A Report on

ENHANCED OIL RECOVERY BY THE METHOD OF CAUSTIC FLOODING

Submitted By

Dhiman Kakati

( 18 July 2016 )

Assisted by : Mr Rahul Saha

Under the guidance of

Dr Pankaj Tiwari

Department Of Chemical Engineering

Indian Institute Of Technology, Guwahati

Guwahati-781039

INTRODUCTION:

In a reservoir oil field, oil recovery mechanisms are split into three categories namely

primary, secondary and tertiary recovery. Primary recovery is the initial production stage

during which the oil is produced from the wellbore to the surface by natural pressure

difference between the rock formation and producing wellbore. Secondary recovery, however

proceeds when the oil present in the reservoir cannot be produced to the surface by natural

pressure difference. It is processed by injecting external fluids such as water or gas into the

reservoir to increase reservoir pressure and displace the oil towards the wellbore formation.

Two-thirds of the original oil in place (OOIP), are still located in the complex region which

cannot be produced by primary and secondary methods, and has to rely on tertiary or

enhanced oil recovery (EOR) methods [1]

Enhanced Oil Recovery (EOR) technique recovers the oil which is trapped in the pores of the

reservoir that cannot be recovered by secondary water flooding. EOR can be performed in

various physical and chemical forms such as thermal oil recovery, gas injection, microbial

injection and chemical flooding[1]. We have confined our interest to chemical flooding in

particular, and in this report we will take a detailed look at one of the most widely used

chemical flooding methods in the extraction of reservoir crude- Caustic Flooding

Caustic Flooding is a process in which the organic acids present in reservoir crude oils (also

called Napthenic Acids) react with the alkali present in the floodwater (which is injected into

the reservoir) to form in-situ surfactants at the oil-water interface, which drastically lower the

interfacial tension (IFT) between the crude oil and the floodwater (by a factor of several

hundred), and under the proper conditions of salinity, pH, and temperature, they change the

wettability of the porous medium to preferentially oil-wet. When the proper alkaline water

and acidic oil flow simultaneously in a porous medium, a viscous oil-external emulsion is

formed. The flow properties of this type of emulsion permit a high, non-uniform pressure

gradient to be generated across the narrow region in the vicinity of the emulsion front. The

pressure gradients are sufficient to overcome the reduced capillary forces and displace the oil

from the pore space. The displacement efficiency can be much improved over ordinary

waterflood efficiencies [2]

The extent to which interfacial tension is lowered depends largely on the properties of the

crude and the floodwater injected. Therefore, it is important to establish the relationship

between interfacial tension and the essential chemical properties of the acidic oil and the

floodwater [3].The process of alkali flooding has been found to work best with heavy or more

viscous oils( Interfacial tension at the oil-water interface was found to decrease at a faster rate

for heavy oils.).

Saline water is preferred over freshwater for the flooding process. The use of saline water

causes the sand to be made oil-wet in the presence of the alkaline water. High salinity also

leads to the formation of a water-in-oil type of emulsion, which does not form in the other

processes. As stated earlier, the most common organic acids in crude oil are naphthenic acids.

The acid content of a crude oil tends to be higher when the base composition of the crude is

high in naphthenic compounds. In cases where acid concentration is low, a bank of oil

containing organic acids could be injected into a reservoir and followed by alkaline water [2]

Mechanism Of Caustic Flooding:-

The mechanisms active at the front where alkaline water is displacing acidic crude oil include

(1) a drastic reduction of oil/water interracial tension, (2) wetting of the matrix grains by oil,

(3) formation of water drops inside the oil phase, and (4) drainage of oil from the volume

between alkaline water drops to produce an emulsion containing very little oil. The low

interfacial tension and the oil-wetting of the matrix result from the forming of soap by an

acid-base chemical reaction at the oil/water interface.The soap is rendered practically

insoluble in the alkaline water by salt present in the water. Under such conditions, soap

promotes oil-wetting of the matrix. (In fresh water, soap is soluble and promotes

waterwetting [2]. Nutting believed that alkaline solutions released residual oil from adherence

to sand surfaces, essentially a wettability change. He also noted that alkali prevented

formation of semisolid, crude oil-water interracial films, but he discounted the importance of

this property in improving recovery. Atkinson believed that residual oil was held within the

spaces between sand grains by forces of capillarity and adhesion in conjunction with oil

viscosity, and that alkaline solutions overcame these forces to release the oil , seemingly

referring to low wettability change combined with oil- water interracial tension reduction[4]

The deficiency of hydrogen ions, which are consumed by hydroxyl ion in aqueous

phase,supports the creation of the soap (AW-), which is an anionic surfactant other than

synthetic surfactant. The creation of such AW- ions will adsorb at the oil-water interface and

reduces IFT. Alkali can react with reservoir rock depending upon the rock mineralogy by

surface exchange and hydrolysis, congruent and incongruent dissolution rock, and formation

of insoluble salt by reaction with hardness ions in the fluid and those exchanged rock surface

[1]. The reversible sodium/hydrogen-base exchange is also a very important mechanism

which indicates the alkali consumption in reservoir rock as indicated in the figure and the

equation is given below.

