Quantitative Evaluation Of Formation Damage Due To Asphaltene Deposition When CO₂Flooding

Asphaltene deposition in crude oil will seriously reduce production during CO2gasinjection. The current experimental and theoretical models are incapable to directly give thereasonable guidance to the strategies of oil field development.The purpose of this thesis is todevelop a systematic method and process to quantitatively describe the formation damage dueto asphaltene precipitation in the process of CO2injection,by conducting permeability damagemodel study, asphaltene phase study and simulation study.Firstly, conventional power function models for permeability damage were summarizedbased on the classic K-Φ relationships of various sand types; An innovative permeabilitydamage model was developed based on the grain-pore-throat model (GPT), which takes intoaccount the effect of adsorption on the surface of grain and the bridge plug in the throat.Secondly, asphaltene phase behavior was studied by gas–liquid-solid model with theexperimental data of crude oil samples from block Dong14, Daqing oilfield and then theobserved bubble point and the amount of asphaltene precipitation under various CO2concentrations were matched by the model. Then, a dynamic model of the pilot area for CO2drive was constructed by integrating geological model, permeability damage model andasphaltene precipitation phase data. The quantitative analysis of the distribution of asphalteneprecipitation, the reduction magnitude of permeability, and the capability of cumulativeinjection and cumulative production were conducted using the model. Moreover parametersensitivity analysis were conducted, including pore throat characteristics of lithologic,asphaltene content, formation pressure, formation temperature and CO2purity of the injectedgas.The following understanding and results are obtained：(1) The conventional powerfunction models for permeability damage were based on grain-sorted results, and thepermeability damage depends on the power function exponent n derived of the the porosityand permeability relationship. The innovative GPT model for permeability damage takes intoaccount the effect of adsorption and bridge plug, which is more serious on the permeabilitydamage than conventional power model as the permeability damage is primarily due to thenarrowing of the throat in the inovative model.(2) A comprehensive quantitative index:reduction rate of accumulative production--RCPI was proposed to describe the formationdamage. With respect to the index the magnitude of reservoir damage can be described. RCPIhas no obvious relationship with permeability, but has a logarithmic relationship betweenRCPI and the exponent n determined by GPT model. RCPI is not senstive obviously to theoriginal formation pressure, but sensitive to the original temperature. Moreover RCPI has asignificant positive linear correlation with CO2concentration.Results of the example applied：Dynamic model of Dong14was established, which wasa pilot area for CO2flooding. The pore volume of the model is2.11×105rm3, and with thesubsurface oil volume1.05×105rm3, and with the oil in place is9.90×104sm3. It’s a5-yearproduction forecast model with17water and gas slugs, and with a total of approximate1HCPV CO2injection. The recovery factor declined from15%to9.5%, when consideringreservoir damage.