Research On Effect Of Oil-gas Influx On Annulus Pressure In Underbalanced Drilling

Underbalanced drilling is the way of controlling hydrodynamic pressure in annulus is under formation pore pressure. The oil-gas reservoir damage is greatly minimized owing to the effect of pressure differential in annulus, the environmental protection type drilling is one of the biggest advantages of underbalanced drilling. In addition, underbalanced drilling is in good manner in improving drilling rate and increasing production of oil and gas.In the process of underbalanced drilling, oil-gas from reservoir flows into the annulus. Bottom hole pressure is changed owing to the influx of the oil and gas from reservoir, therefore differential pressure in bottom is changed also. Oil-gas inflow is affected owing to the change of differential pressure in bottom hole, the differential pressure in bottom hole and oil-gas inflow is mutual affected, the coupling of oil-gas from reservoir and the fluid in the wellbore is involved. In order to precisely control the bottom hole pressure of underbalanced drilling, the effect of mass transfer between wellbore and reservoir to bottom hole pressure should be considered. In the study of other researchers, the mass transfer effect between reservoir and wellbore is ignored.In this paper, based on the summaries and references from the predecessors achievements, the following work is carried:(1)In the process of establishing influx model of horizontal well in underbalanced drilling, the achievements of predecessors mainly concentrated in the case of normal drilling that the flow state was turbulent flow in annulus. However, the influx model of the period of making connections is ignored. In this paper, on the basis of the influx model that annulus circulating fluid is under turbulent flow state, the influx model of the period of making connections is established. The type of established model is under turbulent flow state of annulus circulating fluid. Finally, the effect factors of oil-gas influx are analyzed.(2)The multiphase flow theory in annulus is studied in underbalanced drilling. The coupling model of oil-gas inflow and wellbore is established, the influx model is solved by finite difference method. Then, through the calculation examples, the effect factors of bottom hole pressure is analyzed. At the same time, the impact of annulus pressure considering the oil-gas inflow from reservoir or not is analyzed. Finally, bottom hole pressure change law is revealed considering the coupling of oil-gas reservoir and wellbore.