Laboratory and modelling studies on the effects of injection gas composition on CO₂-rich flooding in Cooper Basin, South Australia. (2009)

This Ph.D. research project targets Cooper Basin oil reservoirs of very low\npermeability (approximately 1mD) where injectivities required for water flooding\nare not achievable. However, the use of injection gases such as CO₂ would not\nhave injectivity problems. CO₂ is abundant in the region and available for EOR\nuse. CO₂ was compared to other CO₂-rich injection gases with a hydrocarbon\ncontent including pentane plus components. While the effect of hydrocarbon\ncomponents up to butane have been investigated in the past, the effect of n-pentane\nhas on impure CO₂ gas streams has not.\nOne particular field of the Cooper Basin was investigated in detail (Field A).\nHowever, since similar reservoir and fluid characteristics of Field A are common to\nthe region it is expected that the data measured and developed has applications to\nmany other oil reservoirs of the region and similar reservoirs elsewhere.\nThe aim of this Ph.D. project is to determine the applicability of CO₂ as an injection\ngas for Enhanced Oil Recovery (EOR) in the Cooper Basin oil reservoirs and to\ncompare CO₂ with other possible CO₂-rich injection gases.\nThe summarised goals of this research are to:\n• Determine the compatibility of Field A reservoir fluid with CO₂ as an\ninjection gas.\n• Compare CO₂ to other injection gas options for Field A.\n• Development of a correlation to predict the effect of nC₅ on MMP for a CO₂-\nrich injection gas stream.\nThese goals were achieved through the following work:\n• Extensive experimental studies of the reservoir properties and the effects of\ninteraction between CO₂-rich injection gas streams and Field A reservoir\nfluid measuring properties related to:\n•\tMiscibility of the injection gas with Field A reservoir fluid\n•\tSolubility and swelling properties of the injection gas with Field A\nreservoir fluid\n•\tChange in viscosity-pressure relationship of Field A reservoir fluid\ndue to addition of injection gas\n• A reservoir condition core flood experiment\n• Compositional simulation of the reservoir condition core flood to compare\nexpected recoveries from different injection gases\n• Development of a set of Minimum Miscibility Pressure (MMP)\nmeasurements targeted at correlating the effect of nC₅ on CO₂ MMP.\nThe key findings of this research are as follows:\n• Miscibility is achievable at practical pressures for Field A and similar\nreservoir fluids with pure CO₂ or CO₂-rich injection gases.\n• For Field A reservoir fluid, viscosity of the remaining flashed liquid will\nincrease at pressures below ~2500psi due to mixing the reservoir fluid with\na CO₂-rich injection gas stream.\n• Comparison of injection gases showed that methane rich gases are\nmiscible with Field A so long as a significant quantity of C₃+ components is\nalso present in the gas stream.\n• There is a defined trend for effect of nC₅ on MMP of impure CO₂. This trend\nwas correlated with an error of less than 4%.\n• Even though oil composition is taken into account with the base gas MMP,\nit still affects the trend for effect of nC₅ on MMP of a CO₂-rich gas stream.\n• An oil characterisation factor was developed to account for this effect,\nsignificantly improving the results, reducing the error of the correlation to\nonly 1.6%.\nThe significance of these findings is as follows:\n• An injection pressure above ~3000psi should be targeted. At these\npressures miscibility is achieved and the viscosity of the reservoir fluid injection\ngas mix is reduced.\n• CO₂ should be compared to gases such as Tim Gas should after\nconsidering the cost of compression, pipeline costs and distance from\nsource to destination will need to be considered.\n• The addition of nC₅ will reduce the MMP and increase the recovery factor,\nhowever the cost of the nC₅ used would be more than the value of\nincreased oil recovered.\n• The developed correlation for the effect of nC₅ on impure CO₂ MMP can be\nused broadly within the limits of the correlation.\n• Further research using more oils is necessary to validate the developed oil\ncharacterisation factor and if successful, using the same or similar method\nused to improve other correlations.