EvaluationEvaluation

VolumetricsVolumetrics

GEOL 4233 ClassGEOL 4233 Class

Dan BoydDan BoydOklahoma Geological SurveyOklahoma Geological Survey

Fall 2011 SemesterFall 2011 Semester

Volumetrics

1) Definitions / Conversions (Handy Facts)

2) Assumptions (The ‘Art’ of Volumetrics)

3) Mechanics (Input Variables)

4) Reserves (Recovery Factors / Probabilistic Calculations)

VolumetricsDefinitions / Conversions

OOIPOGIPRFFVF: (Bo, Bg)Saturations / Residual Saturations (So, Sg, Sw – Soirr, Sgirr, Swirr)EUR

Resources (In-Place) vs. Reserves (Economically Producible)

Definitions / Conversions (I)

14.7 psi = Atmospheric Pressure (@ S.L.)5,280 feet per mile43,560 sq ft per acre640 acres per sq mile = Section (160 ac per quarter section) 247 ac/sqkm3.281 ft per meter (39.37 inches per meter)1.609 kilometers per mile2.54 centimeters per inch35.32 cubic feet per cubic meter7,758 STBarrels per acre-footSpecific Gravity (crude); .80 - .97Btu value for gas: avg ~1 Btu / cubic foot (1000Btu/MCF), rich - higher, a lot of non-hydrocarbons - lowerAPI gravity: 25 = specific gravity .904, 42 = specific gravity .816BOE: 6,000 cubic feet per barrel (average)

Definitions / Conversions (Ia)

Gas Liquids:

Condensate – hydrocarbon liquids that condense from a gas production stream as pressure and temperature are reduced from reservoir to surface conditions. These are collected on the wellsite.

Natural Gas Liquids (NGL) – hydrocarbon liquids that remain in gas at surface temperature and pressure. These must be stripped from the ‘wet’ gas production stream at a central processing facility to bring its heating capacity to pipeline specifications. These are shorter chain hydrocarbons than condensate, consisting primarily of ethane, with smaller amounts of propane and butane.

y = 0.0202x - 19.981

0

2

4

6

8

10

12

14

1000 1200 1400 1600 1800

BTU

Ga

llon

s/M

cf E

tha

ne

+Generalized Conversion of Natural Gas Btu Content to NGL Yield

1200 Btu gas = ~ 4 gallons/MCF ~ 100 Barrels NGL/MMCF

1400 Btu gas = ~ 8 gallons/MCF ~ 200 Barrels NGL/MMCF

1100 Btu gas = ~ 2 gallons/MCF ~ 50 Barrels NGL/MMCF

Courtesy Dr. Jeffery Callard1000 Btu gas ~ 100% methane

Definitions / Conversions (Ib)

Gas – Other Acronyms:

CNG – Compressed Natural Gas: Gas compressed to <1% of its volume at atmospheric pressure, requiring storage at 2,900-3,600 psi. Used as substitute fuel for gasoline/diesel, but because still gaseous has 42% energy equivalency per unit volume.

LNG – Liquefied Natural Gas: Methane gas cooled to -260 degrees F (-162 C) at atmospheric pressure, making it 1/600th the volume as a gas. This makes LNG the preferred global transport method for natural gas. Energy density is 60% that of diesel fuel.

LPG – Liquefied Petroleum Gas / Liquefied Propane Gas: Various mixes of propane and butane used for heating, motor fuel, refrigeration, and aerosol propellants. Derived through the refining process of ‘wet’ gas. Energy density is about 70% that of diesel fuel.

Definitions / Conversions (II)

To calculate pressure (if mud weight balanced precisely):Under vs. Over Balanced

Mud Weight (in ppg) x .052(conversion factor) x depth (in feet) = (BH)Pressure (in psi)

If mud is exactly balanced with formation pressure: Calculated Pressure = BHP (reservoir)

Hydrostatic pressure gradient = 0.43 psi/ft (43 psi/100’)

Volumetric ParametersDefinitions / Conversions (III)

FVFs: Bo - Oil (dead) ~ 1.0 (RSB/STB), oil moderately gassy ~1.2RSB/STB, very gassy ~ 1.4 RSB/STB

Bg – Normally pressured (hydrostatic) FVF = Depth (in ft)/36.9

Example @ 5,000’ FVF = 136 SCF/RCF

Underpressured (Brooken Field example): .23 psi/ft (normal = .43 psi/ft)@ 1,400’ Bgi = 28 SCF/RCF (38 SCF/RCF if normally pressured)

Overpressured

The ‘Art’ of Volumetrics(Assumptions)

• Wells drilled are representative of reservoir as a whole• Average Porosity, Sw, So, and Sg are accurate• Reservoir homogeneous and all parts will be swept• The size, thickness and structure of the reservoir is correctly mapped• The area is calculated precisely

• The OWC and GOC are sharp and known precisely, or …. the porosity saturation cutoffs for pay are accurate, with good sweep above and no feed-in from below these cutoffs

Well Log of Incised Valley-Fill SandstoneOklahoma’s Brooken Field (Booch)

Average Porosity = ?

