reserves estimation and classification

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Reserves Estimation andClassification



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Introduction

Field reservoir engineer is responsible for the estimationand classification of reserves.

Estimation techniques :

material balance calculations

sweep efficiency analysis

decline curve analysis

reservoir simulation

All reserves estimates involve some degree of uncertainty.



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Why Reserve Estimates?

Measure effectiveness of exploration anddevelopment.

Budgeting for drilling and facilities. Unitization.

Purchase / sale of properties.

Bank loans. Taxation.

Government policy and planning.



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Definition

Reserves are estimated quantities of crude oil,condensate, natural gas, natural gas liquids, andassociated substances anticipated to be commercially

recoverable:

- from known accumulations,

- under existing or anticipated economic conditions,

- by established operating practices, and

- under current or anticipated government regulations.



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Original Oil and Gas In place Calculation

Volumetric

OOIP or OGIP = (Rock Volume) x (Porosity) x(1 – Water Saturation)/ ( Formation Volume

Factor )



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Original Oil and Gas In place Calculation

Material Balance

Expanded volume of Volumevolume of = withdrawals - oforiginal injected

reservoir fluidsfluids



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Recovery and Efficiency Calculations

Recovery factors : STB/acre-ft and MCF/acre-ft

Recover efficiencies: fractional recovery of OOIP orOGIP

Methods of estimation:

- analogies - correlations

- water flood design charts - material balance programs

- reservoir simulation



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Analogy Method

The analogous reservoirs should be similar to: . Drive mechanism . Permeability and porosity . Well spacing and pattern

. Size . Relative volumes of oil, gas and aquifer . Degree of permeability and porosity heterogeneity

. Net-to-gross sand ratio . Production practices . Depositional environment and trapping mechanism

. PVT properties

Recovery factor adjustments are made to compensate for differencesbetween the analogous reservoirs and the reservoirs being evaluated.



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Correlations

Depletion Drive Gas Reservoirs:Rec. Efficiency =1 - Pa Zi / Pi Za

Depletion Drive Oil Reservoirs:

1.During under saturated stage : Rec. Efficiency = Ce(Pi-Pb)Boi/Bob Where: Ce=(SoCo+SwCw+Cp)/So

2. During saturated stage : Er = 0.41815 ( Φ(1-Sw)/Bob)˙¹ x (k/1000μob)˙¹(Sw)·³(Pb/Pa)˙²

Water Drive Gas Reservoir: Er=(1-PaZi/PiZa) + ((PaZi/PiZa) EvEd) Where: Ed=(1-Swi-Sgr)(1-Swi)

Sgr= 0.62 -1.3 Φ



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Waterfloods

Mobility Ratio

M = (Krw/ μw) x (Kro/ μo)

Recovery Efficiency

Er = Ea x (Swb – Swi)/(1-Swi)

Areal sweep efficiency, Ea, from a homogenous5-spot water flood for a certain M can beobtained from charts.



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Ultimate Recovery

Volumetric with Recovery Efficiency:

Ultimate Recovery = Er x OOIP or OGIP( from

volumetric)

Material Balance with Recovery Efficiency:

Ultimate Recovery = Er x OOIP or OGIP( frommaterial balance)



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Decline Curve Analysis

Ultimate recovery is the sum of cumulative

recovery to date and remaining reserves.

Remaining reserves can be calculated with

decline curve analysis.



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Reservoir Simulation

Reservoir simulation incorporates a comprehensiveapplication of physical laws governing multiphase fluidflow in porous media.

Reservoir simulation can be summarized in three-stepprocess:

1. Setting up mathematical equations that describe fluidflow.

2. Solving the Mathematical equations.

3. Setting up the numerical model.



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Reserves Classification

Reserves are estimated quantities of crude oil,condensate, natural gas, natural gas liquids, andassociated substances anticipated to be commercially

recoverable:

- from known accumulations,

- under existing or anticipated economic conditions,

- by established operating practices, and - under current or anticipated government regulations

Dr.Helmy Sayyouh



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Reserves Classification

All reserves estimates involve some degree of

uncertainty , depending on :

The amount and reliability of geologic andengineering data available at the time of theestimates.

Interpretation of these data



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Milestones in Reserves Definitions

1944: Frederic Lahee ( API )

1955: Frederic Lahee ( WPC )

1960: American Petroleum Institute

1962: Jan Arps ( SPE )

1965: SPE

1972: V .E. Mckelvey (USGSurvey)

1981: SPE

1985: SPE

1987: SPE

World Petroleum Congress(WPC)

Dr.Helmy Sayyouh



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SPE-Endorsed Definitions of Reserves

Reserves in General 1. Known or discovered accumulations. 2. Estimated volumes: crude oil – condensate – natural

gas – natural gas liquids – associated substances suchas sulfur and carbon dioxide.

