Reservoir Characterisation and Modelling Study Notes

Volumetrics and Uncertainty Analysis

Lecture Outline

•  Prospect Evaluation

• Volumetric Calculation

• Estimation of Resource Uncertainty

• Resource Reporting

Key Concepts in Prospect Evaluation

• Prospect Reserves: Defined by Size & Volume

• Chance of Success (COS): Probability of the prospect being viable

• Project Profitability: Related to economics of the project

Explorer Responsibilities

• Identifying prospects

• Measuring and estimating the following:

  • Reserves distribution, contingent on success

  • Confidence level reflecting chance of success

  • Profitability of ventures in terms of flow rate, price schedule, and economic considerations

Volumetric Analysis Methods

• Deterministic Methods:

  • STOIIP Calculation

  • Reserves definition

  • Ultimate Recovery

  • Net Oil Sands

  • Area Measurements

  • Gross Rock Volume calculations

• Probabilistic Methods:

  • Expectation Curve & Probability of Success

  • Uncertainty analysis

  • Monte Carlo simulations

  • Graph methods

Calculating Oil Volume

• STOIIP Calculation:

  • Deterministic: Single value estimation

    • Quick look estimation primarily yields a single value

    • Sparse data = using field average data

  • Probabilistic: A statistical range of values providing a distribution considering uncertainties.

Hydrocarbon Volumes Understanding

• Relationship defined as:

  • STOIIP/GIIP = Quick look estimation (single value)

• Factors affecting these calculations include:

  • Area or Size

  • Net/Gross ratio (N/G)

  • Porosity

  • Saturation (Oil/Gas)

  • Shrinkage or Expansion Factors

  • Recovery Factors (critical for reserves distinguishing)

Key Volume Calculations

• Bulk Rock Volume (GRV):
  $GRV = A imes H$
  where A = Area and H = Height

• Net Rock Volume:
  $Net Rock Volume = A imes H imes N/G$

• Bulk Pore Volume:
  $Bulk Pore Volume = A imes H imes N/G imes P$
  where P = Porosity

• Net Pore Volume:
  $Net Pore Volume = A imes H imes N/G imes P imes (1 - S<em>w)$ where Sw = Water Saturation

• STOIIP:
  $STOIIP (stb) = GRV imes N/G imes Porosity imes (1-S<em>w) imes rac{1}{B</em>o}$

  • B_o: Oil shrinkage factor or Formation volume factor

  • Ultimate Reserves (UR):
    $UR(stb) = GRV imes N/G imes Porosity imes (1-S<em>w) imes rac{1}{B</em>o} imes Rec ext{ Factor}$

  • Derives reserves using:
    $Reserves (stb) = UR - Cumulative Production$

Representational Methods

• Area-Depth method:

  • Used to calculate reservoir thickness across geographic regions.

• Area-Thickness Method:

  • Engaging graphical tools to visualize the model of volumetrics across a defined reservoir.

Volumetric Uncertainty Analysis

• Identified Uncertainties in volume determinations include:

  • Reservoir structure and petrophysical properties

  • Fluid properties and their distribution

  • Initial state of fluids under varying conditions

Quantifying Uncertainty

• Necessitated to enhance accuracy and manage risk includes identifying uncertainties within:

  • Area or Size

  • N/G

  • Porosity

  • Saturation (Oil/Gas)

  • Shrinkage or Expansion Factors

Estimation Challenges

• All estimations of reserves incorporate uncertainty:

  • Often made without concrete data leading to risk in accuracy.

Probability Density Function (PDF)

• Defined as: A function that describes the likelihood of continuous values falling within a particular range.

• Utilized in analyzing reserve uncertainties and distributions comprehensively.

Resource Classification and Reporting Guidelines

• Standard Terminology:

  • Proved (P1): Lowest Volume - Lowest Uncertainty

  • Probable (P2): Mid Volume - Mid Uncertainty

  • Possible (P3): Highest Volume - Highest Uncertainty

• PETRONAS Definitions for resource classification:

  • Total Petroleum Initially In-Place (TPIIP)

  • Discovered and Undiscovered Petroleum Initially In-Place Definitions

Unit Conversions in Volumetrics

• Standard conversions for volumetric calculations include:

  • 1 square m = 2.471 x 10⁻⁴ Acres

  • 1 square m = 10.76 square ft

  • 1 cubic m = 7758 barrels

  • 1 barrel = 159 liters

Concluding Notes

• Presentation Ending: Thank You

• Recognition of the Highest and Lowest uncertainty in estimations emphasizes the importance of probabilistic approaches in estimating hydrocarbon resources effectively.