‘Sharp’ Fluid Contacts ?

B-184 Horizontal Lateral(Elan Plus Interpretation)

‘Sharp’ Fluid Contacts ?

(Elan Plus Interpretation)

Badak-185 Horizontal Lateral

‘Sharp’ Fluid Contacts ?

Pressure Gradients

‘Sharp’ Fluid Contacts ?Here: + or – 5’

Oil rim estimate: + or – 10%Gas cap estimate: + or – 15%

Transition Zone Transition Zone

Volumetric Mechanics(Equations)

GAS: Area (Ac) x Thickness (Ft) x Avg Porosity (%) x Avg Sgi (%) x Bgi (SCF/RCF) x 43,560 sqft/ac = OGIP (SCF)

OIL: Area (Ac) x Thickness (Ft) x Avg Porosity (%) x Avg Soi (%) / Boi (RB/STB) x 7758.4 Bbls/AcFt = OOIP (STB)

Volumetric Mechanics(Gross Reservoir Volume)

AREA: Productive area (map view), in acresSubdivide overall area into components that are calculated individuallybased on similar average reservoir thickness

THICKNESS: From reservoir or fluid top to contact or saturation cutoff, in feet

SUMMED (AREA(S) X THICKNESS) =

GROSS RESERVOIR VOLUME in AcreFeet

Volumetric Mechanics(Pore Volume)

GROSS RESERVOIR VOLUME (AcFt) x Average Porosity (%) within productive reservoir =

GROSS STORAGE (PORE) VOLUME (AcreFeet)

Volumetric Mechanics(Gross Oil/Gas Volume)

GROSS STORAGE (PORE) VOLUME (AcreFeet) x AVERAGE OIL (Soi) or GAS (Sgi) SATURATION (%) =

GROSS OIL or GAS VOLUME (AcreFeet)

===========================

Conversion to standard units of RBbls or RCF

AcreFeet x 7,758 Bbls/AcreFoot = Oil in Reservoir Barrels

AcreFeet x 43,560 Cubic Feet/AcreFoot = Gas in Reservoir Cubic Feet

Volumetric Mechanics (Oil)(Conversion to Stock Tank Barrels)

FORMATION VOLUME FACTOR (Bo):

Rules of Thumb

‘Dead’ Oil (no dissolved gas): Bo ~ 1.0 (RB/STB)‘Gassy’ (deepish) Oil: Bo ~ 1.4 (RB/STB)‘Typical’ (shallower) Oil: Bo ~ 1.2 (RB/STB)

Oil Volume (RB) / Bo (RB/STB) = OOIP (STB)

Volumetric Mechanics (Gas)(Conversion to Standard Cubic Feet)

FORMATION VOLUME FACTOR (Bg):

Rules of Thumb

• Bg – If normally pressured (hydrostatic)Bg = Depth (in feet) / 36.9 Example: @ 5,000’ FVF = 136 SCF/RCF-----------------------------• Underpressured (Brooken Field example): .23 psi/ft (normal = .43 psi/ft)@ 1,400’ Bgi = 28 SCF/RCF (38 SCF/RCF if normally pressured)-----------------------• Overpressured

Gas Volume (RCF) X Bg (SCF/RCF) = OGIP (SCF)

ReservesFrom OOIP / OGIP

(What can you take to the bank ?)

RECOVERY FACTOR (RF): Function of – • Reservoir Quality, Depth, Pressure, Temperature• Fluid Properties• Drive Mechanism(s)• Reservoir Management

Rules of Thumb

The better the reservoir, the better the recovery factor• Even fluid movement • Larger pore throats (better sweep, more moveable oil/gas)• Better water support (if any to be had)• Better effectiveness in secondary/ tertiary recovery projects

Recovery Factors(Ballpark Rules of Thumb)

OIL: • Poor reservoir (low poro-perm): < 10%• Dual Porosity (low matrix reservoir quality): ~ 20%• Good Poro-Perm (Primary = Secondary): ~ 30%• Excellent reservoir (good water support): ~ 40-50%• Ideal (good reservoir quality, management): ~ 60-70%• Tar Sands (mined): ~ 100%

GAS:• CBM, Shale Gas: < 10% (generally)• Good Quality (depletion): ~ 70% (GOM average)

• Excellent Reservoir (depletion, + compression): 90%+ (Lake Arthur Ex.)