3. Based on interpretation of geologic and engineeringdata.

4. Commercially recoverable under economic, operating

and regulating condition. 5. Time dependent ( production ). 6. Involve degree of uncertainty.

7. Subject to revision.



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Methods of Classifying Reserves

1. Ownership:

Total – Gross – Net

2. Recovery Mechanism:

Primary - Improved

3. Degree of Uncertainty:

Proved – Probable – Possible

4. Development Status:

Developed – Undeveloped

5. Productive Status:

Producing – Non-producing



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Ultimate Potential

Ultimate Recovery

Reserves

Unproved ReservesProved Reserves

Future Additions

Cumulative Production

Possible ReservesProbable ReservesProved Undeveloped

Proved Developed



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Proved Reserves

Proved oil and gas reserves are the estimated quantitiesof crude oil, natural gas, and natural gas liquids which canbe recoverable:

- in future years

- from known reservoirs

- under exiting economic and operating conditions.

A confidence level of 90 to 100% is required.



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Proved Reserves

Proved reserves must have:

The area of a reservoir which includes- that portion delineated by drilling and definedby GOC and / or OWC

- the adjoining portions not yet drilled but

economically productive

The facilities to process .



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Proved Reserves

Proved reserves have been divided into:

Proved Developed Reserves : are expected to

be recovered through exiting wells with exitingequipment and operating methods.

Proved Undeveloped Reserves : are expectedto be recovered from new wells on undrilled

acreage or from exiting wells where a majorexpenditure is require for recompletion.



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Probable Reserves

Probable reserves are less certain than provedreserves and they are more likely to be recovered thannot under mid-trend economic conditions.


Probable reserves have been divided to:

Class 1 : Reserves representing the primary recoveryfrom the delineated area of a known reservoir.



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Probable Reserves

Class 2 : Reserves representing the primaryrecovery which depends on:

a. Lateral extension of the reservoir beyond thelimits assumed for proved or probable class 1reserves due to up dip or down dip extensions.

b. Undrilled fault blocks adjacent to the delineatedarea of a known reservoir.

c. Direct or diagonal offsets to spacing units havingproved or probable class1 reserves.



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Probable Reserves

Class 3 : Reserves representing the primaryrecovery dependent upon the development of

new reservoirs ( not yet produced or tested )within the area of assigned proved reserves.Class 3 reserves occur in a new reservoiroverlying or underlying a proved reservoir.



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Probable Reserves

Class 4 : Incremental reserves where analternate interpretation of actual oranticipated performance or volumetric dataindicates more reserves than can beclassified as proved or probable class1 to 3.

Class 5 : Additional quantities likely to berecoverable through the application,expansion or modification of improvedrecovery techniques.



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Possible Reserves

Possible reserves are less certain than probable reserves and can be estimated with a low degree ofcertainty.

Insufficient to indicate whether they are more likely tobe recovered than not under high-trend economicconditions.


Possible reserves have been divided to:

Class 1 : Reserves representing the primary recoveryfrom the delineated area of a known reservoir.



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Possible Reserves

Class 2 : Reserves representing the primaryrecovery which depends on:

a. Lateral extension of the reservoir beyond thelimits assumed for proved or probable class 1reserves due to up dip or down dip extensions.

b. Undrilled fault blocks adjacent to the

delineated area of a known reservoir. c. Direct or diagonal offsets to spacing units

having proved or probable class1 reserves



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Possible Reserves

Class 3 : Reserves representing the primaryrecovery dependent upon the development of

new reservoirs ( not yet produced or tested )within the area of assigned proved reserves.Class 3 reserves occur in a new reservoiroverlying or underlying a proved reservoir.



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Possible Reserves

Class 4 : Incremental reserves where analternate interpretation of actual or anticipated

performance or volumetric data indicates morereserves than can be classified as proved orprobable class1 to 3.

Class 5 : Additional quantities likely to berecoverable through the application, expansion ormodification of improved recovery techniques.



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Problems in Reserve Classification

Frontier Areas

1. No analogous reservoirs.

2. Sparse subsurface control.

3. Remote from market.

4. High operating costs.

Heavy and Extra Heavy Crude

Thermal stimulation is required and its response ishighly variable.

Dr.Helmy Sayyouh



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Possible Future Development in Reserve Classification

Matrix to describe geologic uncertainty andfeasibility of commercial extraction.

Inclusion with reserves of geologic andengineering bases for estimate

Quantification of probabilities associated withreserve classifications.

reserves estimation and classification

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