Drill stem test or well test is mostly conducted for exploration wells in order to see how much hydrocarbon can be produced for the reservoir, estimate reservoir boundary, determine rock properties, etc.
This topic will teach you how to estimate surface pressure during drill stem test.
First step: Determine Formation Pressure
Formation pressure = 0.052 x Formation pressure equivalent mud weight x Well TVD
Where;
Formation pressure in psi
Formation pressure equivalent mud weight in ppg
Well TVD in ft
Second step: Determine Hydrostatic Pressure of Reservoir Fluid
Hydrostatic pressure of reservoir fluid = 0.052 x Reservoir fluid density x Well TVD
Where;
Hydrostatic pressure in psi
Reservoir fluid density in ppg
Well TVD in ft
Third step: Determine Surface Pressure During Drill Stem Test
Surface pressure = Formation pressure – Hydrostatic pressure of reservoir fluid
Where;
Surface pressure in psi
Formation pressure in psi
Hydrostatic pressure of reservoir fluid in psi
Example: Oil reservoir at 12,000'MD/10,000'TVD and its formation pressure equivalent to 12.0 ppg. A specific gravity of oil in the reservoir is 0.6. What is the pressure at the surface that you will see while performing drill stem test?
First step: determine formation pressure
Formation pressure = 0.052 x 10,000 x 12 = 6240 psi
Second step: determine hydrostatic pressure of reservoir fluid
Reservoir fluid density = oil specific gravity x fresh water density
Reservoir fluid density = 0.6 x 8.33=5.0 ppg
Hydrostatic pressure of reservoir fluid = 0.052 x 5 x 10,000 =2600 pis
Third step: Determine Surface Pressure During Drill Stem Test
Surface pressure = 6240 – 2600 = 3640 psi.
You will see surface pressure of 3640 psi during the test.
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