ABSTRACT
The presence of gas hydrates in Arctic and subsea regions is well documented. Hydrate cores have been obtained from the Gulf of Mexico, the western coast of Guatemala, and in 1972, ARC0/EXX0N recovered a hydrate core sample near Prudhoe Bay, Alaska (Holder et al, 1984). In addition, hydrate deposits in the Canadian and Russian Arctic are known to exist (Makogon, 1965; Bily and Dick, 1974).
Irrespective of the location, drilling through hydrate formations can cause well control problems due to severe mud gasification if proper drilling procedures are not followed. Sizable gas kicks and potential blowout conditions have been reported in the literature (Davidson et al, 1978; Franklin, 1979). The potential for well control problems due to hydrates is expected to increase in the future as further exploratory drilling occurs in known or potential hydrate stability areas; e.g., drilling activities in offshore and onshore Alaska can significantly increase in the future when ANWR (Arctic National Wildlife Refuge) is opened for oil and gas leasing (OGJ, 1986A, B).
Currently, two different methods are used to drill through hydrate containing formations. Predominately, cool drilling fluid with higher mud weight at high circulation rates is used to slow or prevent hydrate dissociation in the formation. Panarctic Oils Ltd., alternatively, tries to promote the dissociation at early times while the drill pipe is in the hole by using low weight muds with proper degassing equipment at the surface (Franklin, 1980).
A thermal simulator developed earlier (Roadifer, et al, 1987) is modified to model the dissociation of hydrates under a wide range of drilling conditions. Major new features include the coupling of wellbore heat transfer and hydraulics with the hydrate dissociation model. Well configuration and makeup are user specified and included in the wellbore calculations. Either arctic or subsea locations and corresponding geothermal and pressure gradients may be specified. For arctic regions, the melting of permafrost may be modeled concurrently with the hydrate dissociation.
The results are presented in a series of nomograms and plots for both arctic and subsea hydrate locations. These nomograms can be used quickly and easily to determine drilling parameters required to keep hydrate dissociation within the limits of the surface gas equipment, or alternatively, given the drilling fluid parameters, design criteria for the surface equipment can be established.
