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Reservoir Engineering II

Waterflooding Gas Reservoir

January, 2014


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Table of contents

List of tables..3

List of figures...........................4

Abstract..5

Introduction......5

1- Reservoir properties considering in waterflooding. ..6

2-Waterflooding gas reservoirs10

2.1 Comparison of water injection in gas reservoirs to oil reservoirs.....12

2.2 Injection Volumes and rates..12

2.3 Timing a waterflood project ..14

2.4 Case study ...16

2.5 Investigation of waterflooding with simulation (Eclipes).17

Conclusion 25

References.. 26


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List of Figures

Figure 1: Injection rate necessary to maintain reservoir pressure.13

Figure 2: Waterflood simulation model diagram...16

Figure 3: Injection rates for waterfloods started in years 4 and 15..19

Figure 4: Volume injected for waterfloods in years 4 and 15...20

Figure 5: Reservoir life for waterfloods started in years 4 and 5..21

Figure 6: Injection rates to obtain recovery wit 19 year teservoir22

Figure 7: Relationship between recovery factor and volume injected for year 423

Figure 8: Relationship between recovery factor and volume injected for year 15..24


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Abstract

The second oil recovery takes place when natural drive energy is too small or

depleted for economic oil recovery. Essential energy must be added to the reservoir to

allow additional oil recovery that additional energy is generally in the form of Gas

injection or Water flooding. The objective of secondary recovery is to preserve

reservoir pressure and to shift hydrocarbons toward the wellbore. The secondary oil

recovery methods are gas injection and water flooding. Normally water is injected into

the aquifer and gas is injected into the gas cap.

1. Introduction

In water flooding projects, water flooding can be studied either by classic material

balance methods or by sophisticated reservoir simulation programs. Although reservoir

simulation is almost always accurate, it requirs loads of variables and data to be

inserted and takes up much time to run models and analyze them. On another

perspective, simulation cannot be available to some operators due to its high cost. On

the other hand, material balance methods are fast and simple but lack advantage like

prediction of gas production rates and also lack the evaluation of important factors as

compression that can greatly help in the estimation of the NPV of the reserves.

Simulation studies were conducted by using Eclipse, to examine recovery optimization

by waterflooding. From water flooding studies it was concluded that the injection rate

is essential to attain a given recovery in a amount of time with a restricted injected

volume goes up over time and that starting water injection in the reservoir`s life can

have a various advantages to performing a waterflood close abandonment .


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1. Reservoir properties considering in Water Flooding.

Reservoir GeometryReservoir geometry is considered one of the most important factors that we must

detect before water flooding because determining the geometry quite well will make

you able to know accurately the location and the number of wells if we are in onshore

fields, but if in offshore fields, the reservoir geometry will make us determine the

location and the number of the platforms needed in this field. Analyzing the reservoir

performance and make an accurate analysis of the reservoir geometry will make you

know if we can use the water flooding as supplying the natural drive mechanism but if

the primary drive mechanism is and active water drive so the project of the waterflooding in this case will be not necessary . (Ahmed, 2010)

Reservoir DepthDetermining the reservoir depth is considered an essential factor that affects the

water flooding projects. As the depth of the reservoir increases we must increase the

injection pressure to reach to our goal without reaching to the fracture pressure and as

also the depth increases it affects the economic and technical issues and increase the

operating costs. So in very deep wells and shallow reservoirs the injection pressure

increases of the water flooding project. In water flooding job, there is a critical pressure

that we must know it is equal to 1psi/ft of depth and this means that if we exceed this

limit we can create fractures in the reservoir and this will need to channeling and cause

many problems during production. We must work on an operational pressure gradient

which is 0.75 psi/ft to provide us with a sufficient safety and to preclude pressure

parting. (Ahmed, 2010)

Fluid Properties


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well completion, and to make a successful water flooding project the injector

and the producer must be at the same lens. A good study for the reservoir

anisotropy must take place and determine the fracture pressure before

determining a proper well planning for suitable water flooding project and for

the flood orientation. (Ahmed, 2010)

.

Primary Reservoir-Driving MechanismsThe oil is produced from the reservoir by primary and secondary and tertiary

drive mechanism:

1- Rock and liquid expansion.

2-Solution gas drive.

3-Gas-cap drive.

4-Water drive.

5-Gravity drainage drive.

6-Combination drive.

