Drill stem test (DST)

Majeed Talal Majeed

Outlines

-what is the drill stem test (DST)?

-what is the benefit of the drill stem test (DST) ?

-Drill stem test tools .

-How to collect and analysis the data from (DST) ?

-Type curve matching method .

- A drill stem test is a test which uses a special tools mounted on the end of the drill string .

what is the drill stem test (DST)?

- It is commonly used to test a newly drilled well, since it can only be carried out while a rig is over the hole .

- Reservoir characteristics that may be estimated from (DST) analysis include; • Average effective permeability • Reservoir pressure • Identify reservoir fluids • Well-bore damage • Barriers, fluid contacts • Radius of investigation • Depletion

ANALYSIS OF THE (DST) REQUIRES SPECIAL TECHNIQUES BECAUSE OF:

1- The flow rate is not constant as the fluid rises in the drill string .

2- Complications also arise to momentum and friction effects.

3- The well condition is affected by recent drilling and completion operations may influence the results.

Drill stem test (DST) tools

- cased hole DST became a standard for many exploration companies, particularly offshore- open hole Drill Stem Testing (DST) based on conventional technology with mechanically set packer was gradually replaced by inflatable packer based DST tools

Disadvantages of open hole:-- some companies still avoid open hole testing based result of: 1-poor success ratios of outdated conventional tools. 2-wireline formation testing has become more frequent due to advances made in wireline test tools.

Advantages of open hole:-1-lower cost of the open hole DST service compared to wireline formation testing. 2-Open hole DSTs are quick and simple to run.  3-Hydrocarbons do not need to be produced to the surface, but a significant volume of reservoir fluid can be produced

Types of DST tools depend on the testing well completion either CASED and OPEN hole

DST Analysis Methods

There is three major methods to analysis the (DST) data:-

◆ Horner’s plot method

◆ Type curve matching method

◆ Computer matching (DST)

The build up and draw down

●Type curves first appeared in oil industry literature  in the seventies. ●Several kinds, as listed below, are used to interpret  a test in a vertical well with an infinite homogeneous  reservoir:

–Ramey Agarwal et al. type curves

–McKinley type curves

–Earlougher and Kersch type curves

–Gringarten et al. type curves

Type curve matching method

Gringarten et al. type curves

• A type curve represents the variations in pressureversus time for a specified reservoir‐wellconfiguration.

• It is calculated using an analytic model andexpressed in dimensionless variables.

• The analytic model used by Gringarten to describe avertical well in an infinite homogeneous reservoir.

THE INTERPRETATION

•The proportionality factor between pD and Δp can  be used to  determine the reservoir’s kh:

kh =141.2qBμ (pd )M/(△p)M

•The ordinate of the match point is measured:-

- in the type curve system of axes: pD – in the field data system of axes: Δp.

pD=(kh / qBμ 141.2) △p (in practical US units)

•In the same way the abscissa of the match point, M, is measured in the type  curve system of axes, tD/CDand in the field data system of axes: Δt..

tD/CD = ( kh 0.000295/ μC ) Δt (in practical US units)

We can also determine :- the wellbore storage.-  CDexp(2S).-  the skin.

THE INTERPRETATION

•Type curves were established for constant flow rate  production (drawdown).

•The curves can be used directly to analyze buildup if:

– Δt << tpafter  constant flow rate;

– Δt << tp(n‐1) after a multirate history

tp(n‐1) duration of the last production period before shut‐in.

•If these conditions do not exist, using the types directly may  lead to  inaccurate results.

Using curves to analysis buildup