Abstract

H-3 Block is an ultra-low permeability reservoir in Changqing oil field, China which had been waterflooded from 2009 and was switched to CO2 flooding in 2013 due to excess water production. However, the nature fractures in NE-SW region have resulted in early CO2 breakthrough and poor production performance. The investigation of CO2 production performance and the method to control CO2 production becomes a key to continue CO2-EOR project.

Outcrop cores are used to perform a series of CO2 flooding experiments at reservoir conditions of pressure, temperature and formation water salinity. Permeability heterogeneity and injection pressure are considered as two variables to affect gas channeling characteristics. It is figured out that producing gas-oil ratio and components analysis of effluent could be used to judge gas channeling and timing to control CO2 production for field use. Starch gel is developed to control CO2 production within nature fractures to improve CO2 swept volume in rock matrix. Ethylenediamine (EDA) is proposed to delay CO2 production within high-permeability zones, and the application boundary as a function of permeability heterogeneity is determined.

Three production stages are clearly stated based on production performance and experimental observation, including gas-free production stage, oil/gas co-production stage, and gas channeling stage. A significant new finding is that oil/gas co-production stage contributes the most to oil recovery. And oil-CO2 mass transfer zone, rather than free CO2, reaches the outlet at this stage, which is proved by color of effluent and chromatographic analysis. For field cases, producing gas-oil ratio and components analysis of effluent at wellhead could help field engineers make a decision: keep producing with caution at oil-gas co-production stage or control the CO2 production at gas channeling stage. The conformance improvement and the increase in injection pressure could remarkably enhance the oil recovery at oil/gas co-production stage. To delay gas channeling and extent oil/gas co-production stage, two-level gas channeling control is presented. A slug of starch gel is first injected to block fractures and then ethylenediamine is injected to react with in-situ CO2 within high-permeability zone. The starch gel, acting as pure viscous fluid, would not leave contamination in rock matrix. And the viscous reactant of ethylenediamine and in-situ CO2 could successfully tune injected CO2 to flood low-permeability zone when permeability ratio is less than 100.

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