Abstract

As the search for petroleum has moved into the high-pressure/high-temperature (HP/HT) environment, the well control problems have continued to increase in complexity due to the small pressure margins and the large temperature-driven variations of mud properties often seen. Recently in western China, a major kick was encountered while drilling to 7874 m with 16-ppg water-based mud in an HP/HT well. Killing of the well was attempted but was not successful. After that, the drillstring broke near the top of wellbore. As a contingency action, the operator abandoned the plan for further killing and put the well into production through diverting the kick to the production pipeline. Many unknowns, which were unsolvable by conventional perceptions, were still intriguing the operator and might be issues in further drilling operations in this field. These unknowns include: (1) Why was the well pressure constant before the gas reached the wellhead while the well was shut-in for almost 2 days? (2) Where was the gas top during shut-in? (3) Why did the killing attempt fail? (4) What killing parameters should have been used in the killing attempt?

To address these unknowns and provide solid guidance for further drilling operations, simulation of the gas kick development and well killing was conducted using a dynamic hydraulic and well control simulator powered by transient multiphase flow model.

Through integration of existing well and reservoir data, the dynamic simulation was able to history match the observed wellhead pressure, casing pressure during the shut-in, and killing operations. The simulation showed that the killing pump rate was not adequate to kill this well for the given gas kick in the wellbore. The dynamic simulations successfully revealed the reason for the gas suspension in the wellbore while shut-in, predicted the gas top, and identified the required pumping rate for a success killing. The simulation also disclosed the root cause of the drillstring failure.

This case study highlights the importance of dynamic simulation during the well engineering design and before any attempt of well killing to improve chance of success for well control and safety of drilling operations.

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