Changing Liquid Level

There are two types of wellbore storage or afterflow phenomenon. These are:

• changing liquid level
• wellbore filled with compressible fluid

In the changing liquid level case, when a producing well is shut-in, the wellbore is not full of liquids and there exists a gas-liquid interface in the wellbore. As afterflow continues, this liquid level rises in the case of a buildup test (falls during a fall-off test). The wellbore storage constant in a changing liquid level case can be expressed as:

(field)

where

Vu = wellbore volumetric capacity, bbl/ft

p = liquid density, lbm/ft3

(metric units)

where

Vu = wellbore volumetric capacity, m3/m

p = liquid density, kg/m3

g = acceleration due to gravity, 9.806 m/s2.

Note:

The wellbore volumetric capacity in the above equation is essentially the effective wellbore area.