Dynamically Coupled Well-Reservoir Model And Field Application In Gas Condensate Reservoirs

Gas condensate resources in China are abundant and have a broad development prospect,which is an important position for increasing gas reserves and production.Complex phase change characteristics exist in both the wellbore and reservoir during the production of condensate gas reservoirs and condensate oil is precipitated in the reservoir area,resulting in gas condensate plugging.The phase change in the wellbore triggers the complicated flow of oil,gas,and water,which makes it difficult for gas to lift liquid underground and thus leads to the liquid loading phenomenon in the wellbore.Reservoir depletion,the complicated flow of oil,gas,and water in the wellbore and reservoir,and the liquid loading phenomenon are all transient flow problems caused by the interactions between the wellbore and reservoir.Therefore,this paper focuses on the dynamic flow problems between the wellbore and reservoir and adopts a theoretical research method to establish a fully implicit wellbore-reservoir coupling model to solve the transient flow problems.Firstly,considering the complex phase change characteristics of condensate gas in the wellbore and reservoir,the PR state equation is proposed to describe the composition of oil and gas,and the Newton iteration method is applied to calculate the gas-liquid equilibrium to obtain the t physical properties of gas and oil.The coupling modeling of the wellbore/reservoir means the coupling of the partial differential equations of the wellbore pipe flow model and the partial differential equations of the reservoir seepage model.Tang et al.2019 drift model is selected as the transient flow model in the wellbore to ensure the stability and convergence of the coupling model.,which achieves the multi-phase flow and reverse flow of fluid in the wellbore under any inclination angle.Secondly,based on the mass conservation of source/sink items at the wellbore/reservoir interface and the quantitative coupling criterion,the meshed wellbore system is taken as the additional grid block of the reservoir model.The fully implicit coupling mechanism is adopted to realize the simultaneous solution of two sets of partial differential equations of the wellbore-reservoir coupling system.Therefore,the construction of fully implicit wellbore-reservoir dynamic coupled models is finished.Then,consider the phase transition of the wellbore and complex oil-water-gas three-phase flow characteristics in the typical problem of liquid loading phenomenon caused by the dynamic interaction between the wellbore and reservoir.Based on the liquid film inversion theory,modeling of gas core and liquid film flow in the wellbore was carried out,and the influence factors such as the inner diameter of the tubing,circular angle,non-uniform film thickness,pressure drop gradient,gas holdup,and fluid properties on the phenomenon of liquid loading were analyzed,then the critical liquid film model was proposed in this paper.Finally,the fully implicit dynamically coupled wellbore-reservoir model is verified by some application case studies: gas well test with the wellhead boundary solution,depletion production of the gas condensate reservoir,and liquid loading phenomenon in gas well.The first case is taking the wellhead as the solution point to analyze the gas well test,the coupled model can record the response characteristic of the wellbore and reservoir to transient pressure during shut-in and start-up.The coupled model can not only reflect reservoir properties and reservoir parameters but also record strong transient phase transformation of condensate gas,fluid properties,and real-time changes of temperature field and pressure field in the wellbore.During the shut-in period,when permeability is 70 and 700 m D,gas flow velocity direction changes,gas production fluctuates obviously,and gas flow time prolongates.With a permeability of 700 m D,the coupled model captures the dynamic flow of gas from the wellbore to the reservoir and then from the reservoir to the wellbore.When simulating the depletion production of the gas condensate reservoir,the coupled model simulates wellbore phase transition and reservoir phase transition synchronously.The simulation results show that with the increase of simulation time,the condensate oil saturation increases and then decreases,the heavy component C7+ in the condensate oil keeps increasing,and the light component C1 and C2 in the condensate gas increases.The condensate gas fraction in the wellbore decreases while the condensate oil fraction increases.The dynamically coupled wellbore-reservoir model can accurately describe the whole cycle of the liquid loading phenomenon from occurrence to completion.After the occurrence of liquid loading,the gas production,water production,and bottom hole pressure have an unstable production period,and the flow pattern and liquid holdup in the wellbore change dramatically.In the unstable period of liquid loading production,the annular flow and churn flow coexist in the wellbore for a long time.The upper part of the wellbore is annular flow and the lower part is churn flow.Furthermore,after the occurrence of liquid loading,the wellbore back pressure on the reservoir increases,leading to the reservoir pressure near the wellbore increasing,and the gas phase saturation increasing too.This paper emphasizes the necessity of the coupled wellbore-reservoir dynamic model to solve the transient flow problems between the wellbore and reservoir and establishes a fully implicit dynamically coupled wellbore-reservoir model based on component simulation.This study can effectively simulate and solves the current problems in the dynamic analysis of well production in gas condensate reservoirs,and provides the theoretical support and technical guidance for solving the transient flow problem between the wellbore and reservoir.