Drill stem test

Drill stem test (DST) is one of the most famous on-site well testing that is used to unveil critical reservoir and fluid properties such as reservoir pressure, average permeability, skin factor and well potential productivity index. It is relatively cheap on-site test that is done prior to well completion. Upon the DST results, usually, the decision of the well completion is taken.

1. Drill Stem Test Prepared by: Nouh Almandhari

2. What is DST Why DST Tools of DST How DST Outline

3. Why DST  Seismic indicates the possible existence of a potential reservoir.  DST indicates the well deliverability and its worthiness for completion.

4. What is DST temporary completion of a wellbore that provides information on whether or not to complete the well. -> Done after installing the casing and before hooking-up the well. Good DST yields: F luid samples R eservoir pressure ( P *) F ormation properties, including permeability ( k ), skin ( S ), and radius of investigation ( r i ) P roductivity estimates, including flow rate ( Q ) -> Analysis of the DST transient pressure data can provide an estimate of formation properties and wellbore damage.

5. Before we start the DST Planning: The key to successful testing depends upon planning and teamwork between the geoscientist and the engineer. Potential pay zones should be identified before drilling commences so that the drilling program can be designed to accommodate the test. Safety: Running a DST is one of the most dangerous jobs in the oil field because the well is essentially uncontrolled during the test. All fire fighting equipment and the blowout preventers should be inspected and tested before starting a DST. Hydrogen sulfide (H2S) equipment should be on hand if anticipated conditions are sour. No test should be initiated at night or during an electrical storm . No smoking should be allowed on the drill floor or near any flow lines or surface test equipment.

6. How DST It is done in 4 stages Short production period (Initial Flow-IF)  5-10 minutes (A) Short shut-in period (Initial Build-up –IBU)  30-90 minutes (B) Longer flow period (Final Flow-FF)  1-5 hours (C) Longer shut-in period (Final Build-up-FBU)  3-10 hours (D) Time Pressure A typical pressure profile observed during DST test A B C D

7. Why is it like this The initial flow period removes the “supercharge” effect of mud filtrate near wellbore. The first build-up is run to determine a valid P * (reservoir pressure) provides the “guiding light” for determining the proper slope found using buildup #2. The second flow period is used to collect a fluid sample and create a pressure disturbance at a distance beyond any damaged zone The final build-up is used to evaluate reservoir transmissibility, damage, and radius of investigation (Build-up data analysis ).

8. Tools of DST

9. Tools of DST Testing tool and packer are mounted below the drill collars . Below the testing tool, a perforated pipe ( anchor pipe ) is mounted . Inside anchor, an upper pressure gauge is situated above perforations, reading pressure below the testing tool . Below the anchor pipe, another pressure gauge, which reads the pressure in the annulus between the sand face and anchor pipe.

10. Good to Know The wellbore is filled with mud. During the DST, the string is partially filled with Cushion . The testing tool isolates the overlying DCs and DPs from the mud present in the well. • An empty string remain empty as it is lowered.

11. Why we need Cushion fluid ? To protect the drill string from collapse . To reduce the inertia of formation fluid as they enter the pipe.  For unconsolidated formation, pressure exerted by cushion is 400 psi difference (or less) from formation pressure (to prevent sand production, gravel-pack plugging, fracture proppant production ). Cushion Water Diesel Nitrogen Natural Gas

12. DST P ressure Profile Pi

13. Warning Since the DST result analysis requires the knowledge of Pressure Build-up (PBU) analysis Lets first review PBU Then for DST data apply the PBU knowledge and equations!

14. Review of Build-up Data Analysis

15. Review of Build-up Data Analysis B and µ known from fluid analysis h is known from logging q is the constant flow rate of the flow period done before the shut-in By having m , the average reservoir permeability (K) can be easily found.

16. Review of Build-up Data Analysis Permeability: Skin:

17. Review of Build-up Data Analysis Flow rate must reach the surface to reach steady-state flow regime thus having constant q

18. Thanks   ‘ Today’s Pain is Tomorrow’s Power ’