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**

Tq fr this awesome informative site, im studyng petroleum, the explanation and equations are very applicable fr my revision

Dear Sir,

The way, surface pressure during drill stem test has been explained , really it is

appreciable. Any one can clear his concept about formation and reservoir pressure.

thanks

Fresh water density = 8.3 ?

as far as i know, fresh water density = 1.0.

correct me if i’m wrong.

Hi Dany,

8.3 ppg (pound per gallon) is fresh water density. Specific gravity of fresh water is 1.0 (unitless).

Regards,

Shyne

Thx for these valuable information