Study Of Gas Kick Rule And Migration Model During Underblance Drilling

Maintain stable voltage value was underbalance pressure drilling′s core, in gas underbalance drill, bottom-hole pressure value would be sharp increased, which against borehole stability, even aroused well blowout and well kick, because the formation gas. Through the mud density or aggravate add back pressure gas handling problems, cannot achieve purpose of underbalance pressure drilling. So accurate calculation gas invasion and air bubbles in the ring of migration rule was the key to the rational and safety underbalance pressure. The reservoir conditions and drilling parameters on the influence of the gas invasion were considered, computational model of bubbles formed dimensions in different formation were established, the migration and rule of bubble rings air were analysed, according to the bubble dynamics theory, and therefore accurate scientific basis for the design of reasonable underbalance pressure in air cushioning were provided.First, according to unstablefiltration theory, the reservoir condition and the penetration rate had a strong effect on gas invasion were considered, calculating model underbalance with drilling were established, drilling rate had impacts on gas drilling were analysed, the requisite parameters were provided for bubbles formed dimensions and the moving rule.Second, model of bubble formation were established, bubble formation was divided into two phases of inflation and departure during bubble formation on force analysis. According to the influence of the instantaneous gas bubbles formation, the calculating model of critical gas were established by bubble coalescence conditions, the geological conditions and drilling parameters were considered, computational model of bubbles formed dimensions were established, bubbles from frequency of the different pore body radius had be determined. Model of the rising velocity of small bubble were established, the bubble size and flow rules were considered, the relations of the rising velocity of bubble and air bubble diameter was obtained, which laied a sound foundation in calculation of bottom underbalance pressure for air cushioning.Again, turbulence modeling and Euler multiphase model were applied, dynamics rule of the single bubble were calculated, and the effects of the drilling fluid propertiesthe on the bubble deformation were analysed. At the same time, the fusion mechanism of bubbles from bubble dynamics were analyed, bubbles in the ring together with the condition and fusion rate, which reflected in the air bubbles during migration, and therefore accurate scientific basis for fluid type of compartmentalizing were provided.Finally, the debris content and flow pattern of gas cut drilling fluid were considered. A bottom hole pressure calculation model was established. According to the established calculation model of gas invaded volume while drilling, the calculation model of gemerated diameter for central air bubble, the speed calculation model of bubble fusion and the calculation model of bottom hole pressure during gas cut, an example of calculation for a well under balanced drilling process was carried out in Daqing area. The gas quantity and well backpressure variation by the present model were well agreed with the actual defects, the model of gascut-while-drilling and the model of bubbles formation, migration and deformation could be provides necessary basis for the rational design of underbalance pressure in gas underbalance stage, the theory was of great theoretical and practical importance.