design concepts in waterflood processes

8/10/2019 Design Concepts in Waterflood Processes

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DESIGN CONCEPTS IN WATERFLOOD PROCESSE

SELECTION OF OPTIMUM FLOOD PATTERN

RESERVOIR FILL-UP

WATER INJECTIVITY

INJECTION ALLOCATION

RESERVOIR VOIDAGE

TYPE OF FLOOD PATERN


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Peripheral floods

Suitable for dipping, relatively homogeneous reservoirs

Require adequate lateral continuity and high transmissibility

Require careful control of withdrawal from up-structure wells and shutting-in of high

water cut wells Uniform flood patterns If well drilling cost is low, utilize smaller uniform patterns with equal distances

between injectors and producers such as four, five and seven spot

Choice between normal and inverted patterns should be based on observed

injectivity

Selected pattern should provide optimum injection and production capacity

Selected pattern type, pattern size and injection rate should be consistent with

available fluid lifting, rock fracturing pressure and well injectivity

Guidelines for Pattern Selection

Example

An oil reservoir is considered for waterflooding with a desirable flood life of 10 years and

total water injection of 2.5 pore volumes.

Given data: Porosity 28%

Net reservoir thickness 64 ft

Reservoir depth 2200 ft

Water injectivity 1.65 Bbl/day/psi

Maximum lifting capacilty 700 BFPD

Average reservoir pressure 900 psia

Expected operating days per year 350


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Fracturing pressure gradient 0.85 psi/ft

Water formation volume factor 1.02 RB/STB

Using a maximum bottomhole injection pressure of 90% of fracturing pressure, and

assuming zero voidage rate, determine the appropriate flood pattern for the proposed

waterflood.

Assume that pattern size = A acre

Pore volume per pattern = 7758xAx64x0.28 = 139A MBbl

Total volume of water injection = 2.5x139A = 348A MBbl

Desired Injection rate = 348Ax1000 / (10x350) = 99.4A BWPD/pattern

Fracturing pressure = 0.85x2200 = 1870 psia

Maximum injection rate = 1.65x(1870x0.9900) = 1292 BWPD

Hence; Pattern size A = 1292 / 99.4 = 13 acreRequired lifting per pattern = 1292x1.02 = 1318 BFPD

Number of producing wells required per pattern = 1318 / 700 = 1.9

Therefore;

An inverted seven spot (with size of 13 acre) is recommended since this type of pattern

provides a producer-to-injector ratio of 2.

Reservoir simulation models can help in selecting the flood pattern type and size to

achieve maximum oil recovery with minimum injected water.

Selected flood pattern should utilize as many as possible of the existing producing

wells.

Some existing producing wells can be converted to injectors. It should be

remembered that poor producers also make poor injectors. Hence; before deciding

on converting a poor producing well to an injector, some analysis is required to

determine the reasons for poor productivity.

If anisotropy or natural fractures exist, pattern alignment and utilization of elongated

patterns should be considered in order to avoid premature water breakthrough.

Reservoir simulation models can help in selecting optimum pattern variations. In flood patterns within dipping reservoirs, injectors should be located off center

closer to the up-dip side to delay the breakthrough time in down-dip producing

wells.

The shape and size of flood patterns located near fault planes or flow barriers

should be properly adjusted to in order avoid lack of communication between

injectors and producers in the same pattern


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Reservoir Fill-up

A fill-up period is required if free gas exists in the reservoir before waterflood

Oil production response in usually starts after fill-up period

During fill-up period, a significant amount of free gas goes back into solution

Waterflood design should allow for the fill-up period and its effect on production

performance and injectivity

Reservoir simulation models automatically account for fill-up effects

Reservoir engineering calculations can also be made using conceptual models to

provide approximate values for fill-up effects

Filled-Up Volume

If production occurs during fill-up:


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Wif= (Vp Sgi/ Bw) + (Npf Bo/ Bw) + Wpf

If no production occurs during fill-up:

Wif= Vp Sgi/ Bw

FILLED UP TIME

tif= Wif/ qinj = [(Vp Sgi/ Bw) + qo tif{(Bo/ Bw) + WOR}] / qinj

Solving for tifrequires an iterative procedure if qoand WOR are functions of time

Example

Calculate the volume of injected water required for fill-up, length of the fill-up period and

volumetric sweep efficiency for a waterflood pattern with the following characteristics:

Pattern size 20 acre

Gross reservoir thickness 72 ft

Net-to-gross ratio 0.86

Porosity 26%Initial free gas saturation 15%

Initial water saturation 31%


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Water saturation at breakthrough 63%

Oil production rate 158 BOPD

Water-oil ratio 0.7

Water injection rate 2500 BWPD

Oil formation volume factor 1.22 RB/STB

Water formation volume factor 1.03 RB/surface Bbl

Example, continued

Pore volume Vp= 7758x20x72x0.86x0.26 = 2498 MBbl

First iteration: Wif= 2498x0.15 / 1.03 = 364 MBbl

tif= 364000 / 2500 = 145.6 days

Second iteration:

Wif= (2498x0.15 / 1.03) + (158x145.6 / 1000)[(1.22 / 1.03) + 0.7] = 407.1 MBbl

tif= 407100 / 2500 = 162.8 days

Third iteration:

Wif= (2498x0.15 / 1.03) + (158x162.8 / 1000)[(1.22 / 1.03) + 0.7] = 412.3 MBbl

tif= 412300 / 2500 = 164.9 days

Fourth iteration:

Wif= (2498x0.15 / 1.03) + (158x164.9 / 1000)[(1.22 / 1.03) + 0.7] = 412.9 MBbl

tif= 412900 / 2500 = 165.2 days

Hence; Fill-up volume = 413 MBbl and Fill-up period = 165 days

Volumetric sweep efficiency at fill-up = 413x1.03 / [2498(0.630.31)] = 53.2%

Water Injectivity

Water injection rates play an important role in project design and economics Injection rates directly impact surface facilities and flood life

Water injection rate into a given well depends on:

-- Fluid viscosity and density

-- Fluid saturation distribution

-- Water quality

-- Reservoir depth

-- Injection tubing size and roughness

-- Bottomhole pressure in injection wells Pinj

-- Bottomhole flowing pressure in producing wells Pw-- Reservoir permeability

-- Flood pattern shape and size

-- Relative permeability characteristics

Water injectivity Jwis defined as:

Jw= qinj/ P

where P = PinjPw

Jwcan be estimated from Darcys Law and can be measured from well tests

Procedure to estimate Jwdepends on the flood stage:

-- From start till interference

-- From end of interference till fill-up


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-- From end of fill-up till breakthrough

-- From breakthrough till abandonment

Injectivity calculations:

First stage; Based on radial flow around injection wells

Third and fourth stages; Based on pattern shape, mobility ratio and areal sweepefficiency

Second stage; Use average between end of first and beginning of third stages

Note: First and second stages apply only for reservoirs with initial free gas

saturation Sgi

First Stage: From Start till Well Interference

where: k = absolute permeability, md

krw= water relative permeability at Swbt

kro= oil relative permeability at Swi

h = net reservoir thickness, ft

w= water viscosity, cp

o= oil viscosity, cp

Bw= water formation volume factor

S = skin factor

First stage applies as long as: rob< D /2

When oil banks from adjacent injectors meet: robmax= D / 2 and the second

stage starts

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First Stage: Example

Flood pattern 20-acre five-spot


Porosity 24%

Permeability 174 md

Initial water saturation Swi 28%

Oil relative permeability at Swi 0.86

Average water saturation at breakthrough Swbt 62%

Water relative permeability at Swbt 0.15

Initial gas saturation 12%

Oil viscosity 1.3 cp

Water viscosity 0.5 cp

Water formation volume factor 1.02 RB/surf Bbl

Wellbore radius 0.4 ft

Bottomhole pressure in producer 600 psia

Bottomhole pressure in injector 1300 psia

Skin factor +0.9

Estimate:Time required to inject 30 MBbl of water per pattern at flood start Injected

volume and injection rate at start of well interference

First Stage: Example, continued

Oil bank outer radius rob= [1.787x30000 / (54x0.24x0.12)]0.5= 186 ft

Water bank outer radius rwb= 186x[0.12 / (0.62 0.28)]0.5= 111 ft

P = 1300 600 = 700 psi

Injection rate = = 2103 BWPD

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Distance between adjacent injection wells D = (20x43560)0.5= 933 ft

