Low salinity water flooding Experimental experience and challenges Force workshop Ingebret Fjelde

Outline Questions Experiences Challenges Concluding remarks

Questions Are the mechanisms in low salinity water flooding understood? Can low salinity water flooding be used in all sandstone fields? If not, when can it be used? Will low salinity water flooding improve the oil recovery in all sandstone reservoirs? Can oil recovery be estimated by existing reservoir simulators?

Low salinity water flooding can not be understood by classic EOR No miscible process No reduction in IFT Mobility control? Favorable mobility control if M<1 Reducing krw or mo Increasing kro or mw Non of these seems to be dominating When all other explanations have failed put the blame on wettability

Literature Reported to improve oil recovery in sandstones Core experiments and reservoirs Reported to depend on, e.g. Multicomponent ion exchange important Clay content (sometimes fines produced) Composition formation water (Ca2+, Mg2+) Oil composition (crude oil, no effect with white oils) Initial water saturation required pH increase (≈ alkaline flood, but no correlation with AN)

Tulsa SPEIOR 2008 SPE 113480, Endicott field Improved oil recovery shown in core floods and single well tracer test Kaoline content important SPE 113976 Mechanisms Multiple-component ionic exchange (MIE) between adsorbed crude oil components, cations in the in situ brine and clay mineral surfaces Single well tracer test Alaskian reservoir High salinity water and produced water Sor=0.300.02 Non optimised low salinity brine Sor=0.280.02 Optimised low salinity brine Sor=0.200.02 Composition of optimised brine not given

Irreversible? SCA2006-36 Re-aged cores higher oil recovery at high salinity, but no oil production in tertiary low salinity water flooding Not likely due to multiple-component ionic exchange

Spontaneous imbibition limestone Formation water vs low salinity water Low salinity water can also increase oil recovery in limestone Probably not only clastic clays that are important for low salinity water flooding of sandstone

Adsorption surface active components onto surfaces Surface charge pH, salinity and brine composition Adsorption density depends on salinity and brine composition Desorption Depend on the same parameters as adsorption By decreasing salinity adsorption density decreases Change of brine composition can also change surface charge Electrical double-layer expands with decreasing brine salinity

Adsorption of acidic crude oil components onto chalk Acid number in effluent during injection of crude oil to chalk

Surfactant adsorption Decreases with decreasing salinity Adsorption equilibrium Usually reversible Reduction of salinity will give desorption Similar expected for surface active components in crude oils Asphaltenes may be an exception Somasundaran, P. and Hanna, H.S., ”Physico-chemical aspects of adsorption at solid/liquid interfaces,” in Improved Oil Recovery by Surfactant and Polymer flooding, ed. D.O. Shah and R.S. Schechter, Academic Press, New York, 1977, pp. 2005-74

Drilling fluids Water in drilling fluids can give swelling of clay and shale and reduction of permeability Solved by using inhibitive drilling fluids Water based drilling fluid with high salinity Emulsified mud with high salt concentrations Swelling clay: Bulk study Low salinity water flood may not be carried out in all oil reservoirs

Mechanisms Alteration of flow saturation functions Description of mechanisms required

Challenge Time effects Analyses of effluents necessary to confirm that adsorption equilibrium has been established Preparation of initial state at high salinity Preparation of final state at low salinity Studies of mechanisms Time effects can be important in the laboratory, but not in the fields But laboratory results will be used as inputs to simulators for estimations of oil recovery potential

Time effects 1 Adsorption can be slow Oil components Aging of core plugs Chemicals Surfactants Polymers I.Fjelde, T. Austad and J. Milter, ”Adsorption VII. Dynamic adsorption of a dual surfactant system onto reservoir cores at sea water salinity,” J. Petr. Science & Eng., 13 (1995), 1993-201.

Time effects 2 Desorption surface active components can be slow Interfacial tensions (IFT) between effluent samples and formation water during back-production of mud filtrate with crude oil Fjelde, I., “Slow retention and release processes during drilling with emulsified drilling fluids,” 18th International Oil Field Chemicals Symposium, Geilo, Norway, 25-28 March 2007. IFT (crude oil – brine) ≈ 30 mN/m Mud filtrates

Challenge Crude oils Complex mixtures of surface active oil components Many types of surface active components Different crude oils different types and concentrations Interactions between crude oil components Interaction between resins and asphaltenes important for asphaltene solubility / dispersion Simplified systems may not give good enough description Concentrations of surface active oil components and their solvency different in stock tank oil and live oil Some oil components soluble in water

Challenge Crude oils cont. Mixing of crude oils with low aromatic synthetic oil Will reduce concentrations of surface active oil components Can reduce solvency of some of the surface active components, e.g. will increase aggregation of asphaltene molecules in oil phase and on surfaces

Challenge Crude oils / Oxidation Oxidation of crude oil will increase concentrations of acidic components Oxidation products can have other properties than original polar components Oxidation products can be less soluble, e.g. oxidation asphaltenes Important to compare concentration of surface active components in used crude oil vs in original reservoir oil Isolation of polar components from crude oil especially critical After short term storage, often difficult to dissolve all of the polar components because of oxidation

Challenge Correct sampling in laboratory Proposed mechanisms are known to depend on Conditions Temperature, pressure and pH Different properties can be affected, e.g. Solubility (ions and oil components) Interactions (ions, oil components and rock surfaces) Sampling should be carried out at test conditions Alternative, have to verify that sampling can be carried out at other conditions, e.g. room temperature and 1 atm

Challenge Reservoir conditions High temperature and high pressure Characterisation of mechanisms can be difficult at reservoir conditions, e.g. Interactions between surfaces and oil components and ions Zeta potential Contact angles

Challenge Potential estimates Parametic study Identify mechanisms Modelling experiments Simple but good enough description Preliminary studies in simplified fluid and rock systems, but final validation at reservoir conditions Extension larger scale Behaviour larger scale Potential estimate Optimisation reservoir process, e.g. inj. strategy

Concluding remarks Low salinity water flooding an environmentally friendly EOR method Open the route for alkaline flooding and alkaline/surfactant flooding Need best practice for low salinity water flooding potential evaluation Description of mechanisms and determination of recovery potential should be confirmed using reservoir fluids and reservoir rocks at reservoir conditions