Forensic AnalysisWhy Did This Field Die?

Presented at

Western Australia Section of SPEJune 19, 2012

Perth, Australia

Dr. Bill CobbWilliam M. Cobb & Associates, Inc.

Petroleum Engineering & Geological ConsultantsDallas, Texas

Every Field is Different!

Reservoir Drive Mechanism PRIMARY

Rock & Liquid Expansion Above the Bubble Point SOLUTION GAS DRIVE Initial Gas Cap Expansion - Size Aquifer Influx – Size

WATERFLOODING CO2, Steam, Polymer, Other

Why Did This Oil Field Die?

PRIMARY RECOVERY VS WF

Solution gas drive requires the reservoir pressure to be constantly decreasing

WF is a displacement process and is most efficient when reservoir pressure is maintained or increased

When converting from primary to waterflooding or any type of fluid injection, the reservoir recovery mechanism changes.

Consequently reservoir evaluation and reservoir management procedures generally need to be changed

Can we learn lessons that may help us resuscitate the WF or guide us if we move to a tertiary project?

Why did the project die? Where do we start? What does the evidence say?

THIS PRESENTATION IS THE FORENSIC ANALYSIS OF A DEAD OR DYING WATERFLOOD WHERE PRIMARY

PRODUCTION WAS FROM SOLUTION GAS

WHAT ARE THE KEY FACTORS THAT DRIVE THE OUTCOME OF A WATER INJECTION

PROJECT?

NP = Cumulative Waterflood Recovery, BBL.

N = Oil in Place at Start of Injection, BBL.

EA = Areal Sweep Efficiency, Fraction

EV = Vertical Sweep Efficiency, Fraction

ED = Displacement Efficiency, Fraction

 

WATERFLOOD RECOVERY FACTOR

EA = f (MR, Pattern, Directional Permeability, Pressure Distribution, Cumulative Injection &

Operations)

EV = f (Rock Property variation between different flow units, Cross-flow, MR)

EVOL = Volumetric Sweep of the Reservoir by Injected Water

ED = f (Primary Depletion, So, So, Krw & Kro, μo & μw)

RF N

Np DVA E

E

EE

VOL

**RF

TO MAXIMIZE WATERFLOOD OIL RECOVERY

Recognize the recovery mechanism has changed.

The driver of the waterflood is the injection well.

The injector is the Quarterback of the operation.

Efficient utilization of the injector maximizes volumetric sweep. (The fraction of the reservoir contacted by the injected water).

THE QUARTERBACK OF ALL INJECTION PROJECTS IS THE INJECTION WELL

Properly located injection wells are A significant key to A successful waterflood:

They deliver the water where it needs to be injected

They deliver the water at the correct time They deliver the water in the proper volume Effective utilization of injection wells is an

important key to optimizing the WF by allowing EA and EV values and RF to be maximized

Failure to properly locate and manage the injectors will result in a less efficient injection project and could lead to failure.

DID THIS WATERFLOOD DIE AND FAIL TO PRODUCE EXPECTED WF OIL?

Did this project meet a premature high water cut death? If the answer is yes, some of the reasons for failure are likely due to:

Lack of significant movable oil saturation at the start of injection – low ED

High initial free gas saturation leading to a small or negligible oil bank and long gas fill-up time

Floodable net pay is overstated (same porosity cutoff used for WF as used for primary depletion)

Poor volumetric sweep – low EVOL

NORTH AMERICALIQUID EXPANSION - SOLUTION GAS DRIVE

                       

Pi = 4400 Psi Pbp = 4000 Psi P = 400 Psi   

               

          Sg = 36%        RF = 1%     RF = 19%             

  So = 76% So = 76%                     

          So = 40%                 

               

               

  Swc = 24% Swc = 24% Swc = 24%                   

 Boi = 1.75 Bobp = 1.78 Bo = 1.15   

OOIP = 100 MMSTBO OIP = 80 MMSTBO *L.P. Dake – Fundamentals of Res Eng. – Eq. 3.22 Pg. 86  