Maximum value of rob: robmax= 933 / 2 = 466.5 ft

Corresponding value of rwb= 466.5x[0.12 / (0.62 0.28) ]0.5= 277 ft

Hence; at start of well interference:

Volume of injected water = 54x0.24x0.12x(466.5)

2

/ 1.787 = 189400 Bbl

Injection rate = = 1843 BWPD

Third and Fourth Stages: After Fill-up

M = 1 and Sgi= 0

Five spot pattern:

d = distance between injector and producer

Line drive with (d/a) 1:

d = distance between rows

a = distance between producers

Seven spot pattern:

d = distance between wells

Third and Fourth Stages: After Fill-upM = 1 and Sgi= 0

Nine spot pattern: d = half the length of pattern side

R = ratio of producing rate of corner to side wells

P is based on bottomhole flowing pressure of corner well

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and if P is based on bottomhole flowing pressure of side well

Third and Fourth Stages: After Fill-up

For unit mobility ratio M = 1; hence; kro/ o= krw/ w

Injectivity for this condition is designated as base (initial) injectivity Jw0

For example, for Five spot pattern:

For M = 1: As Eaincreases, Jwremains equal to Jw0

For M < 1: As Eaincreases, Jwdeclines

For M > 1: As Eaincreases, Jwincreases

Conductance ratio:

= Jw/ Jw0= qinj P

0/ qinj0 P

is a function of mobility ratio M and areal sweep efficiency Ea

can be used to estimate changes in injectivity with time

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Third and Fourth Stages: Example

Estimate the water injection rate initially and after cumulative injection reaches 350

MBbls for a waterflood that has the following characteristics:

Flood pattern 20-acre five-spot


Porosity 24%Permeability 174 md

Initial water saturation Swi 28%

Oil relative permeability at Swi 0.86

Average water saturation at breakthrough Swbt 62%

Water relative permeability at Swbt 0.15

Vertical sweep efficiency at breakthrough 80%

Initial gas saturation 0

Oil viscosity 1.3 cp

Water viscosity 0.5 cp

Water formation volume factor 1.02 RB/surf Bbl

Wellbore radius 0.4 ft

Bottomhole pressure in producer 600 psia

Bottomhole pressure in injector 1300 psia

Skin factor +0.9

Third and Fourth Stages: Example, continued

Distance between injector and producer:

d = (20x43560 / 2)0.5= 660 ft

Base injectivity:

Jw0

= 0.003541x0.86x174x54 / [1.02x1.3x(ln(660 / 0.4)0.619 + 0.9)]= 2.81 Bbl/day/psi

Initial injection rate = 2.81x(1300600) = 1967 BWPD

Mobility ratio M = (0.15x1.3) / (0.86x0.5) = 0.45

Pore volume per pattern = 7758x20x54x0.24 = 2011 MBbl

After injection of 350 MBbl:

Volumetric sweep efficiency Ev= 350 / [2011(0.620.28)] = 0.512

Areal sweep efficiency Ea = 0.512 / 0.8 = 0.64

From the correlation, Conductance ratio = 0.65

Hence; Water injection rate = 0.65x1967 = 1278 BWPD

Injection Allocation

Allocation of injected water is required in order to assure a uniform oil displacement

and optimum oil recovery

This is a key step in waterflood optimization and requires cooperative effort from

geologists and reservoir engineers

Injection allocation consists of two parts:

Balancing the injection rate and cumulative injection between various patterns

according to their pore volume

Achieving a uniform injection profile covering all reservoir flow units within

waterflood interval


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Continued monitoring is required to assure that allocated injection rates and

injection profiles are implemented

Balanced injection also:

Prevents fluid migration across pattern boundaries

Results in uniform fluid lifting requirements in producing wells

Minimizes premature water breakthrough

Injection rates for various patterns are calculated as follows:

Injection rate for pattern n qinjn= qinj

tVpn/ Vp

t

Where: qinjn= injection rate for pattern n

qinjt= total injection rate for the waterflood

Vpn= net pore volume for pattern n

Vpt= total net pore volume for waterflood area

Example

Total injection rate = 30000 BWPD

Total pore volume = 54750 MBbl

Pattern 1 2 3 4 5 6 7 8 9

Pore volume 5246 4246 6689 6027 7635 7988 3899 5866 7155

Injection rate 2875 2327 3665 3302 4183 4377 2137 3214 3920

Actual injected volumes can deviate from design values due to:

Unknown reservoir heterogeneity

Presence of natural fractures and thief zones

Formation damage in injection wells Non-uniform initial fluid saturation distribution in the reservoir

Non-uniform reservoir pressure distribution

Irregular pattern shapes

Monitoring and suitable remedial work should be conducted

Pattern voidage maps, Halls plots, production bubble maps and performance plots are

useful in this regard

Original injection allocation is usually revised based on actual performance and updated

reservoir studies

Maintaining uniform injection profile in all injection wells is a difficult task

Layer heterogeneity, shale breaks and thief zones affect injection profiles

Dual tubing strings with packers, twin injection wells and limited entry techniques

can help obtaining uniform injection profiles

Frequent spinner surveys, tracer surveys and use of observation wells are helpful

in determining actual injection profiles and water front movement

Cased-hole logging and 4-D seismic surveys also are done in some waterflood

projects to provide insight about fluid distribution and oil displacement

Note that these techniques are expensive, time consuming and require experience

and high technical capability

Relationship to Reservoir Pressure


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After reservoir fill-up, the next step is to raise average reservoir pressure to a

reasonable value

Selection of the pressure value is usually guided by fluid lifting conditions, available

water pumps, fracturing pressure and bubble point of reservoir oil

In general, a pressure value within 10 - 20% tolerance below the initial bubble point

is reasonable Raising average reservoir pressure is generally combined with fil-up period

Water injection and fluid withdrawal rates should be controlled in order to achieve a

negative reservoir voidage rate for a calculated period of time

After the desired reservoir pressure is reaches, waterflood is operated at zero

voidage rate to maintain the pressure

Voidage definition

Cumulative voidage =

NpBo+(GpNpRs)Bg+WpBwWinj-We

Voidage rate =

qo[Bo+(RpRs)Bg+WORBw]qinjBw-we

If voidage rate = 0 Reservoir pressure remains constant

If voidage rate > 0 Reservoir pressure will decline

If voidage rate < 0 Reservoir pressure will increase

Injection-Withdrawal Ratio

Defined as Injection rate / fluid withdrawal rate

IWR = qinjBw/ [qo{Bo+ (RpRs) Bg+ WOR Bw}]

IWR > 1 during reservoir fill-up period

IWR = 1 during pressure maintenance period


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Note that IWR does not take the water influx rate (we) into account due to the difficulty in

its estimation

If the water influx rate is known, the modified IWR is:

(IWR)m= (qinjBw+ we) / [qo{Bo+ (RpRs) Bg+ WOR Bw}]

