Drscussiox has often taken place as to whether or not gas can enter a drilling well in opposition to a pressure (due to a column of mud fluid) in excess of the rock pressure in the gas-sand. It has been argued that, especially if the formation contains comparatively wide "drainage channels," bubbles will escape into the mud and rise to the surface under the action of gravity no matter how heavy the mud column is, and further, that if some gas escapes in this way, the mud will become "gas cut" and lighter, and that eventually a blowout may occur from a sand the rock pressure of which is less than the original hydrostatic pressure of the mud column. 2. Whether or not this be so, it is certain that gas is often present in drilling mud and that, even if the quantities of gas be very small, these may in part be re-circulated in a well. The present note, therefore, deals only with the effect of stopping the circulation in, and closing the casing head of a well the drilling mud of which contains some gas, however or whenever it got there. 3. Let us consider a well 4000 feet deep in which is being circulated a drilling mud weighing about 100 lbs. per cubic foot, and containing a small percentage of gas bubbles, and let us suppose that for some reason circulation is stopped and the well is shut in (say by a blow-out preventer). Unless the mud is so viscous as to inhibit movement, the gas bubbles in the mud column are certain to rise to the top of the column under the influence of gravity, the rate of rise being largely dependent on the viscosity of the mud fluid. A bubble which rises from a depth of 4004 feet to a point close to the surface of the well would, if the well were not closed in at the top, increase in volume nearly two hundred times. As the well is closed in at the, top, the bubble cannot increase in size except to, the extent that by increase of pressure it can drive mud -fluid back into the strata at the bottom of the well, a performance which is in practice likely to be difficult or impossible, because of the relationship between viscosity/sealing effects of mud fluid and permeability of strata. To the extent that mud cannot be forced back into the strata, the expansion of the, bubble of gas as it moves up to the top of the well will result in an increase of pressure at the top of the well, and this increase will be recorded on the pressure gauge at the casing head. 4. This effect can be demonstrated in the laboratory, a simple method and apparatus being as described in the attached Appendix I.

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