An Experimental Investigation into Explosive Decompression of Progressing Cavity Stator

Elastomers Due to Gas Infiltration

Society of Petroleum EngineersPCP Applied Technology Workshop

September 16, 2002

Presented by: Mike Guidry, Jr.

September 16, 2002PCP Applied Technology Workshop

Definition

Explosive Decompression

– Occurs when the internal pressure applied to a material by a trapped fluid exceeds the tear strength of that material upon the reduction of external pressure.

September 16, 2002PCP Applied Technology Workshop

History

Field experience

– Well bore gases can infiltrate the elastomers in down hole service.

– Severe explosive decompression damage can result when there is rapid pressure loss.

• Uncontrolled shut down situations

• Ordinary well service

September 16, 2002PCP Applied Technology Workshop

Theory

This discussion only concerns materials which are inertrelative to each other.

September 16, 2002PCP Applied Technology Workshop

Definition

Infiltration (Permeation)

– The migration of fluid into or through a lattice.

September 16, 2002PCP Applied Technology Workshop

Theory

Elastomers are permeable.

– Permeability varies with the compound structure, properties of the contact fluid, and environmental conditions.

September 16, 2002PCP Applied Technology Workshop

Theory

Typical Elastomer Structure

– Elastomer compounds are generally an amorphous lattice of long chain molecules.

– The long chain molecules are sparsely cross-linked by other atoms.

September 16, 2002PCP Applied Technology Workshop

Theory

Typical molecular structure of a carbon-hydrogen back bone elastomer with sulfur cross-links

September 16, 2002PCP Applied Technology Workshop

Theory

Two highly compressible fluids common topetroleum wells are:

– Methane

– Carbon Dioxide

September 16, 2002PCP Applied Technology Workshop

Theory

Methane

– Methane is a tetrahedral molecule

September 16, 2002PCP Applied Technology Workshop

Theory

Tetrahedral molecular structure of methane

September 16, 2002PCP Applied Technology Workshop

Theory

Carbon Dioxide

– Carbon dioxide is a linear molecule

• Linear molecular structures have the smallest cross-section of the typical molecular structures.

September 16, 2002PCP Applied Technology Workshop

Theory

Linear molecular structure of carbon dioxide

September 16, 2002PCP Applied Technology Workshop

Theory

Fluid pressure exceeding the pressure applied bythe elastomer typically results in one or more of thefollowing:

– Deflection

– Tearing

– Infiltration

September 16, 2002PCP Applied Technology Workshop

Theory

Carbon dioxide infiltrating the elastomer lattice

September 16, 2002PCP Applied Technology Workshop

Theory

Explosive decompression occurs when:

– The rate of decompression exceeds the rate of permeation

– The pressure of the infiltrate fluid exceeds the tear strength of the elastomer

September 16, 2002PCP Applied Technology Workshop

Testing

Can explosive decompression damage be minimizedthrough compound selection and field implementationtechnique?

September 16, 2002PCP Applied Technology Workshop

Testing

Test Method

– Samples of different 5 compounds were prepared in the form of a progressive cavity stator.

– 1 sample of each compound was loaded into 5 separate test cylinders.

– Each cylinder was then loaded with the test fluid and rotated for the duration of the test.

September 16, 2002PCP Applied Technology Workshop

Testing

Figure 1. Test Sample

September 16, 2002PCP Applied Technology Workshop

Testing

Test Parameters

– 1000 PSI (6.895 MPa)

– 75°F (24°C)

– 71 hours

September 16, 2002PCP Applied Technology Workshop

Testing

Fluid Composition by Volume Fraction

– Cylinder A water 100%

– Cylinder B water 80% carbon dioxide 20%

– Cylinder C water 70% carbon dioxide 30%

– Cylinder D water 60% carbon dioxide 40%

– Cylinder E water 70% carbon dioxide 30%

September 16, 2002PCP Applied Technology Workshop

Testing

Decompression Rates

– Cylinders A-D -- controlled decompression rate

– Cylinder E -- maximum decompression rate

September 16, 2002PCP Applied Technology Workshop

Results

Cylinder A -- 1 Hour Post Decompression

Compound 1 Compound 2 Compound 3

Compound 4 Compound 5

September 16, 2002PCP Applied Technology Workshop

Results

Cylinder C -- 1 Hour Post Decompression

Compound 1 Compound 2 Compound 3

Compound 4 Compound 5

September 16, 2002PCP Applied Technology Workshop

Results

Cylinder C -- Post Decompression Time Dependence

1 hour 6 hours 70 hours

Com

poun

d 4

C

ompo

und

1

September 16, 2002PCP Applied Technology Workshop

Results

Post Decompression Volume Fraction Dependence

0% Carbon Dioxide

20% Carbon Dioxide

Compound 1 Compound 4

September 16, 2002PCP Applied Technology Workshop

Results

Post Decompression Volume Fraction Dependence

30% Carbon Dioxide

40% Carbon Dioxide

Compound 1 Compound 4

September 16, 2002PCP Applied Technology Workshop

Results

Compound 4 -- Decompression Rate Dependence

1 hour

70 hours

Cylinder C Decompression Rate Cylinder E7 psi/min 400 psi/sec

September 16, 2002PCP Applied Technology Workshop

Conclusion

Explosive Decompression Damage Factors

– Volume fraction of compressible, infiltrate fluid

– Rate of decompression

– Duration at lower pressure

– Elastomer compound

September 16, 2002PCP Applied Technology Workshop

Conclusion

Reducing Explosive Decompression Damage

– Select the proper elastomer compound

– Minimize the rate of decompression

– Minimize the duration at low pressure