Use Dynamic Simulation to Diagnose the Startup Failure of a Long Horizontal Well in the North Sea

Abstract

Gas breakthrough for horizontal oil producers can sometimes be an advantage for keeping the well producing, particularly in late life when the reservoir pressure is low and the water-cut is high. However, this additional gas may be unavailable after a long shut-in due to the gas front migrating upwards from the near wellbore. This results in a much lower inflow of gas during well start-up, which can make it difficult to overcome the hydrostatic head to kick off the flow. This paper is intended to summarize the study of such a case where a high GOR horizontal well failed to start up after a long shut-in. The study considered all given potential causes of the start-up failure such as mechanical restrictions and the presence of a hydrate plug in addition to the assumption that only the low GOR oil rim would contribute to the inflow during the early phase of the start-up. A commercially available advanced transient multiphase flow simulator was used to simulate each scenario and the results were then compared with the field measurements. It was determined that the missing reservoir gas influx at the start-up was the most likely reason for the failure of the well to restart. This conclusion has provided valuable input to the well intervention decision making. After the study was completed, a light well intervention was performed on the well and it was confirmed that there was no mechanical blockage or restriction in the wellbore.