High-rate, multistage hydraulic fracturing in unconventional wells results in high internal burst loads, which are normally considered during production string design; yet, recent production casing failures exhibit casing collapse after several stages of a fracturing operation. Post-analyses of presented case studies demonstrate that production strings can be exposed to high-fracturing collapse loads caused by inadequate cement bonds around the casing.
In this study, a fracturing operation was initially simulated to obtain an actual thermal profile for casing-stress analysis with a thermal wellbore simulator. Based on available data, bottomhole treating pressure (BHTP) was calculated to match the actual field data. A load case was run to check the collapse integrity of production casing with the assumption that a casing section below the top of cement (TOC) is fully exposed to the simulated BHTP because of channels created in the casing annulus. Additionally, a sensitivity analysis was performed to identify the threshold BHTP limit that can start collapse failure. Another scenario was simulated by assuming a good cement bond for comparison with previous simulations.
Two case studies are presented wherein production casing collapse failures occurred during multistage fracturing operations. Both wells had horizontal trajectories and planned for 20 and 28 frac stages, respectively. Results based on actual field data demonstrate that the collapse safety factors were less than the design factors all the way from TOC to the casing shoe depth in case of exposure to BHTP in the casing annulus. Additionally, the sensitivity analysis highlighted that the casing collapse rating can be exceeded if exposed to even lesser BHTP than simulated and actual BHTPs. Simulations without any pressure communication to the annulus also confirmed that the potential of collapse failure becomes minimal with adequate cement bond under multistage-fracturing operations. Based on these findings, it was derived that casing collapse failures in both wells were the result of BHTP transfer into the annulus while fracturing operations. Such failures could be avoided in the future with additional design considerations during the planning phase of such wells.
This work shows the importance of additional collapse loads for production strings to improve the integrity of unconventional wellbores that are designed for multistage hydraulic fracturing operations. It is possible to ignore this load analysis if all potential operational risks are addressed with appropriate contingency designs.