The oil recovery by any of the above driving mechanisms is called primary

recovery. The term refers to the production of hydrocarbons from a reservoir by

the natural energy of the reservoir such as rock and liquid expansion. The

primary drive mechanism and anticipated ultimate oil recovery should be

considered when reviewing possible waterflood prospects. The approximate oil

recovery range is tabulated below for various driving mechanisms. These

calculations are approximate and, therefore, the recoverable oil found in this

table may be out of these ranges according to the condition found in the

reservoir.(Ahmed, 2010)


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Driving Mechanism Oil Recovery range %

Rock and liquid expansion

Solution gap

Gas Cap

Water drive

Gravity drainage

Combination drive

3-7

5-30

20-40

35-75


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The two prime fluid characteristics which distinguish the study of gas recovery from

oil recovery are compressibility and mobility. When an oil pore space is saturated is

swept by water and all of the mobile oil is displaced. The oil that is left behind is a

residual saturation that won't flow. Gas included in a residual saturation, it will extend

if the pressure of reservoir is dropped. (Walker, 2005)

Theoretically, the ideal gas law shows us that as the pressure is reduced by half

therefore the gas volume will be doubled. Critical gas saturation happens once gas

enlarges enough to make a continuous stage. If Critical saturation is reached, gas will

flow more than the liquid phase. This action is well authenticated in oil reservoirs and

can else happen in a gas zone that has been swept via water. (Walker, 2005)

There is an experiment where 2 sandstone cores were flooded by water and

subsequently depressurized. The permeability of cores is 200 and 1500md and residual

gas saturations is 0.415 and 0.35 respectively. Gamma neutron reaction measurements

showed that during gas saturations blow down had to increase with 0.04 and 0.14

above in the residual amounts in the cores of 200 and 1500 md for gas to become

movable again. (Fishlock, 1986)

In a same experiment, it was observed that for the three cores utilized the gas

saturation had to increase from residual values from 0.3 to 0.4 for gas to remobilize.

The permeability of cores were 1445, 1792 and 1915 md.. The residual gas saturations

were resolved by history matching the experiments by rather than direct measurement.

(Firoozabadi, 1987) It seems to be fairly nearly agreement with the data collected by

Fishlock.

The reasoning for gas mobilization delaying is that the permeability of gas submit tohysterias during blow down and the relative permeability to gas after absorption is not

like as it was during primary drainage. (Fishlock, 1986)

Gas permeability that was measured by Fishlock at gas saturation 0.58 to be0.001. In spite of low relative permeability, the gas phase become mobile the


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fractional flow of gas increased quickly with more increase in gas saturation. It

was deduced that the relative permeability to water decreased as a consequence

of gas expansion and when relative permeability of both are low the viscosity

ratio encourages gas flow. Moreover, the blows down results were dependent on

rock and the magnitude of the difference between mobilization saturations and

residual may not be typical of reservoir rocks. (Walker, 2005)

Experimental data specifies that there is a difference between remobilization and residual

saturations. (Fishlock, 1986), (Firoozabadi, 1987). It is significant to notice that there is no field

evidence toconfirm these observations. (Ancell, 1990)

Moreover, gas saturation is essential to increase 5-15% to become mobile, unless

both trapping pressure and residual saturation are low, remobilization have to be

possible Then, if trapping pressure and residual saturation are low, primary recovery

shall be high and remobilization may not be essential.

2.2 Injection Volumes and Rates

At a gas well particularly at low reservoir pressure, Water breakthrough might cause

the well to load up or else water out completely. Injecting water until breakthrough

shouldn`t decrease the recovery, if the reservoir is just over the abandonment pressure

Though, if the average pressure is higher than abandonment pressure injection should

be reduced before the expected breakthrough. It attitudes to reason that there is some

maximum water volume, that can be injected into group of wells or a given well

without affecting the adjacent producers.

Near abandonment , should be nearly equal to the displaceable pore volume of

a circular injection manner whose radius equivalent to the distance between the injector

and producer. During injection of water, the displacement should be closely piston like

due to the very favorable mobility ratio. (Agarwal, 1965)

If injection is stopped, some factors as gas expansion and gravity segregation leads

water to acclimate (sag) and laterally spread. This phenomenon should be considered

when determining, at high reservoir pressures.


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Previous studies have examined waterflooding gas reservoirs at or close abandonment.

The common problem with waiting to start injection close abandonment is that the gas

compressibility is almost as high as it can obtain. The minimum injection rate that

needed to sustain reservoir pressure is . Figure 1 is a graph of minimum

injection rate verses pressure for various gas production rates. The production rates are

low; when reservoir pressure falls below 1000 psia the minimum injection rate starts to

increase drastically. Figure1 clarifies how is inversely proportional to reservoir

pressure. As in the figure, a production rate of 500 MSCFD reservoir pressure has

dropped from 3,000 to 1000 psia and has increased from 500 to 1500 BPD.

(Walker, 2005)

The major advantage of starting injection before abandonment is that the reservoir

pressure does not should be accurately maintained. There is no instant threat of falling

under the abandonment pressure if the injection rate is not achieved. In the Early life of

the reservoir, is the rate required to inject before the abandonment pressure

is reached. If is a constant value then increases along the life of the reservoir

and gets its maximum value at abandonment.


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Figure 1:

2.3 Timing a waterflood project

All preceding water flooding studies and field projects have included gas reservoirs

close abandonment. The most well authenticated waterflooding of a gas reservoir

located in st. Martin Parish, Louisiana in the D-1 reservoir of the Duck Lake Field.

