Effect of crude oil ageing on low salinity and low salinity surfactant flooding

摘要

Injection of brine with lower salinity than the connate brine has proven to give a moderate increase in oilrecovery in sandstones. Recent research has shown that this process will significantly benefit fromintroducing surfactant optimised for low salinity environment.The mechanisms underlying increased recovery by low salinity brine injection are not yet fully understood.However, research to date suggests that they are related to complex crude oil/brine/rock interactions. Withthis in mind, the present paper investigates primarily how the extent of oil recovery from Berea sandstonessubjected to long term exposure of crude oil is influenced by (1) low salinity water injection and (2) combinedprocess low salinity water injection with surfactant flooding.Core displacement tests were conducted on four Berea cores (30 cm), two in a natural state and two that hadbeen subject to extensive crude oil ageing at high temperature. Results obtained from different flooding stepsare discussed in terms of oil recovery and effluent properties including turbidity, pH- and ion analysis (Na+,Mg~(2+), Ca~(2+)). The results effectively illustrate that oil recoveries from the aged cores are higher during bothlow salinity water injection and low salinity water injection combined with surfactant flooding. Anassessment of how tertiary oil recovery in aged and unaged cores varies with surfactant concentration is alsopresented.Effluent ion analysis from low salinity water floods showed that Mg~(2+) ions were strongly retained in the agedcore while Ca~(2+) ions were being produced from both aged and unaged cores. The latter was attributed mainlyto calcite dissolution. Results obtained from pressure profiles, effluent ion analysis and turbidity tests suggesthigher production and elution of fine particles from the unaged core.