Changes in reservoir pressureP = ( V / Vpct)

ct= cp+ Swcw + Soco+ Sgcg

Where: P = change in reservoir pressure, psi

V = cumulative reservoir voidage in RBbl

Vp= reservoir pore volume, Bbl

ct= total system compressibility, 1/psi

Sw, Soand Sgare water, oil and gas saturations

cw, coand cgare water, oil and gas compressibilities, 1/psi

cpis pore volume compressibility, 1/psi

After reservoir fill-up:

ct= cp+ Swcw+ Soco

ctafter fill-up


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Average producing WOR 1.8

Average producing GOR 405 SCF/STB

Oil formation volume factor 1.28 RB/STB

Gas formation volume factor 1.5 RB/MCF

Solution gas-oil ratio 368 SCF/STB

Expected water influx rate 8600 RB/day

Total compressibility ct= [6.8 + 0.38x3.2 + 0.62x14]x10-6= 16.7x10-6psi-1

Required change in reservoir pressure = 970635 = +335 psi

Required cumulative negative voidage = 187x106x335x16.7x10-6= 1046 MRBbl

Required voidage rate = 1046000 / (6x30) = 5812 RB/day

Current withdrawal rate = 14860[1.25+(674-315)x0.0022+0.87x1.03] = 43627 RB/day

Hence; Required water injection rate = 4362713500 + 5812 = 35939 BWPD

(IWR)m= (35939 + 13500) / 43627 = 1.13

Fluid withdrawal rate during pressure maintenance =

18500[1.28 + (405 - 368)x0.0015 + 1.8x1.03] = 59006 RB/day

Hence; Required water injection rate = 590068600 = 50406 BWPD

(IWR)m= (50406 + 8600) / 59006 = 1

Notes: -- Calculated injection rate during pressure maintenance period is quite sensitive to

the GOR and WOR values

-- It is recommended that reservoir engineers keep material balance to provide

reliable water influx estimates

Voidage Maps

Voidage analysis based on entire waterflood area is sometimes misleading Some waterfloods could have adequate voidage control as a whole but the

distribution for various parts may not be acceptable, i.e. some patterns may have

positive voidage while other patterns have negative voidage

Reservoir engineers should calculate voidage for individual patterns and prepare

appropriate voidage maps

Voidage maps (based on cumulative or current rate) provide visual illustration of

injection and withdrawal distribution

Voidage maps provide guidelines for making suitable changes to achieve optimum

oil displacement and recovery

Allocation factors

Calculating cumulative voidage or current voidage rate for a pattern requires the

application of well allocation factors:

Applied to injectors in normal Applied to producers in inverted patterns

Simple method:

Allocation factor = Angle of contribution / 360

Examples:

Corner well in nine-spot pattern = 90/360 = 0.25


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Side well in nine-spot pattern = 180/360 = 0.5

All wells in four-spot pattern = 60/360 = 0.167

All wells in five-spot = 90/360 = 0.25

All wells in seven-spot = 120/360

More accurate allocation factors are based on angle of contribution iand weighting

factors wirelated to reservoir characteristics

Appropriate weighting factors are usually estimated by engineers and geologists

familiar with the reservoir

Approximate weighting factors:

wi= (kh)ifor voidage rate

wi= (h)ifor cumulative voidage

Fi= wii / wi I

Allocation factors for peripheral wells are estimated based on their location and primary

production


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Allocation factors, Example

Allocation factors from the eight producing wells in this nine-spot pattern are:FA= 90 w5/ (90 w1+ 90 w2+ 90 w4+ 90 w5)

FB= 180 w5/ (180 w2+ 180 w5)

FC= 90 w5/ (90 w2+ 90 w3+ 90 w5+ 90 w6)

FD= 180 w5/ (180 w4+ 180 w5)

FE= 180 w5/ (180 w5+ 180 w6)

FF= 90 w5/ (90 w4+ 90 w5+ 90 w7+ 90 w8)

FG= 180 w5/ (180 w5+ 180 w8)

FH= 90 w5/ (90 w5+ 90 w6+)

Uses of voidage maps

Provide guidelines in making operational decisions to:

Increase or decrease water injection rates

Modify lifting capacities in certain wells

Drill additional infill wells


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Example of voidage maps

This voidage map indicates that:

1. Several patterns in the center of flood area need additional injection2. Need to decrease injection rate, modify fluid lifting or add infill producing wells in the

eastern part of flood area

design concepts in waterflood processes

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