Probabilistic Volumetrics(Because there is no single answer)

• Calculate a range of values based on confidence in variables.P = Probability Factor

P 100 – dead certaintyP 70 to 90 – high confidenceP 10 to 30 – low confidence

• For each variable with significant uncertaintyAssign P 90 , P 50, and P 10 values to create distributionExample: Productive area – P 90 = smallest reasonable area, P 50 = most likely area, and P 10 = maximum area (but not unreasonable)

• Qualitative (‘fudgability’ - what do you want it to be ?)Usefulness a function of experience in areaRequires objective assessmentMost beneficial when comparing large projects in which data is sparse

Probabilistic Reserves(Taking Credit Now for Future Additions)

(P + P + P)

• Proved.Highest level of certainty (assigned $ value)

PDP – Proved-Developed-Producing (decline curve)PUD – Proved-Undeveloped (Nonproducing)

• Probable.Undrilled, but based on known areas has high likelihood of producingExamples:

Undrilled fault-block in area where faults do not sealArea adjacent to existing production with quantifiable DHI

• Possible.Higher risk, but based on incomplete information meets known requirements for production

Volumetric Computations(1)

Prerequisites –

Net Pay Isopach (which requires)Structure Map (on top of the pay)Elevation of fluid contactsNet Reservoir IsopachAccurate Pay Cutoffs (Porosity, Sw, Shale Content ie: k measure)

Knowledge of Potential Flow-Barriers (each compartment calculated separately)

Structure Map - identify isolated fault blocksCross-Section(s) – identify potential stratigraphic barriers

Volumetric Computations(2)

Mechanics –

Work Station (high-tech, but still just a tool)

Log analyses, tops, net pay thicknesses are usually digital and internalComputer-generated maps/cross-sections must be ‘truthed’ and editedAdvantage – can sift vast amounts of data and quickly analyze wide range of possibilitiesDisadvantage – GIGO (garbage in, garbage out) – but it’s nice looking garbage

Paper (much slower, but often results in better geological understanding )

PC computer aid only, interpretation on paper (hand-contouring & log analysis)

Planimeter usually used for calculating areas, or………….Eyeball entire pay map with an average pay thickness, or box-out into bite-size

chunksGiven the assumptions – the experienced eyeballer always has the edge

Reservoir VolumeMechanics

(Work station’s crashed &/or planimeter’s been stolen)

Bite-Size Chunks Technique

• Box out areas into rectangles-triangles• Calculate areas• Assign each area an average thickness• Sum the volumes calculated

Reservoir VolumeMechanics

Slab and Wedge Technique(Useful in areas of shallow dip)

• Reservoir thickness ~ constant• Area inside of where water contact is at reservoir bottom assigned full thickness value• Area outside of this, to the edge of the water contact, is assigned half of the full thickness value

Net Pay zero-line

Net Pay maximum line

Slab Area

Wedge Area

Assume OWC @ Base of reservoir

Slab Area + Wedge Area / 2= Gross Reservoir Volume

Blanket 40’ Reservoir with 80’ of Closure

Net Oil Reservoir Isopach(Well control good, Zero line conforms to OWC)

Planimeter 2-3 areas: ~ 0-20, 20-30, 30+

Volumetric Map SetRigorous ‘By the Book’

(This is usually overkill)

Brooken Field Net Sandstone Isopach

Trapping Fault

Reams Southeast FieldMiddle Booch Structure Map

Reams Southeast Field StudyPS-0 Net Sand Isopach

Reams Southeast Field StudyPS-2 Net Sand Isopach

Reams Southeast FieldMiddle Booch Net Sandstone Isopach

(Showing Combination Trap)

Fault Contact

Water ContactReservoir Limits

Reams Southeast Field Study Volumetric Input

Reams Southeast Field Study Gas Volumes

ExercisesExercises

Exercise 1a:

Calculate OGIP

Exercise 1b:(Alternative Interpretation)

Calculate OGIP

Exercise 1c:(Yet another alternative Interpretation)

Calculate OGIP

Exercise 1Exercise 1(Sparse Data)(Sparse Data)

Volumetrics Sensitivity: Volumetrics Sensitivity: • Gross Reservoir VolumeGross Reservoir Volume - varies by a factor of 4 (at least) in 3 - varies by a factor of 4 (at least) in 3 reasonable interpretations that honor all data. This is made possible reasonable interpretations that honor all data. This is made possible both by changing the productive area and the thickness within it. If the both by changing the productive area and the thickness within it. If the porosity cutoff (8%) for reservoir were moved up or down, results porosity cutoff (8%) for reservoir were moved up or down, results would vary even more.would vary even more.• PorosityPorosity - for each percent the average value goes up or down, the - for each percent the average value goes up or down, the OGIP estimate is changed by 10%. In heterogeneous reservoirs the OGIP estimate is changed by 10%. In heterogeneous reservoirs the porosity range can be large (8 - 18% not unusual).porosity range can be large (8 - 18% not unusual).

