Grade P110 Tubing Shredded by Downhole Detonation
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A failure analysis was performed on a length of API 5CT Grade P110 tubing that failed while being used in a drilling operation. Approximately 10 feet of the tubing was shredded by axial slits, and there were collapsed zones above and below the failure. The slits were at 45° to the tubing surface and showed very fine microvoid coalescence. Metallographic examination showed that the slits had formed along adiabatic shear bands. The presence of adiabatic shear bands indicated an extremely high strain rate. That, combined with the orientation of the slits, indicated that the failure was caused by a detonation of gasses in the annulus surrounding the tubing. Well operations responsible for the detonation will be briefly addressed.
KeywordsGrade P110 tubing Detonation Intermittent collapse
The upper collapsed zone was separated from the shredded portion by an upset connection, which remained visibly intact, indicating that the collapse damage had occurred on two separate lengths of tubing.
Laboratory Examination and Testing
Because the collapse did not extend to the lower end of the shredded tubing, a portion of apparently intact tubing was available for testing. Tension test results, wall thickness, and diameter were satisfactory for the specified size, weight, and grade of tubing. No manufacturing defects were identified.
Scanning Electron Microscopy
The characteristics of the failure indicated it was caused by a detonation in the annulus surrounding the tubing. Specifically, (1) the multitude of axial slits indicated a high-energy failure, (2) the presence of adiabatic shear bands indicated high strain rates , (3) the collapse damage indicated extremely high pressures in the annulus (differential greater than 19,000 psi, based on the rated collapse resistance of the tubing ), (4) the intermittent and interrupted nature of the collapse damage above and below the shredded portion was consistent with a short-duration pressure wave, and (5) the 45° angle of the slits closely resembled damage created in laboratory settings by explosives wrapped around a hollow cylinder .
The features of the failure indicated that the impulsive loading responsible for the failure was in the annulus surrounding the tubing, where air and hydrocarbons were mixed, rather than inside the tubing, which contained only compressed air. The collapse damage was clearly caused by excessive pressure in the annulus, and the 45° orientation of the axial fractures indicated that they were generated by shear stresses from circumferential compression of the tubing. Under less dynamic loading conditions, excessive external pressure will cause an instability and collapse. However, the high strain rate from the impulsive loading caused the strain in the region closest to the detonation to be localized in adiabatic shear bands. As a result, that portion of the tubing shredded rather than collapsed.
The appearance of the collapse damage above and below the shredded portion of the tubing also suggested impulsive loading. In upset tubing, a collapse failure caused by excessive external pressure typically initiates at a single point and propagates rapidly in both directions until it arrests at the thicker upsets at each end, or until the deformation causes the tubing to leak, allowing the pressure to equalize. If the tubing is ductile enough that it flattens without rupturing, additional lengths of tubing above and below it may collapse as well. Because a collapse propagates so rapidly; however, it is unusual to have two separate collapsed zones in a single length of tubing, as was observed in both the damaged length and the length above it. The unusual appearance of the collapse damage thus suggested it was caused by a pressure wave rather than by sustained external pressure.
A common hazard associated with air drilling in the presence of hydrocarbons is a downhole fire , but downhole fires generate extreme heat and result in very ductile torsional failures with associated microstructural changes. This failure showed a very different appearance: there was little evidence of torsional yielding, and the microstructure showed no evidence of the overheating that is typical of downhole fires. The microstructural evidence of impulsive loading indicated the failure was caused by a detonation, not a fire.
Drilling with compressed air continues to enjoy vast popularity. It is often the application of choice in dry, hard rock because the drilling fluid medium is inexpensive and, compared to water or weighted drilling mud, respectable rates of penetration can be achieved. In terms of economic considerations with respect to drilling fluid expenditures, air drilling has virtually no equal. However, air drilling is not without its impediments, not the least of which is its reactivity with hydrocarbons.
A typical mechanism proceeds as follows: compressed air exits the bit or downhole hammer, a productive pay is exposed usually producing condensate, a mud ring forms uphole creating an annular packoff between the drill pipe and the protective casing. The air compresses in that space below the annular packoff increasing the partial pressure of oxygen and supplementing the heat already in the confined space through adiabatic compression. Add fuel from the condensate, and a possible ignition source such as a friction spark from the drill bit or downhole hammer—the result is a downhole detonation, very similar to that created in a diesel engine.
The resulting detonation can be so violent and intense that it collapses (actually flattens) and twists drilling tubulars. Often times, there is little need to run a junk basket after a downhole detonation because the bottomhole assembly and bit disintegrate downhole.
Downhole fires and detonations can be effectively prevented by using compressed nitrogen rather than compressed air as the drilling fluid when entering hydrocarbon zones.
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