MH+N a+¿+OH−¿⬄MNa+H2 O ¿¿

Where M denotes a mineral-base exchange site

Figure above illustrates the alkali recovery process( deZabala,1982) [1]

LITERATURE REVIEW

There is an ocean of literature available on the process of caustic flooding, enumerating the

favourable conditions and the experimentally determined responsive samples in great detail.

Dranchuk et al , reporting on laboratory caustic floods of a viscous Lloydminster crude oil,

observed that stable oil-in-water emulsions were produced in situ and that there was evidence

for reduced water mobility during displacement. Because no significant reduction in ultimate

residual oil saturation results from this mechanism, it is directed primarily toward viscous oils

or oils in heterogeneous reservoirs where sweep efficiency is poor. Under these

circumstances, an improvement in vertical and areal sweep efficiency through improved

mobility ratio can be much more important economically than recovery of residual oil from

only the small volume of the reservoir normally swept [4]

Samanta et al (2011) also investigated the interaction between alkali and crude oil and its

effects on enhanced oil recovery. The presence of carboxylic acid group in the crude oil was

analyzed by FTIR, which forms an in-situ surfactant after interacting with the alkali.

Considering the characteristic interaction between alkali and crude oil, a concentration of 0.5-

1 wt% NaOH of alkali slug was injected and maintained. Three sets of flooding were

performed in sand-pack systems to determine the effectiveness of alkali on enhanced oil

recovery. Increment in oil recovery was observed with the increase in alkali slug

concentration and an additional oil recovery of more than 15 % OOIP over the recovered by

conventional water flooding (∼50%) was recovered, which is due to reduction of IFT,

solubilization of interfacial film, emulsification of oil and water and wettability reversal[1]

Wagner and Leach(1959) presented laboratory tests showing improved oil recovery through

injection of water solutions that reverse rock wettability from oil-wet to water-wet. They

demonstrated this could be accomplished by added chemicals that changed injection- water

pH. These chemicals included acids, bases, and some salts. They reasoned, as others before

them, that the injected chemical always would be preceded by displaced connate water so that

treated water would encounter only residual oil left behind the untreated, connate water-flood

front. Since residual oil in a waterwet porous medium is discontinuous and immobile, as

compared with the continuous residual oil phase in an oil-wet porous medium, Wagner and

Leach concluded that water-wet rocks could not respond to a wettability reversal mechanism.

Therefore, they limited application to oil-wet reservoirs where wettability could be reversed

from oil-wet to water-wet. This represents the second of the four different mechanisms for

caustic flooding, that of wettability reversal (oil-wet to water-wet) [4]

Haihua Pei et al (2013) conducted a series of experiments in micro-model and Sandpack

flood test to observe the potential of alkali flooding in enhancing the recovery of Binnan

heavy oil. Micro-model test shows the penetration of alkali solution into the crude oil

forming water drops inside the oil phase thus resulting in water-in-oil (W/O) emulsion which

reduces the mobility of water due to the blockage of permeability zone caused by preceding

water flooding and diverts it to the unsweep region thus increasing the overall sweep

efficiency.For sandpack flooding the parameters varied was alkali concentration, alkali slug

size and injection rate which is in the range of 0.1-1 wt% NaOH, 0.25-2.5 PV and 0.1-1

ml/min respectively. The sandpack flooding conducted indicates the tertiary oil recovery

which is about 20% of the initial oil in place (IOIP) using 1.0% NaOH and the recovery

increases further with increase in alkali concentration. Water droplets number and its size in

W/O emulsions increases with increase in alkali concentration resulting in higher recovery;

however an optimum slug size and injection rate was observed which gives the highest

recovery. The optimum condition was 1 wt% NaOH, 0.25 ml/min injection rate and 0.5 PV

slug size. They also investigated the injection pattern and found that continuous alkali

injection gives better oil recovery rather than cyclic injection pattern [1].

