of record:
1 January 2011
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