Gas Migration

If a gas kick enters a well, the pressure of gas at that point is formation pressure (PF), (ignoring any skin damage effects).

Gases are compressible and thus store energy for later release. Considerable energy can be stored in this way. Liquids, in contrast, are virtually incompressible, and because little or no change in volume occurs little energy can be stored.

This characteristic of gas means that when a well is shut in with a gas kick, the gas in the well retains the prevailing pressure at the moment of shut in, if no expansion of that gas is allowed.

Gases are generally lighter than the drilling muds in use, and tend to 'float' or percolate up the hole toward the surface. So long as the well stays shut in, and nothing breaks down, the gas bubbles will stay the same total volume and hence at the same pressure while moving up the hole. This is usually called 'Gas Migration'.

It is a basic principle that in an unbroken, sealed in well the pressures at any point must balance each other. If a gas bubble of constant volume and constant pressure is slowly moving toward the surface, there must be progressively less and less mud hydrostatic head above it, to balance its pressure. Thus in order to maintain a balance as the bubble rises, the annulus, or casing, pressure must rise.

In this case, an increase in casing pressure must result in an increase of bottom-hole pressure, since the total hydrostatic pressure of fluid in the annulus has not changed.

This increased bottom hole pressure must itself in turn result in an increased drillpipe pressure, since the hydrostatic head of fluid in the drillpipe has not altered.

A slow continuous increase of shut-in drillpipe and shut-in casing pressures together, after their initial stabilisation, is a positive indication of gas in a kick.

400 psi

EFFECTS OF BLEEDING DRILL PIPE PRESSURE TO ORIGINAL STABILISED VALUE WHILE GAS IS MIGRATING

Gas 5,800 psi

5,000 ft

Gas 4,640 psi

BHP 5,800 psi

7,500 ft

BHP 5,800 ps

5,000 ft

2,864 psi

BHP 5,800 psi

2,864 psi

BHP 5,800 psi

FORMATION pRESSURE 5,8°0 pSI

400 psi U

EFFECTS OF NOT BLEEDING PRESSURE WHILE GAS IS MIGRATING

400 psi U

EFFECTS OF NOT BLEEDING PRESSURE WHILE GAS IS MIGRATING

BHP 5,800 psi

3,100 psi

BHP 5,800 psi

Mud Hyd. 2,700 psi

5,800 psi

Mud Hyd. 2,700 psi

5,800 psi

BHP 8,500 psi

Mud Hyd. 5,400 psi

11,200 psi

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Responses

  • tesmi
    How gas kick effects bhp?
    2 years ago

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