6 Alkaline Flooding.ppt Student

Oil Recovery by: Alkaline Flooding Prof. Attia M Attia By Prof. Attia M. Attia

Alkaline Flooding

Introduction ; Historical • Alkaline chemical is also called Alkalis for improving oil recovery dates back to the early 1917s with the issuing of a patent to squires who maintained that oil displacement may be more complete by introducing an alkali into the water.

• Alkaline salt ,such as sodium carbonate ,instead of sodium hydroxide to diminish the adverse effect of rock dissolution. • The acting mechanisms were not clearly understood at that time, but the hypothesis of wettability change combined with oil –water interfacial tension (IFT) lowering was emitted. • Start alkaline flooding in 1925s using Sodium Carbonate In USA

Introduction Cont. Pertaining to an aqueous solution, has a more hydroxyl ions (OH-) than hydrogen ions (H+) and PH greater than 7. An enhanced oil recovery technique in which an alkaline chemical such as sodium hydroxide, sodium orthosilicate or sodium carbonate is injected during polymer flooding or water flooding operations. The alkaline chemical reacts with certain types of oils, forming surfactants inside the reservoir. Eventually, the surfactants reduce the interfacial tension (IFT) between oil and water and cause an increase in oil production.

• Alkaline flooding is not recommended for carbonate reservoirs because of the large quantity of calcium : the mixture between the alkaline chemical and the calcium ions can produce hydroxide precipitation that may damage the formation

Mechanisms

Fluid –Rock Properties Fluid- fluid interaction

Fluid and Rock Properties 1. 2. 3. 4. 5.

Lowering IFT Wettability Clay Gypsum and Hydride Vertical and horizontal permeability

PH PH = Log [ H+] H+ Ionic concentration of hydrogen With increasing OH leads to decrease the concentration of H+

Alkalinity • Alkalinity is defined as the amount of acid required to neutralize the alkali. • It is expressed as the percentage of Na2O(K2O) contained in each chemical.

Fluid-fluid interactions 1. Lowering of the water- crude oil interfacial tension. 2. Emulsification 3. Precipitation of salts and hydroxides 4. Effect of alkalis on the chemical stability of surfactants and polymers

Lowering of the water- crude oil interfacial tension.

• The lowering of the crude oil-water interfacial tension by using alkaline agents was noted a long time ago, and this observation is the basis of the use of such chemicals to improve oil recovery. • It is now firmly established that some of the components contained in crude oils may generate surface active material when they react with alkaline agents.

The acid number

The acid number gives the amount of potassium hydroxide (in mg) required to neutralized one gram of oil. In alkaline flooding acidity number should be more than 0.8 mg KOH/gm crude oil

Emulsification and entrapment • This reaction results in the formation of emulsifiers which reduces the interfacial tension between oil and water and hence forming an emulsion. • The emulsion droplets are entrapped in the pore constrictions which are smaller in size than these droplets. This willl lead to a lower injected water mobility and higher sweep efficiency.

Emulsification and Entrainment • As mentioned before the reaction of NaOH and the organic acids in some crude oils form emulsifying soap which reduce interfacial tension between oil and water resulting in formation of oil- in water emulsion. This emulsified oil is entrained (pulling) in the following alkaline solution. The actual movement of the oil may be better described by coalescence (union) behaviors than entrainment.

i.e • residual oil undergoing mobilization is emulsified and entrained into the flowing alkaline solution and is produced as a fine emulsion.

By Prof. Attia M. Attia

•Precipitation of salts and hydroxides

• Effect of alkalis on the chemical stability of surfactants and polymers

2.Rock-Fluid interactions

1. Wettability alteration 2. Dissolution of mineralsin by alkalis 3. Effect of alkalis on the retention of surfactants and polymers

Wettability Reversals • The wettability reversal from oil-wet to water – wet may be more favorable with sodium orthosilicate. • This may be due the formation of polysilicate anions which alter the surface properties of both soild-phase and aqueous-phase. The addition of some chemicals to injection water alters the rock wettability.

Environment requirements for alkaline flooding 1. Crude oil Composition 1. Existing of acidic components such as carboxylic acids, carboxyphenole or asphaltene fractions in the crude oil plays a key role in determining if alkaline will be useful and what slug concentration and size can be efficient.

2.Formation water composition • Presence of multivalent cations such as magnesium and calcium in significant concentrations in the reservoir brines reduces the effectiveness of alkaline flood. • When Alkaline contacts these cations, precipitates of magnesium and calcium hydroxide carbonate or silicate may form depending on : • PH, • concentration, • Temperatures… • …Also sodium chloride can helpful in altering the effect of alkaline on wettability.

