Abstract
The development of mature fields has led operators to use underbalanced drilling to benefit from advantages such as reducing formation damage and increasing penetration rate. In gasified liquids, a light oil can be used as the liquid phase and nitrogen with almost 5 percent oxygen as the gas phase. Due to the simultaneous presence of oil and oxygen in the drillstring, safety considerations are a major concern in these operations.
To study and simulate the occurrence of explosions, oil base liquids were aged at 150°C and 14 MPa initial pressure in rocking and stationary reactors for 2.5 and 5 days in the presence of a gas with different concentrations of oxygen. The presence of drilling cuttings was simulated by addition of calcium carbonate to the base liquids. To cover the real penetration rates encountered in the drilling industry, volume percentages of 0.3, 0.6, 1.2, 2.4, and 4.8 of calcite were employed.
The experimental results of the samples that experienced explosions show that a delay time of approximately 0.5 hours was observed. In the period leading up to an explosion, the vapor phase temperature remained relatively constant while the liquid phase temperature first increased gradually and then more rapidly. At the time of the explosion, a rapid rise in the vapor phase temperature accompanied by a sudden increase in the reactor pressure was observed. This behavior indicates that the explosion was triggered by liquid phase oxidation reactions but that the sudden energy release event was associated with combustion reactions in the vapor phase.
This study signifies the important impact of the liquid phase temperature on explosion occurrence. The liquid phase acts as an initiator or an ignition source for explosions. Furthermore, it can be used as a parameter to warn of an impending explosion.
