Chemical EOR is an increasingly employed approach used to enhance oil recovery by combining changes in fluids mobility, macroscopic sweep, interfacial tension, etc. to essentially improve, or extend the economic life of a water flood. It includes flooding with polymer, surfactant, alkaline/surfactant, alkaline-surfactant-polymer (ASP), CO2 and / or other miscible gases which is often combined with waterflood (e.g., CO2 WAG) etc. However, the improved oil recovery is often accompanied by physical and chemical changes in the produced fluids that cause many production-chemistry (PC)-related challenges when fluids subsequently arrive in the production system, including exacerbation of scale and naphthenates deposition, carboxylate deposits associated with injected polymer, enhanced corrosion and separation issues, etc. Understanding and predicting the production chemistry challenges at producers are further complicated by chemical changes as the fluids propagate through the reservoir such as reaction with reservoir formation minerals, chemical retention, chemical degradation and hydrolysis, etc. More importantly the implications for the production system and processing facilities are not always accounted for and proactively managed.
The paper evaluates the main chemical changes that occur in the system for each EOR approach –– and shows how these changes, including in situ reservoir reactions and the stability/instability of the EOR packages themselves can exacerbate a range of PC-related challenges especially when considering the likely production of up to three different fluids: formation water, the EOR flood medium and any previous flood water from previous secondary recovery
The paper includes modelling results, laboratory results to validate model predictions as well as examples from field case studies to illustrate the impact of the chemical changes referred to above. Specific highlights include the impact of the use of either high- or low-pH EOR fluids on scale control, corrosion control and asphaltenes control; for scale it examines both inhibitor performance per se as well as retention onto rock during squeeze treatment. Also illustrated are the risk of carboxylate-based deposit derived from polymer flood, and the phenomenon of carboxylate-based solids and soaps, which can exacerbate the separation of an already highly challenging system.
The overall conclusion is that chemical EOR can have significant impact on PC and that these should not just be considered at the design stage and not just for the injection system but also to take into account the impact these may have on production wells following breakthrough of flood waters, showing that essentially each new or exacerbated PC issues can be predicted or at least anticipated with the required degree of confidence before implementation of EOR.
