This paper demonstrates an application of failure prediction and a cost effective solution to integrity problems of safety critical elements in wells in a high pressure, high temperature (HP/HT) gas field. Lekhwair field is a gas development in Northern part of the Sultanate of Oman producing approximately 2.8 MMSM3/D from the Upper Mafraq (UM) formation and Lower Mafraq (LM) formation. The LM formation has CO2 content in the range of 2-5 vol% which requires all the Lekhwair wells producing from this formation to be completed with stainless steel (FF) x-mas tree and tubing hangers.
At the onset of field exploration and appraisal, some wells in Lekhwair field were completed with carbon steel (DD) x-mas trees and tubing hanger which is incompatible with the CO2 component of the formation fluid. This has led to severe corrosion in some of wells. Consequently, a x-mas tree retrieval and inspection campaign confirmed the metal loss rate was standing at ~ 5mm/year.
To predict the meantime to failure of the DD tubing hanger and the x-mas tree, a robust stress analysis model was built to determine failure prognosis for the tubing hanger. The model was calibrated with actual data by integrating the measured metal loss observed on the existing x-mas tree after retrieval. This process has been embedded as part of reliability and failure analysis modelling for wells with DD x-mas trees and tubing hangers.
The outcome of the failure modelling showed that the tubing hangers would fail within 3 years if the hanger is consistently flow-wetted. This was used as a basis to develop a well integrity management plan and remediation solution for the Lekhwair wells with DD tubing hangers. To remediate the problem, the conventional solution applied in the industry is to execute a work-over to pull the existing tubing hanger and recomplete the well using the rig at a cost of 2 MMUSD/well with a 50% probability of success. This innovative new solution was devised by the team to execute an intervention using a slick-line unit and installing a through-tubing straddle across the existing tubing hanger to isolate it. This effectively prevents the tubing hanger from being flow-wetted and stops the tubing hanger from further corrosion. The solution has been deployed in the Lekhwair wells, and has been proven to lead to a cost and time saving in excess of 1.5 MMUSD/well and 10 days rig time, respectively.