(Cason, 1983).

The original gas in place of D-l reservoir was valued to be 681 by using amaterial balance. The initial formation volume factor was estimated to be 456 .

The volume of water equal to 130 injected Duck lake reservoir. This

identical to an average injection rate 33,000, the author determined with material

balance that water injection was accountable for increase in production.


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2.4 Case Study

Table 2: Water flood simulation model properties.


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Figure 2

.

2.5 Investigation of waterflooding with simulation (Eclipse)

To examine the theory and practice of waterflooding gas reservoirs, a reservoir

simulation was done with Eclipse, The waterflood simulation model properties and

waterflood simulation model diagram are shown in table1 and figure2 respectively.

The main objective of this study was to examine the difference between early water

injecting in the life of reservoir and waiting till it is close abandonment. Though, the

effects of some factors as starting time, injection rate, and volume injected on recovery

and production life were also investigated.

The reservoir was first produced to abandonment in order to define reservoir life and

the base recovery. As soon as the base production life was determined an abandonment

waterflood was started at the last time phase. Water injection waspersistent till the end


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minimum requirement. An extra 2% of recovery over pre-abandonment floods can be

achieved by injection rates higher than 5,000 for the abandonment flood. The

more efficient process is to allow occurrence of gas expansion before sweeping the

reservoir than trapping the gas then let it to expand. Yet, it requires an additional 10-15

of water injection and 7-10 years of production. Because of complex relations

between starting time, injection rate, recovery and time of the project. The engineer

must have a decision how starting time, injection rate, and producing life impact the

value of the project. It is valuable to hold some of the values constant to understand

them better. A sequence of simulation runs began injection in different years, and for

each run 20 was injected before year 14. The rate of injection was regulated

until the end of all of the runs during year 19 and the recoveries were almost equivalent

to what could be achieved. Figure 6 shows the injection rates and recovery factors.

From year 2 to year 10 the desired injection rate tripled while the recovery went down

by 0.5% and the life of the project increased by half a year.

The required injection rate that is to get a given recovery factor in a given amount of

time, with a limited injection volume rises significantly over time. A high injection rate

will be essential to avoid having to accept less recovery or a longer production life.

As shown in figure 7, there was a strong linear connection between recovery factor

and the volume of injected water when the waterflood was began in year 4, this

behavior was idealistic up to year 10. In figure 8, this relation became quadratic in

nature close abandonment. These two graphs indicate that the factor which is most

directly related to recovery is the amount of water injected. However, in order to reach

this optimal recovery, a waterflood must have an associated blow down phase before

abandonment.

For most of the reservoir life, the ideal injection volume is more or less constant. As

shown in Figures 4 and 7 the ideal volume for the simulation model was about

20. At any point in time there is a minimum rate that will hit injected target

with enough time to blow downbefore production stopped. (Walker, 2005)


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.

Figure 3:


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Figure 4:


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Figure 5:


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Figure 6:


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Figure 7:


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Figure 8:

ConclusionSimulation studies were conducted by using Eclipse, to investigate optimization of

recovery by waterflooding. From waterflooding studies it was concluded that the

injection rate is essential to attain a given recovery factor in a given amount of time

with a limited injected volume goes up over time and that early in the life of a reservoir

starting water injection can have many advantages to carrying out a waterflood near

abandonment. Water rates, volumes and starting/stopping times can attain same

recoveries, but affect the project life and then NPV differently. The main goal of earlyinjection in a reservoirs life is pressure maintenance rather than displacement. There

are many production scenarios when injection is started early. Various options can

achieve same recovery results even though the volumes, injection rates and beginning

times are different.


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References

Agarwal, R. A.-H. (1965). The Importance of Water Influx in Gas Reservoirs. SPE

1244 presented at SPE Annual Fall Meeting. Denver.

Ahmed, T. (2010). Principles of waterflooding. In T. Ahmed, Reservoir Engineering

Hand Book (Fourth Edition ed.).

Ancell, K. F. (1990). Remobilization of Natural Gas Trapped by Encroaching Water.

SPE 20753 presented at the 65th Annual SPE Technical. New Orleans.

Cason, L. D. (1983). Waterflooding Increases Gas Recovery. SPE 12041 presented at

the SPE Annual Technical Conference. San Francisco.

Firoozabadi, A. O.-R. (1987). Residual Gas Saturation in Water-Drive Gas Reservoirs.

SPE 16355 presented at the SPE California Regional Meeting . Ventura.

Fishlock, T. S. (1986). Experimental Studies on the Waterflood Residual Gas Saturation

and its Production by Blowdown. the 61st Annual SPE Technical Conference .

New Orleans.

Walker, T. (2005). ENHANCED GAS RECOVERY USING PRESSURE AND

DISPLACEMENT.

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