NET PAY

Static OOIP Dynamic OOIP

Drive mechanism – primary vs secondary Waterflood net pay cutoffs controlled by:

Water Cut Economic Limit Permeability Distribution between Flow Units

(Dykstra-Parson Coefficient) Oil/Water Relative Permeability Mobility Ratio (Oil and Water Viscosity) Fluid Saturations at Start of Injection (So, Sg, Swc)

* See SPE #48952 and SPE #123561

WF VOLUMETRIC SWEEP USING OIL PRODUCTION DATA SINCE START OF

INJECTION

* See SPE #38902

Cont'd – WATERFLOOD FAILURES MAYBE DUE TO:

VRR less than 1.0 and pressure declines High variation in permeability between geological

layers causing the high permeability layers to water out the producing wells before the lower perm layers Contribute meaningful waterflood production

Failure to develop a pattern Failure to recognize and honor KX and KY trends Failure to keep fluid levels pumped off in producing

wells Mechanical integrity of injection wells to control

vertical distribution of injected water

ASIAN WATERFLOOD

SOLUTION GAS DRIVE (WEAK WATER INFLUX)

Pi = Pbp = 2250 Psi P = 2100 Psi - At Start Of Injection

Rsi = 550 SCF/STBO Swc = 29%

Boi = 1.39 RB/STB Sg = 3%

µoi = 0.44 CP MR = 0.30

PRF W/O H2O Current RF EUR VRR Since

AREA % % % Start of Inj.

1 15-18 18 27 0.51

2 15-18 21 31 0.63

3 15-18 25 33 0.71

4 15-18 31 44 1.09

WATER INJECTION RESPONSE

SCHEMATIC CROSS SECTION

WF WAS NOT SUCCESSFULWHAT SHOULD BE CONSIDERED?

Compute So and Sg at start of waterflood Compute WF volumetric sweep efficiency Re-evaluate net pay cutoffs Check for high permeability thief zones Check VRR since start of injection Consider infill drilling Realign pattern – this could violate the 11th

commandment Is it a candidate for horizontal producers?

SUMMARY OF LESSONS LEARNED

WATERFLOOD LESSONS LEARNED

1) Every field is different2) There are almost no analogy floods3) Need high movable oil saturation4) Need low free gas from solution5) Prefer low oil viscosity (high viscosity oils can be

produced by cycling large volumes of water).6) Pattern highly desirable7) Honor KX/KY 8) Effective VRR since injection start equals 1.0 or more for

each pattern and key geological zones

LESSONS LEARNED Cont'd

9) Careful attention given to selection of net pay cutoffs. NET PAY FOR WATERFLOOD IS LESS THAN NET PAY FOR PRIMARY DEPLETION.

10) Keep fluid levels in producing wells pumped off11) Accurately test each producing well monthly for oil,

water and gas.12) Maintain production plots on a well by well basis.

Combining wells into groups is equivalent to combining “The Good, The Bad and The Ugly”.

LESSONS LEARNED Cont'd

13) Forecasting the future production should be performed on a well by well basis. Typical plots of oil rate vs time or oil rates vs cumulative

oil produced should be used with extreme caution because oil rates are directly related to injection rates and injection rate changes and stratification.

Semi-log plots of WOR vs cumulative oil produced are likely to be more reliable because WOR is largely independent of injection rate and injection rate changes.

Analysis of WOR plots is most reliable when forecasts are made using WOR values which exceed 2.0 to 3.0

14) Reliable well forecasts require accurate well tests

LESSONS LEARNED Cont'd

15)Conduct regular injection profiles to monitor the amount of water going into various zones.

16)Thief zones must be identified and isolated or oil production will occur at high water cuts.

