Research On Management Of Gas Influx In Constant Bottomhole Pressure MPD

Gas influx in drilling can be detected in time when the Constant Bottomhole Pressure Managed Pressure Drilling is adopted.Shutting in wells is no longer the only choice for well control,because gas influx could be brought under controlled quickly if effective methods were applied,such as increasing wellhead back pressure or turning up displacement.Although the CBHP MPD can quickly and effectively control a certain amount of gas invasion,safety risk increases at the same time due to more well control equipment,thus leading to the increasement of difficulties in well control decision.MPD technique has begun to applied in China and relevanting domestic devices has been developed simultaneously,however,theoretical researches about MPD remains comparatively rare.Accordingly,gas invasion control during CBHP MPD should be studied in further.Based on the theory of CBHP MPD and the basic principle of fluid mechanics,a flow model for gas-liquid two-phase flow in annular when gas influx occurs is established.Then a parameter analysis is conducted,aiming at varying patterns of bottom pressure for different stages of gas influx and the state after shut-in measures are carried out.Finally,using field data,safety risks under dynamic well control and bottom pressure variations during the time from gas influx arises to all the gas is totally discharged are analyzed.The simulation results show that flow-rate monitoring can detect the gas influx and it is also relatively accurate to reflect the kick rate and quantity of gas influx during a period of time;Controlling gas influx by increasing wellhead pressure in a short time leads to the rapid increasement of pressure at well bottom and casing shoe,as a result,the quantity of returning drilling fluid at well head decreases constantly until the pressure at bottom hole restores its balance once again,after which inlet rate equals to outlet rate;Controlling gas influx utilizing high displacement,the effect is limited when drilling fluid density is low even though with higher dynamic shear and viscosity of drilling fluid;When conventional shut-in method is adopted,stopping pumps will result in a sudden drop of bottom pressure,while MPD devices is beneficial to prevent it from dropping in further;Wellhead back pressure will continue to rise up,and it goes up sharply when gas flows up to wellhead;Improving circulating displacement contributes to bring down wellhead back pressure;At the beginning of circulation,flow rates of gas drilling fluid increase slowly while they reach its peaks successively when gas moves near to wellhead;Flow rate of gas and drilling fluid will increase with increasing displacement,thus the capacity of gas-liquid separator should be taken into account before turning up pump displacement;The problem of bottom pressure control under CBHP MPD when gas influx occurs has been studied in this paper,which is promising to enrich well control theory of pressure controlled drilling technique.