0 2 4 M iles

0 4 Kilometers2

1:50000

118° 40' 118° 45' 118° 50'

5° 50'

5° 45'

118° 45'118° 40'

5° 45'

5° 50'

ARCO

PhillipinesNymphe Area

JW/DB Dec, 1999

NYMPHENORTH 1

NYMPHE 1

KUDATERBANG 1

NYMPHESOUTH 1

BENRINNES 1

Trapping StyleTop M2 Depth Structure

C.I. = 200 m

Spill Point

N

Real Life Example(One penetration)

Interpretation based on inferred environment of deposition and analog comparisons (in some cases seismic DHI’s can help)

With production history, the geologic model can be refined(and then used as a template elsewhere)

From Fredrick RobeliusUppsala Universitet, 2005

Exercise 2:

Calculate OGIPNorth Dome Field

(Qatar/Iran)

Regional Location Map

Ghawar Field

North Dome Field

North Dome Field:

Productive Area: ~ 40 x 70 mi Average Thickness: ~ 510’Average Porosity: ~ 20%Average Swi: ~ 20% DEPTH ~ 11,000’ (assume normal pressure)Carbonate reservoir

Calculate:

OGIP_______________

Reserves (assuming 65% RF)

__________________________

Exercise 2

Get ready for a lot of zeros

Exercise 3

Location Map

Greater Ghawar Field

Area: ~ 110 x 15 milesAvg thickness: ~ 185’Avg porosity: ~ 18%Average Swi: ~ 11%Boi – 1.32

Avg perm: ~ 350 mdAPI-32 degreesGORi = 550 Depth -6600’OWC

Calculate:

OOIP________________

EUR_________________

(given various RF’s)

Exercise 3

Get ready for a lot more zeros

Assume: Depth ~ 8,000’ (normally pressured)Reservoir – 20’ blanket SS (no wedges)Avg por – 15%, Avg Sw 10% (gas cap), 20% (oil rim)Bo – 1.20 RB/STB

Calculate:OGIP (up/downthrown)OOIP

Exercise 4

Exercise 4Schematic Cross-Section

Exercise 5

20’

Lessons Learned:Lessons Learned:

• Outcome sensitive to reasonable changes to inputOutcome sensitive to reasonable changes to input• Where data are sparse, a wide range of OGIP/OOIP values possibleWhere data are sparse, a wide range of OGIP/OOIP values possible• Structural Issues: attic oil, undrained fault blocksStructural Issues: attic oil, undrained fault blocks• Stratigraphic Issues: depositionally or structurally isolated ‘pods’Stratigraphic Issues: depositionally or structurally isolated ‘pods’

• How to improve the quality of volumetrics ? How to improve the quality of volumetrics ? (The Value of Experience)(The Value of Experience)• Mapping of analog areas where more data availableMapping of analog areas where more data available• If in rank area, may need to go far afieldIf in rank area, may need to go far afield• Comparison to fields with production history (material balance ?)Comparison to fields with production history (material balance ?)• Improved understanding of reservoir architectureImproved understanding of reservoir architecture

• Thickening ratesThickening rates• Reservoir heterogeneitiesReservoir heterogeneities• Pay cutoffsPay cutoffs• Recovery factorsRecovery factors

Geological Objectivity (Ethics)Geological Objectivity (Ethics)• The company needs drillable prospects / reserve adds, but……..The company needs drillable prospects / reserve adds, but……..• The play you’re assigned is weak economicallyThe play you’re assigned is weak economically

Be Objective Without Being PessimisticBe Objective Without Being Pessimistic• Understand your area as completely as possibleUnderstand your area as completely as possible

• Geologic history (petroleum system)Geologic history (petroleum system)• Environments of deposition (log-core-outcrop)Environments of deposition (log-core-outcrop)• Reservoir properties (keys to pay quality)Reservoir properties (keys to pay quality)• Successful explorationists understand and map producing fieldsSuccessful explorationists understand and map producing fields• Integrate geological interpretation into engineering dataIntegrate geological interpretation into engineering data

• PressuresPressures• Drive mechanisms Drive mechanisms • Fluid properties (do they change ?)Fluid properties (do they change ?)

• Justify and document all assumptions (data mining)Justify and document all assumptions (data mining)• Keep an eye out for ‘upside’Keep an eye out for ‘upside’

• Explaining anomalies is the key to new geologic plays Explaining anomalies is the key to new geologic plays • Shallower objective(s)Shallower objective(s)• Deeper objective(s)Deeper objective(s)• A different way to drill, complete (?)A different way to drill, complete (?)

Remember: Quality work will be recognizedRemember: Quality work will be recognized

May Mother Nature Smile Upon You

Cushing Field