Degassing Analysis Of Gas-liquid Two-phase Flow In Underbalanced Drilling Wellbore

Underbalanced drilling plays an important role in improving drilling rate and protecting oil and gas reservoir.In drilling in oil reservoir of under-balanced drilling construction,the formation of crude oil into wellbore and in the air flow from bottom hole to wellhead,shaft ring as pressure,temperature,dissolved gas continuous precipitation resulting in decrease of non-equilibrium gas liquid two phase flow in the wellbore formation,the special wellbore flow of precise calculation for underbalanced drilling construction of bottom hole pressure prediction,flow parameter control,wellbore with rock,wellhead control plays an important role.Based on the comprehensive investigation of domestic and foreign literatures,this paper studied the physical parameters of oil and gas in the wellbore and the gas-liquid two-phase flow in the wellbore,corrected the gas solubility of the wellbore in the non-equilibrium flow,and calculated the gas-liquid two-phase flow with Mathcad combined with the construction case.The main achievements of this paper are as follows:(1)Based on the analysis of thermodynamic non-equilibrium degassing process,the h-k model is introduced into the gas-liquid two-phase flow theory to distinguish the flow pattern characteristics of non-equilibrium flow,and non-equilibrium degassing velocity and other physical quantities are introduced to describe the specific characteristics of gas-liquid two-phase flow in the wellbore.(2)Optimized the calculation method of physical parameters such as wellbore gas solubility,degassing speed,bubble point pressure and gas compression factor;Differential method and equilibrium constant method are used for degassing velocity.Experimental method,empirical formula method and chart method are given to calculate bubble point pressure.For the gas compression factor,the calculation methods of three different pressure grades of low pressure,medium pressure and high pressure are established,and the adaptability of each algorithm is clarified.(3)The influence of non-equilibrium gas-liquid two-phase flow on gas solubility,bubble point pressure and degassing velocity was investigated.According to the characteristics of non-equilibrium system,the therinodynamic model of gas solubility of non-equilibrium gas-liquid two-phase flow was established,and the degassing law of saturated gas-oil in non-equilibrium gas-liquid two-phase flow was analyzed by the model.(4)The degassing process of crude oil with saturated gas under equilibrium and non-equilibrium conditions was simulated in the laboratory.The mixed system in the reaction kettle was degassed by step-down pump.The different processes of non-equilibrium degassing and equilibrium degassing were implemented by adjusting the working state of step-down pump.The calculation equation of the gas solubility of the non-equilibrium gas-liquid two-phase flow was determined through the experiment,and the difference analysis of the non-equilibrium degassing process and the equilibrium degassing process under different working conditions was made.(5)Based on X well casing pro gram and drilling parameter,using Mathcad pro gramming and numerical simulation of different geothermal gradient and wellhead back pressure and different density of drilling fluid and drilling fluid displacement under no equilibrium conditions and equilibrium conditions of wellbore flow characteristic parameters,such as wellbore pressure distribution,equivalent density,gas phase velocity,flow pattern transition rule,etc.The theory and model established in this study can accurately describe the flow pattern change and degasification law of formation crude oil returning along the wellbore annulus after the underbalanced drilling meets the oil layer,which has important guiding significance for field well control.