Study On Process Calculation Method Of CO₂ Flooding Surface Gathering System

Low permeability oilfield in Daqing using CO₂ flooding to enhance oil recovery, which has achieved good results. These blocks’ surface gathering system generally use cyclic mixing watering gathering process. With the increase of CO₂ content in the wellhead production fluid, will cause the pipeline temperature drop increases, resulting in oil ring blocking accidents. Therefore, it is necessary to aiming at the problems of CO₂ flooding ground gathering system to carry out systematic research, find out the bottleneck problem.This paper established CO₂ flooding oil mixture phase equilibrium calculation model, CO₂ flooding wellbore’s thermal and hydraulic calculation model, CO₂ flooding branch-annular mixing water pipe network thermal and hydraulic calculation. In the CO₂ flooding oil mixture phase equilibrium calculation model, we used the modified mixing rule of BWRS equation as the state equation. In the wellbore thermal and hydraulic calculation model, we considered the fluid friction of the wellbore wall and Joule-Thomson effect factors. On this basis, we proposed a wellbore thermal hydraulic numerical calculation method. In the branch-annular mixing water pipe network’s thermal and hydraulic calculation model, we considered different pipeline thermal and hydraulic calculation model, network node calculation model. On this basis, we proposed a network thermal hydraulic numerical calculation method. We have applied the C# language to compile the software of the "CO₂ flooding surface gathering and transportation process calculation software", which can make the CO₂ flooding oil mixture phase equilibrium calculation, CO₂ flooding wellbore thermal and hydraulic calculation, CO₂ flooding branch-annular mixing water pipe network thermal and hydraulic calculation and analysis. From the five aspects of oil production, gas-oil ratio, oil gathering network’s length, ambient temperature and CO₂ mole fraction, the effects of these five factors on the temperature of the ground gathering and transportation network are analyzed. In which the gas-oil ratio, the length of oil gathering ring and the CO₂ mole fraction of the increase can lead to oil gathering ring temperature drop increases, the lower oil production and environment temperature can cause the temperature drop of oil gathering ring to be increased.