reserves estimation and classification · 2021. 1. 24. · reserves are estimated quantities of...

Reserves Estimation and Classification

Dr. Helmy Sayyouh

Petroleum Engineering

Cairo University

Introduction

Field reservoir engineer is responsible for the estimation and classification of reserves.

Estimation techniques:

material balance calculations

sweep efficiency analysis

decline curve analysis

reservoir simulation

All reserves estimates involve some degree of uncertainty.

12/28/2017 DR.HELMY SAYYOUH

Why Reserve Estimates?

Measure effectiveness of exploration and development.

Budgeting for drilling and facilities.

Unitization.

Purchase / sale of properties.

Bank loans.

Taxation.

Government policy and planning.


Definition

Reserves are estimated quantities of crude oil, condensate, natural gas, natural gas liquids, and associated 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.


Original Oil and Gas In place Calculation

Volumetric

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


Recovery and Efficiency Calculations

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

Recover efficiencies: fractional recovery of OOIP or OGIP

Methods of estimation:

- analogies

- correlations

- water flood design charts

- material balance programs

- reservoir simulation


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 differences between


Correlations

Depletion Drive Gas Reservoirs:

Rec. Efficiency =1 - Pa Zi / Pi Za

Water Drive Gas Reservoir:

Er=(1-PaZi/PiZa) + ((PaZi/PiZa) EvEd)

Where: Ed=(1-Swi-Sgr)/(1-Swi)

Sgr= 0.62 -1.3 Φ


Water floods

Mobility Ratio

M = (Krw/μw) x (Kro/μo)

Recovery Efficiency

Er = Ea x (Swb – Swi)/(1-Swi)

Areal sweep efficiency, Ea, from a homogenous 5-spot water flood for a certain M can be obtained from charts.


Ultimate Recovery

Volumetric

Ultimate Recovery = Er x OOIP or OGIP( from volumetric)

Material Balance

Ultimate Recovery = Er x OOIP or OGIP( from material balance)


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.


Reservoir Simulation

Reservoir simulation incorporates a comprehensive application of physical laws governing multiphase fluid flow in porous media.

Reservoir simulation can be summarized in three-step process:

1. Setting up mathematical equations that describe fluid flow.

2. Solving the Mathematical equations.

3. Setting up the numerical model.


Reserves Classification

Reserves are estimated quantities of crude oil, condensate, natural gas, natural gas liquids, and associated 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


Reserves Classification

All reserves estimates involve some degree of uncertainty, depending on :

The amount and reliability of geologic and engineering data available at the time of the estimates.

Interpretation of these data


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)


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 such as sulfur and carbon dioxide.

3. Based on interpretation of geologic and engineering data.

4. Commercially recoverable under economic, operating and regulating condition.

5. Time dependent ( production ).

6. Involve degree of uncertainty.

7. Subject to revision.


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


Proved Reserves

Proved oil and gas reserves are the estimated quantities of crude oil, natural gas, and natural gas liquids which can be recoverable:

- in future years

- from known reservoirs

- under exiting economic and operating conditions.

A confidence level of 90 to 100% is required.


Proved Reserves

Proved reserves must have:

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

- the adjoining portions not yet drilled but economically productive

The facilities to process .


Proved Reserves

Proved reserves have been divided into:

Proved Developed Reserves: are expected to be recovered through exiting wells with exiting equipment and operating methods.

Proved Undeveloped Reserves: are expected to be recovered from new wells on undrilled acreage or from exiting wells where a major expenditure is require for recompletion.


Probable Reserves

Probable reserves are less certain than proved reserves and they are more likely to be recovered than not under mid-trend economic conditions.


Probable reserves have been divided to:

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


Probable Reserves

Class 2: Reserves representing the primary recovery which depends on:

a. Lateral extension of the reservoir beyond the limits assumed for proved or probable class 1 reserves 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.


Probable Reserves

Class 3 : Reserves representing the primary recovery 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 reservoir overlying or underlying a proved reservoir.


Probable Reserves

Class 4 : Incremental reserves where an alternate interpretation of actual or anticipated performance or volumetric data indicates more reserves than can be classified as proved or probable class1 to 3.

Class 5 : Additional quantities likely to be recoverable through the application, expansion or modification of improved recovery techniques.


Possible Reserves

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

Insufficient to indicate whether they are more likely to be recovered than not under high-trend economic conditions.


Possible reserves have been divided to:

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


Possible Reserves

Class 2: Reserves representing the primary recovery which depends on:

a. Lateral extension of the reservoir beyond the limits assumed for proved or probable class 1 reserves 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


Possible Reserves

Class 3 : Reserves representing the primary recovery 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 reservoir overlying or underlying a proved reservoir.


Possible Reserves

Class 4 : Incremental reserves where an alternate interpretation of actual or anticipated performance or volumetric data indicates more reserves than can be classified as proved or probable class1 to 3.

Class 5 : Additional quantities likely to be recoverable through the application, expansion or modification of improved recovery techniques.


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 is highly variable.


Possible Future Development in Reserve Classification

Matrix to describe geologic uncertainty and feasibility of commercial extraction.

Inclusion with reserves of geologic and engineering bases for estimate

Quantification of probabilities associated with reserve classifications.


reserves estimation and classification · 2021. 1. 24. · reserves are estimated quantities of...

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