Jennings et al proposed yet a fourth mechanism by which caustic injection can improve oil

recovery. Their laboratory experiments showed that if interfacial tension were low enough,

residual oil in a preferentially water-wet core could be emulsified in situ, could move

downstream with the flowing caustic, and could be entrapped again by pore throats too small

for the oil emulsion droplets to penetrate. This mechanism of emulsification and entrapment

results in a reduced water mobility that improves both vertical and areal sweep efficiency.

This is especially important in waterflooding viscous oils where waterflood sweep efficiency

is notoriously poor[4]

MATERIALS AND METHODS

Crude Oil sample(Type1) has been used in the experiment, which has been purchased from

Oil India Limited(OIL).Alkali that has been included in the experiment is Anhydrous Sodium

Carbonate(99.9% purity), purchased from Merck Specialties Private Limited, India

The interfacial tension between chemical and oil phase can be performed by using a spinning

drop method with Spinning Drop Tensiometer – SITE100 (Make: Kruss, Model: Site100),

and the results obtained through the following equation:

σ = r3 ω2 (ρH – ρL)/4, L /D ≥ 4

Where σ is the interfacial tension (mN/m), ρHis the density of the heavy water phase (g/cm3),

ρLis the density of the oil phase (g/cm3), ω is the rotational velocity (rpm), D is the measured

dropwidth (mm), L is the length of the oil drop (mm) and n is the refractive index of the

water phase. The Measuring range is up to 10-6 mN/m with rotation speed up to 15,000 rpm

(optionally up to 20,000) and Temperature range of 0 to 100°C [1]. We have maintained a

fixed rotational speed of 2000 rpm while performing this experiment(at room temperature)

Figure: Schematic Diagram Of a Spinning Drop Tensiometer

EXPERIMENTAL OBSERVATIONS

The following readings are the result of an experiment performed to determine the interfacial

tension between the interface of Assam Crude Oil(Type 1) and a 1% (by weight) aqueos

solution of Sodium Bicarbonate(in addition a graph has been plotted between IFT and

time).The readings have been taken for a fixed rotational velocity of 2000 RPM at room

temperature.

Time(in

minutes)

Drop

Diameter(D)

IFT(mN/m) Rotational

Speed(RPM)

0 0.56 0.0263 2000

2 0.53 0.0217 2000

4 0.54 0.0237 2000

6 0.54 0.0234 2000

8 0.54 0.0230 2000

10 0.53 0.0224 2000

12 0.53 0.0227 2000

14 0.53 0.0237 2000

16 0.53 0.0234 2000

18 0.54 0.0234 2000

20 0.54 0.0234 2000

22 0.55 0.0248 2000

Figure: Plot between Time(X-axis) and IFT(Y-axis)

We can observe from the above plot that the IFT assumes a near constant value( with

extremely minor fluctuations) for a fixed rotational speed.Also we can observe a slight

increase in the IFT value for a corresponding increase in drop diameter D.

RESULTS AND DISCUSSIONS

From the graph plotted in the previous section we can see that interfacial tension remains

constant for a fixed RPM supplied at a constant temperature. It is also evident from the

observations that the value of IFT increases as the drop diamteter D increases inside the

spinning drop tensiometer.

CONCLUSION

The following test was performed on Type 1 Assam Crude Oil, whose API gravity is

higher than 15 and whose viscosity falls in the range of 15-35 centipoise.Hence we

can conclude that Assam Crude Oil (Type 1) will respond satisfactorily to the method

of Caustic Flooding( from the observations in the Spinning Drop Tensiometer

experiment)

Using saline water in place of fresh water for the flooding process helps in altering the

wettability, which is an important factor in extracting maximum oil from the

reservoirs.

Caustic Flooding is an effective process in extracting maximum oil from the

reservoirs, as the surfactant used for the purpose of IFT reduction is minimised.

REFERENCES

[1] Rahul Saha, “Optimum Formulation Of Chemical Slug For Enhamced Oil Recovery of

Assam Crude Oil”, State Of The Art Seminar,Department Of Chemical Engineering,IIT-

Guwahati

[2] C.E Cooke Jr, R.E Williams, P.A Kolodzie, “Oil Recovery By Alkaline Waterflooding”,

SPE-AIME, Exxon Production Research Co.

[3] T.S Ramakrishnan, D.T Wasan, “A Model for Interfacial Activity of Acidic Crude

Oil/Caustic Systems for Alkaline Flooding”, SPE Journal

[4] C.E Johnson,Jr., “Status Of Caustic and Emulsion Methods”,SPE-AIME,Chevron Oil

Field Research Company