3.Reservoir rock reactivity • The presence of clays ,anhydrite (CaSo4) or gypsum (CaSO4.2H2O) in the rock may react with the alkaline resulting in the formation of insoluble Ca(OH)2 and consumption of the alkaline.

4.Reservoir Temperature • Higher temperature leads to a higher rate of consumption of the alkaline by the reactive reservoir rock. • Another effect of temperature is the reversal of wettability from oil-wet to water wet and hence increase in water permeability.

Screen Criteria

For Alkaline

What is the mean of screen criteria? screen criteria can be defined as the criteria which should taken in consideration when we design EOR process . These criteria include the factors which can affect on this process and cause problems or reduce the efficiency of this process.

What is the important of screen criteria? Screen criteria is important in case of determine the EOR process can be used in certain conditions and this cause:Save money as we do not use process which will fail Save time as we do not use the process and test the effect

Screen criteria for alkaline We must know that as the alkaline process affect on oil, formation water and rock properties also these things affect on the process (+ve or –ve) We will study the effect of:Oil Formation water Rock mineralogy

Effect of oil properties 1) Amount of residual oil in place Amount of residual oil is important in design and evaluation of any recovery mechanism . In alkaline flooding it is suggested that the amount of residual oil must be at least [ 400 BBL/acre-ft (40%)] . Higher oil saturation is obviously better , lower oil saturation may not be displaced by this process

2) Viscosity & gravity Chemical flooding are applicable for oils the viscosities of which in range of applicability of water flooding . The over limit of viscosity is about [100 c.p ] The gravity should be in the range [13-35º API ] for best results . Very heavy oils or very volatile oils make poor prospects.

3) Chemical character of oil Oil must have acidic components that can react with the caustic to form surface active agent . The acidic number desirable is [ 0.5 mg KOH/gm crude oil ] ,also it is preferably for oil to not contain gases like CO2 and H2S as it consume the alkaline

4)Compressibility High compressible oils are more suitable

Effect of formation water 1) Connate water saturation the initial connate water saturation should be less than [ 50 % ] to reduce alkaline consume

2) Formation water salinity The most important factor which affect on any EOR process . IFT is sensitive to Nacl

The salinity should be such as to produce the minimum IFT , in an average situation formation water salinity less than [200000 PPm] is desirable

3) Formation water chemistry There are several chemical exist in water which can affect strongly in all chemical floods . Ca+2 and Mg+2 can deplete the injected alkaline slug by precipitation of in soluble hydroxides.

Also the presence of Ca+2 increase IFT ,also -2 components such as HCO3 , SO4 ,Cl and H2S in the water that consume the alkali also must be considered

Rock mineralogy 1) clay having large surface area and high ion-exchange capacity can rapidly deplete the injected alkaline

2) Presence of anhydrite and gypsum in greater than tracer quantities is highly undesirable because they cause precipitation of insoluble Ca(OH)2 and decrease alkalinity of slug , so we do not use alkaline flooding in carbonate reservoirs and only used for sandstone 3) The reservoirs should not be very heterogeneous

4) There should be no extensive fractures or thief zones causing the alkali to bypass the bulk of rock with little or no response to injection 5) Large degree of layering and shale barriers of large areal , the net pay thickness should be extent and sufficiently thick about (20-30 ft) 6) Water drive reservoir is not suitable for alkaline flooding

7) Permeability and porosity should be well developed . Permeability at least [50-100mD]

8) Reservoir should not have a gas cap because oil will move up to resaturate gas cap 9) Temperature is a critical parameter in process . It is found that alkali consume increases rapidly at high temperatures . And reservoir temp. less than [200°f]is recommended for these conditions the reservoir depth is about [6000 ft] or less is preferable

summary Property Residual oil sat.{Sor} Oil viscosity

Criteria 400BBl/acre-ft [40% ] <100 c.p

Acidic number

Range 13-35 °API {sp.gravity 0.95 } > 0.5 mg KOH/gm oil

Oil compressibility

High compressible oil

Connate water sat.{Swi}

Less than 50%

Oil gravity

Property Water chemistry

Rock mineralogy

Permeability

Criteria No Ca+2 &Mg+2& Hco3No clay &anhydrite& gypsum &carbonate rock 50-10mD

Reservoir mechanism

Water drive and gas cap

Temperature

Less than 200°f