Surface Pressure During Drill Stem Test

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.

Reference book: Formulas and Calculations for Drilling, Production and Workover, Second Edition