Polymer Chemical Structure and its Impact on EOR Performance

摘要

Polymer flooding is a mature EOR technology that has seen an increasing interest over the past decade.Co-polymers of Acrylamide(AMD)and Acrylic Acid(AA)have been the most prominent chemicals to beapplied,whereas sulfonated polymers containing 2-Acrylamido-tertiary-butyl sulfonic acid(ATBS)havebeen used for higher temperature and/or salinity conditions.The objective of this study was to generateguidelines to aid in the selection of appropriate polyacrylamide chemistry for each field case.Our main focuswas in sandstone fields operating at the upper end of AMD-AA temperature tolerance,where it needs to bedecided whether sulfonation is required.The performance of the polymer throughout the whole residencetime in the reservoir was considered since the macromolecule can undergo some changes over this period.Several key properties of nine distinct polymer species were investigated.The polymers consisted ofAMD-AA co-polymers,AMD-ATBS co-polymers and AMD-AA-ATBS ter-polymers.The polymers werestudied both in their original state as they would be during the injection(initial viscosity,initial adsorptionand in-situ rheology)as well as in the state which they are expected to be in after the polymer has agedin the reservoir(i.e.in a different state of hydrolysis and corresponding viscosity retention and adsorptionafter ageing for various time periods).We note that the combination of viscosity retention and adsorptionduring the in-situ ageing process has not been typically investigated in previous literature,and this is akey novel feature of this work.Each of the above parameters has an impact on the effectiveness and theeconomic efficiency of a polymer flooding project.The content of ATBS was limited to 15 mol%.BuffBerea sandstone was applied in the static and dynamic adsorption experiments.The majority of the work was carried out in seawater at temperature,T = 58°C.Under these conditionsAMD-AA samples showed maximum viscosity and lowest adsorption when the content of AA was moderate(20 mol%).When the AMD-AA polymers were aged at elevated temperature,the AA content steadilyincreased due to hydrolysis reactions.When the AA content was 30 mol% or higher,the viscosity startedto decrease,and adsorption started to increase as the polymer was aged further.Thermal stability improvedwhen ATBS was included in the polymer structure.In addition,sulfonated polyacrylamide samples showedincreasing initial viscosity yields and decreasing initial adsorption with increasing ATBS content.For most of the samples,the maximum observed apparent in-situ viscosity increased when the bulk viscosity andrelaxation time of the sample solution increased.The information generated in this study can be used to aid in the selection of the most optimalpolyacrylamide chemistry for sandstone fields operating with moderate/high salinity brines at the upper endof AMD-AA temperature tolerance.