Flow Characteristics And Numerical Simulation Of Polymer Flooding In Heavy Oil Reservoirs

The heavy oil reservoirs in China are rich in geological reserves,and their rational,efficient and economical exploitation can relieve the pressure of crude oil import from other countries.Compared with onshore heavy oil reservoirs,the development of offshore heavy oil reservoirs is restricted by space,environment,etc.Among development methods,polymer flooding is considered as one of the primary methods for enhancing oil recovery in offshore heavy oil reservoirs due to its good adaptability.Therefore,it is of great significance to study the polymer flooding to improve the recovery factor of offshore heavy oil reservoirs.In this thesis,the Block A of Bohai Oilfield is taken as the research object.Firstly,the rheological properties of heavy oil and its flow in porous media are investigated,and the polymer flooding is evaluated indoors by physical experiments.Then,based on the experimental research,a three-dimensional three-phase full-implicit numerical simulator for non-Darcy flow in the polymer flooding in heavy oil reservoirs is designed and validated.Moreover,a KPOD-ITPWL model reduction method is proposed by combining the Krylov enhanced Proper Orthogonal Decomposition(KPOD)method and the improved Trajectory Piecewise-Linear(ITPWL)method.The reduced order acceleration methods for the designed simulator are proposed by applying the Proper Orthogonal Decomposition(POD),Proper Orthogonal Decomposition-Trajectory Piecewise-Linear(POD-TPWL)and KPOD-ITPWL model reduction methods in the numerical simulation of non-Darcy flow in polymer flooding in heavy oil reservoirs,respectively,and their application effects are analyzed.Finally,the numerical simulation of non-Darcy flow in polymer flooding in heavy oil reservoirs and its KPOD-ITPWL reduced order acceleration method are applied to study the influencing factors of polymer flooding in heavy oil reservoirs and the development optimization of an actual reservoir.The key conclusions and understandings of this thesis are as follows:(1)The results of heavy oil rheological experiments show that the heavy oil in the target reservoir has the characteristics of a Bingham fluid.The threshold pressure gradient of heavy oil is measured by the improved micro-flow method,and its experimental results show that the threshold pressure gradients of heavy oil are not equal even when the viscosity of the heavy oil and the permeability of the porous media are the same.The threshold pressure gradient of heavy oil cannot be characterized by only two parameters of viscosity and permeability.Moreover,the relationship between the threshold pressure gradient of heavy oil and the component content of heavy oil,permeability and temperature is characterized by experiments.The threshold pressure gradient of heavy oil increases with an increase in asphaltene content or resin content,and decreases with an increase in aromatics content,saturates content,permeability or temperature.Within the research scope of this thesis,the descending order of influence of various factors on the threshold pressure gradient of heavy oil is: temperature,permeability,asphaltene content,saturates content,aromatics content and resin content.It has also been found that the flow of heavy oil in porous media no longer follows Darcy's law.(2)The first-order dynamic degradation rate constant of polymer is determined by the designed indoor experiment of polymer dynamic degradation under reservoir conditions.It is found that the first-order dynamic degradation rate constant of polymer is greater than its first-order static degradation rate constant under the same conditions.Moreover,the effect of a flow rate and permeability on the first-order dynamic degradation constant of polymer is quantified by the designed polymer dynamic degradation indoor experiments.The first-order dynamic degradation constant of polymer increases with an increase in the flow rate and with a decrease in permeability,and the influence of permeability is greater than that of the flow rate.(3)The designed simulator can not only reasonably reflect the threshold pressure gradient of heavy oil and polymer degradation,but also distinguish the static and dynamic degradation of polymer,and has high stability and accuracy.(4)The accuracy of the POD method is high,but its acceleration is limited.Compared with the POD method,the accuracy of the KPOD-ITPWL method is not much different,but the KPOD-ITPWL method is more efficient.Compared with the POD-TPWL method,the KPOD-ITPWL method has advantages in accuracy and acceleration.(5)The ultimate recovery factor of polymer flooding in heavy oil reservoirs is negatively correlated with the threshold pressure gradient of heavy oil,the viscosity of heavy oil,the first-order degradation constant of polymer,the polymer adsorbance and the water cut during the polymer injection,and positively correlated with the flow behavior index of polymer solution,the inaccessible pore volume of polymer,the residual resistance factor,the injected polymer concentration and amount.Through economic evaluation,it is found that the optimal development adjustment plan for the target reservoir is: additional drilling of 7 new production wells,the polymer concentration of 1750 mg/L in injection wells,and a polymer injection period of 3years.In this thesis,physical experiments,theoretical analysis and numerical simulations are applied to investigate the polymer flooding in heavy oil reservoirs in depth.This thesis solves some problems existing in the current research,lays a theoretical foundation for enhanced oil recovery in heavy oil reservoirs,and further promotes the rational and efficient development of heavy oil reservoirs in China.