Feasility Study On CO₂ Flooding In High Water Cut Oilfields

Most of major oilfields in China have reached high water cut stage,but still,they contribute to 70%of domestic oil production.In petroleum engineering,how to extract more oil from mature oilfields has become a hot topic.Carbon dioxide flooding has been recognized as one of the most effective options for oil recovery enhancement in depleted or mature oilfields.Most of the researches have been focused on CO2 flooding,while very few feasibility studies on high water cut oilfields are seen in the literature.A set of theory and methodology should be established to evaluate the potentials of CO2 flooding in high water cut oilfields,while serving as a guide for the application and spread of CO2-EOR projects.This study was based on CO2 enhanced oil recovery mechanism.By applying reservoir simulation and experimental design,feasibility study was carried out on CO2 flooding in high water cut oilfields.Primary achievements could be concluded as follows.a)Evaluating indices were defined to assess the technical and economic efficiency of CO2 flooding.Based on the regression of relative permeability ratio kro/krw and water saturation Sw in high water cut stage,a new water flooding characteristic curve was derived which is more applicable for high water cut oilfields.The calculation results proved it to be a practical tool in the prediction of production indices in the late development stage of oilfields.b)CO2 water-alternating-gas flooding was considered to be more effective than continuous gas flooding.The orthogonal experimental design method was utilized to investigate the most effective operational scheme with a consideration of CO2 slug size,ratio of CO2 slug size to water slug size(WAG ratio),CO2 injection rate,and voidage replacement ratio.By applying data analysis software,SAS 9.3,evaluation models considering Improved Recovery Factor and Gas Replacing Oil Ratio were established with various operation parameters,respectively.Furthermore,the weighting factors of operation parameters were determined.c)The most effective operational scheme proposed above was used to perform reservoir simulations for technical and economic analyses of CO2 flooding.Five reservoir parameters and two development parameters were taken into account.Average horizontal permeability,permeability variation coefficient,ratio of vertical to horizontal permeability,rhythm of sand body and net thickness of formation were discussed as reservoir parameters;while well-spacing pattern and percentage of recoverable reserves by water flooding were studied as development parameters.d)The D-optimal design was applied to build the Rapid Assessment Model of CO2 flooding in high water cut oilfields.Five quantitative variables were considered,including the average horizontal permeability,the permeability variation coefficient,the ratio of vertical to horizontal permeability,the net thickness of formation and the percentage of recoverable reserves by water flooding.It could be used as a guide for the application and spread of CO2-EOR projects.e)The weighting factors of five quantitative variables and their interactions were analyzed on Improved Recovery Factor and Net Present Value of CO2 flooding.Using the Rapid Assessment Model of CO2 flooding in high water cut oilfields,type curves were established and analyzed for the evaluation of technical and economic efficiency of CO2 flooding in high water cut oilfields.