Hydraulic fracturing treatments in unconventional wells increase stress levels in API couplings and have been one of the key contributing factors that caused many costly failures in the field in the recent years. The combination of these overstressed connections; material susceptibility and presence of hydrogen have led to a Hydrogen stress-cracking phenomenon, with brittle longitudinal cracks in the couplings.
This paper presents the experimental work done with different API 5CT P110 materials under high levels of stress in a very demanding environment that was created by a solution of HCl acid with none to different levels of inhibition. The analyzed conditions were simulated for short time intervals, in similar way that they are experienced in most plug & perf completions typically performed in the majority of shale’s wells.
Shale Drilling for both natural gas and oil has increased dramatically in North America over the last years. The development of these resources requires extensive hydraulic fracturing treatments. Many failures have been reported in API 5CT P110 connections during stimulation operations, all of them with similar morphology (brittle longitudinal splits in the couplings, Figure 1).
Environmentally assisted cracking (hydrogen stress cracking) has been identified as the root cause of those failures. This failure process involves the presence of three simultaneous factors to occur:
- Level of stresses
- Source of hydrogen
- Susceptibility of the material
A fit-for-purpose (FFP) test was conducted to simulate the conditions that the materials were facing. To support the selection of the testing parameters, a detailed analysis of the actual situation was performed, as summarize below.
Level of Stresses (API Connections):
API connections, both LTC and BTC (Figure 2), in particular in the OD range 4 ½” - 5 ½” (which is the common range used as production Casing in most Shale wells) by design have a high level of diametrical interference. This element, together with the thread compound, is the main sealing mechanisms of this type of connections. This interference creates high hoop stresses on some areas of the couplings (even after make-up); to a level that exceeds the yield stress of an API 5CT P110 grade material. This stress is increased even further when internal pressure is applied during hydraulic fracturing.
This problematic was addressed by API (Work Item 3076), that investigated this problem and proposed a change to the coupling OD for the range 4 ½” - 5 ½” that will be incorporated in the API 5CT 10th edition. As a summary of API work, below is presented the stress situation on a 5 ½” BTC connection under different load steps.