Effect of Non-Hydrostatic Pore Pressure from the Depth to the Base of the Hydrate Stability Zone

Robert W. Lankston
Geoscience Integrations, Missoula, MT

The pressure term in the hydrate stability equation can be converted to depth to facilitate estimating the vertical extent of the hydrate stability zone (HSZ). The conversion from pressure to depth is generally made assuming that hydrostatic conditions exist downward from the surface, and a gradient of 0.0100 MPa/m is often applied. While being a convenient factor, it is about 5% less than the widely accepted value for hydrostatic pressure, i.e., 0.0105 MPa/m. This gradient can be expressed as 8.94 lb/gallon mud weight.

Using the larger factor for the gradient yields a higher pressure at any given depth, which means that hydrate would be stable at a higher temperature at that depth. For a given geothermal gradient, the base of the HSZ is deeper with higher pressure gradients.

Gas is commonly trapped above hydrostatic pressure. Higher pressure in the reservoir causes the base of the stability zone in that reservoir to become deeper. This effect may help to explain why the base of the hydrate stability zone reported for the Hot Ice well on the North Slope was approximately 120 m shallower than the gas hydrates that dissociated contributing to the 1992 blowout of the Cirque 1 well, which was just a few miles to the west. Reported mud weights from the Cirque 1 well were between 9.1 and 9.4 lb/gallon. Mud weights from Cirque 1 and nearby Cirque 2 can be used to estimate a base of hydrate stability in the reservoir that is approximately 300 m deeper than would be estimated based on a hydrostatic assumption.

In the case of deepwater hydrates, varying gas pressure in reservoir sands helps to explain why sets of bright reflections in steeply dipping sands, interpreted as indicating the base of the HSZ, do not track the sea floor reflection as a BSR might be expected to do.

AAPG Search and Discovery Article #90078©2008 AAPG Annual Convention, San Antonio, Texas