17) REMEMBER THE QUARTERBACK

HONOR THE DATA

Every field is different The reservoir speaks to us – are we listening? Data costs money Lack of quality geological, reservoir and production data

usually leads to inefficient operations or early failure. Lack of relevant data puts the geoscientist, reservoir or

operational analyst into a position where he/she must trust experiences, instincts, hunches and wishful thinking.

“People are not always truthful – the evidence does not lie.” – from Gil Grissom of CSI

Become an OFI – Oil Field Investigator – follow the evidence

DON’T CHANGE ITDON’T CHANGE IT

THEN, THERE IS NO PROBLEMTHEN, THERE IS NO PROBLEM

YES NO

DID YOU TRY TO FIX IT?

DID YOU TRY TO FIX IT?

YOU IDIOT!YOU IDIOT!

DOES ANYBODYKNOW ABOUT IT?

DOES ANYBODYKNOW ABOUT IT?

HIDE ITHIDE IT

YES

YOU POOR BASTARD!YOU POOR BASTARD!

YES

NOCAN YOU

BLAME SOMEONEELSE?

CAN YOUBLAME SOMEONE

ELSE?

NO

YES

ARE YOUGOING TO BE IN

TROUBLE?

ARE YOUGOING TO BE IN

TROUBLE?

NO

YES

PRETEND YOU DON’T KNOW ABOUT

IT!

PRETEND YOU DON’T KNOW ABOUT

IT!

NO

WHEN ALL ELSE FAILS, A POTENTIAL WATERFLOOD ANALYSIS METHOD IS:

DOES IT FUNCTION?

DOES IT FUNCTION?

Forensic AnalysisWhy Did This Field Die?

Presented at

The Dallas E & P ForumDecember 13, 2011

Sponsored By SIPES and SPE

Dr. William M. CobbWilliam M. Cobb & Associates, Inc.

Petroleum Engineering & Geological ConsultantsDallas, Texas

TWO ADDITIONAL POINTS

Assume Gas fill-up has been achieved

(reservoir Contains oil and water Reservoir pressure is approximately constant

(Bo is constant) Steady state flow prevails (approximately)

Conclusion Effective Water Injection = Liquid Production

(at Reservoir Conditions)

DECLINE CURVE ANALYSIS

DECLINE CURVE ANALYSIS

* (1 )

*

w o w wo

o o

w ww

w

i f i fq

B B

i fq

B

ConclusionOil and Water Production Rates are directly related to injection rates. Therefore, DCA of qo vs t or qo vs NP must be evaluated only after giving consideration to historical and projected water injection rates.

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100

1000

10000

WATERFLOOD EXPONENTIAL DECLINE

BO

PD

EL

Start Water Injection

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 600000

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

OIL RATE VS CUMULATIVE OIL PRODUCED

Cumulative Oil Production (MMBbls.)

BO

PD

EUR 49 MMBO

Start Water Injection

EUR 53 MMBO

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 600000

1000

2000

3000

4000

5000

6000

7000

8000

9000

10000

0

2000

4000

6000

8000

10000

12000

14000

16000

18000

20000

OIL RATE VS CUMULATIVE OIL PRODUCED

Cumulative Oil Production (MMBbls.)

BO

PD

EUR 49 MMBO

Start Water Injection

EUR 53 MMBO

WOR IS INDEPENDENT OF INJECTION RATE

0

. .

*

* (1 )

(1 )

( ) *(1 )

w

w w

w w

w

w

w oSTD COND

w w

qWOR

q

i fWOR

i f

fWOR

f

f BWOR

f B

Conclusion WOR is independent of injection rate

Other Points WOR should be applied to individual wells and

not field WOR should be applied using values greater

than 2.0

0 5000 10000 15000 20000 25000 30000 35000 40000 45000 50000 55000 600000.1

1

10

100

WATER OIL RATIO VS CUMULATIVE OIL

Cumulative Oil Production (MMBbls.)

WO

R

EUR 55 MMBO

49