Well Bore-formation Coupled Heat And Mass Transfer Model For SAGD Quick Start Process And Its Application

SAGD is a relatively cutting-edge technology for the current exploitation of heavy and super-heavy oil.Before the SAGD production stage,the injector and producer and the areas around them are preheated.During the preheating process,the viscosity of the crude oil is reduced and the two wells will achieve a hydraulic and thermal communication.This stage is called the SAGD warm-up stage or the SAGD start-up stage.There are a variety of heating methods can be used for preheating in the SAGD warm-up phase or the SAGD start-up phase,one of which is steam circulation.Due to high viscosity of crude oil,it takes too much time for steam circulation in preheating process,which is costly and will eventually affect final profits.As a result,some scholars put forward a new method to speed up the preheating called dilation assisted preheating.High-pressure steam or hot water is firstly injected into the formation to form dilation region around the wells,after that,steam circulation is conducted.The aim of dilation is to improve the porosity and permeability of the region around the wells,so that heat transfer process can change from only heat conduction into both heat conduction and thermal convection which will shorten the time to reach a certain temperature.In this way,shortening warm-up time by preheating in dilation region is called quick start.At present,there are some problems with the studies of heat and mass transfer in SAGD start-up process:(1)almost all of the existing steam circulation models did not consider the steam entering the formation;(2)this studies did not take into account the existence of dilation region and did not consider the influence of the parameters such as shape,size,permeability of the dilation regions on steam circulation process;(3)most of the existing calculation models only take single well steam circulation process into consideration which forget the interaction between the two wells that may exist when two wells circulation at the same time.For these problems,and based on theories of heat transfer,thermophysics,mechanics of fluid flow in porous media,this thesis conducts a systematic research and analysis of heat and mass transfer in SAGD quick start process.Firstly,taking the long and short pipe string structure as the research object,heat and mass transfer of multiphase flow in single well and two well is analyzed with considering the influence of steam entering the dilation region.The formulas for calculating the steam flow and heat transfer in the long tubing and the annulus are given,and the mathematical model of heat and mass transfer of multiphase flow is established.Secondly,combined with the real situation of oil field,a simplified scheme of dilation region is put forward.On the basis of this,the heat and mass transfer between wellbore and formation in single well and two well steam circulation process is analyzed with considering the influence of dilation region.The formulas for calculating pressure and temperature of dilation region and original formation are given,and the wellbore-formation coupled heat and mass transfer model is established.Thirdly,combined with the established single well and two well steam circulation models,the intermediate variables needed to be identified and acquired in the models are analyzed one by one,the ideas and processes of solving the models are given,and the models are solved according to the actual parameters of a block in Xinjiang Oilfield.Finally,based on the results of models,the characteristics and time-varying characteristics of heat and mass transfer in the wellbore,between the wellbore and the dilation region,and between the dilation region and the original formation in the single well and the two well steam circulation processes are analyzed.The factors that affect the effectiveness of the expansion zone and the preheating time are analyzed,and the main controlling factors are found out.The applications of both models are pointed out at the same time.The research results of this thesis are an important supplement to the current study of heat and mass transfer in SAGD start-up process.It has significant meaning in further understanding of heat and mass transfer process from wellbore to formation in steam circulation when there are dilation regions.