Original Gas in Place

Test Your Knowledge

Quiz: Understanding Original Gas in Place (OGIP)

Instructions: Choose the best answer for each question.

1. What does OGIP stand for? a) Original Gas In Place b) Oil Gas In Production c) Oil and Gas Industry Partners d) Original Gas Industry Production

2. Which of these is NOT a factor influencing recoverable reserves? a) Reservoir permeability b) Production technology c) The color of the reservoir rock d) Economic factors

3. OGIP represents: a) The total volume of gas that can be extracted from a reservoir. b) The amount of gas that is currently being produced. c) The total volume of gas originally present in a reservoir at standard conditions. d) The maximum amount of gas that can be extracted from a reservoir using current technology.

4. Why is OGIP an important concept in oil and gas exploration? a) It helps determine the best location to build a gas station. b) It helps estimate the potential profits from a project. c) It helps determine the best type of gas to extract. d) It helps determine the best time to start drilling.

5. Which of these is a step involved in estimating OGIP? a) Determining the reservoir's porosity. b) Analyzing the gas's flavor. c) Determining the reservoir's aesthetic appeal. d) Analyzing the gas's ability to conduct electricity.

Exercise: OGIP Calculation

Scenario:

You are an exploration geologist working on a new gas field. You have gathered the following information:

• Reservoir volume: 10 million cubic meters
• Porosity: 20%
• Gas Saturation: 75%
• Gas Specific Gravity: 0.6
• Reservoir Temperature: 100°C
• Reservoir Pressure: 300 bar

Task:

Using the information provided, estimate the OGIP of this gas field.

Assumptions:

• Standard conditions are 15°C and 1 atm.
• Use the following formula: OGIP = (Reservoir Volume x Porosity x Gas Saturation x Gas Specific Gravity x Reservoir Pressure) / (Standard Pressure x (1 + (Reservoir Temperature - Standard Temperature) x Gas Expansion Coefficient))

Note: You will need to find the gas expansion coefficient for the specific gas. You can research this online or use a reference book.

* Convert reservoir pressure to atm: 300 bar * 1 atm / 1.01325 bar = 296.07 atm
* Convert reservoir temperature to Kelvin: 100°C + 273.15 = 373.15 K
* Convert standard temperature to Kelvin: 15°C + 273.15 = 288.15 K
* Assume the gas expansion coefficient is 0.0035/K (This is a typical value for natural gas, but you should always consult specific data for the gas in question).

* OGIP = (10,000,000 m³ x 0.2 x 0.75 x 0.6 x 296.07 atm) / (1 atm x (1 + (373.15 K - 288.15 K) x 0.0035/K)) 
* OGIP ≈ 3,280,000,000 m³ of gas at standard conditions.

**Therefore, the estimated OGIP of this gas field is approximately 3,280,000,000 cubic meters of gas at standard conditions.**

Techniques

Understanding Original Gas in Place: A Crucial Metric in Oil & Gas Exploration

This document expands on the concept of Original Gas in Place (OGIP), breaking down the topic into key areas.

Chapter 1: Techniques for Estimating Original Gas in Place (OGIP)

Estimating OGIP relies on a combination of geological interpretation and engineering calculations. Several key techniques are employed:

1. Volumetric Method: This is the most common method, particularly for simpler reservoir geometries. It involves:

• Defining Reservoir Geometry: Using seismic data, well logs, and geological mapping to determine the reservoir's volume (area x thickness).
• Determining Porosity: Analyzing core samples and well logs to measure the pore space within the reservoir rock. Various techniques such as sonic logs, density logs, and neutron logs are used.
• Measuring Gas Saturation: Well logs are used to determine the fraction of the pore space filled with gas. This may involve using techniques like NMR logging or capillary pressure measurements.
• Calculating Gas in Place: The formula used is: OGIP = (Volume)(Porosity)(Gas Saturation)(Gas Formation Volume Factor) The gas formation volume factor accounts for the gas expansion from reservoir conditions to standard conditions.

2. Material Balance Method: This approach uses pressure and production data over time to estimate OGIP. It is particularly useful for mature fields with substantial production history. The method involves:

• Monitoring Reservoir Pressure: Continuous monitoring of reservoir pressure decline is essential.
• Tracking Cumulative Production: Accurate records of gas production are crucial.
• Applying Material Balance Equation: A mathematical model relates pressure decline, cumulative production, and initial gas in place. The equation's complexity depends on the reservoir's characteristics (e.g., presence of water or condensate).

3. Decline Curve Analysis: This method predicts future production based on historical production data. While not directly estimating OGIP, it can provide an indirect estimate when combined with other techniques. The analysis helps determine the reservoir's depletion characteristics and extrapolate to estimate the initial gas volume.

4. Reservoir Simulation: This sophisticated technique uses numerical models to simulate fluid flow and pressure behavior within the reservoir. It incorporates various reservoir properties and allows for modeling different production scenarios to estimate OGIP and predict future performance. It's computationally intensive and requires detailed reservoir data.

Chapter 2: Models Used in OGIP Estimation

Several models are employed to estimate OGIP, each with its strengths and weaknesses:

1. Volumetric Models: These are relatively simple models based on the geometric volume of the reservoir. They are suitable for simple reservoirs with homogeneous properties. Limitations include the difficulty in handling complex reservoir geometries and heterogeneities.

2. Material Balance Models: These models account for changes in reservoir pressure and cumulative production. They are more complex than volumetric models but provide a better representation of reservoir behavior. However, they require accurate and comprehensive pressure and production data.

3. Decline Curve Models: Empirical models that describe the decline in production rates over time. They are useful for forecasting production, but OGIP estimation is an indirect outcome.

4. Reservoir Simulation Models: These are the most comprehensive models, using numerical methods to simulate the fluid flow and pressure distribution within the reservoir. They can handle complex reservoir geometries, heterogeneities, and fluid properties. However, they require extensive data and computational resources.

5. Analytical Models: These simplified models provide quick estimates, often based on specific assumptions about reservoir geometry and properties. They are less accurate than numerical models but offer faster solutions for preliminary assessments.

Chapter 3: Software for OGIP Estimation

Various software packages are available to assist in OGIP calculations, ranging from simple spreadsheets to sophisticated reservoir simulators:

• Spreadsheet Software (e.g., Excel): Can be used for basic volumetric calculations, but lack the capabilities of specialized software.
• Specialized Reservoir Engineering Software: Packages like Petrel, Eclipse, CMG, and others provide advanced functionalities for data management, geological modeling, reservoir simulation, and OGIP estimation. These often incorporate sophisticated algorithms for material balance calculations and decline curve analysis.
• Geostatistical Software: Software like GSLIB or Leapfrog Geo are used for spatial analysis and modeling of reservoir properties, providing crucial input for OGIP estimation.
• Python Libraries: Libraries such as pandas, NumPy, and SciPy can be used for data manipulation and analysis, providing customized tools for specific OGIP calculation needs.

Chapter 4: Best Practices in OGIP Estimation

Accurate OGIP estimation requires careful consideration of several factors:

• Data Quality: High-quality data from various sources (seismic surveys, well logs, core analysis) is crucial for accurate results. Data validation and quality control are paramount.
• Geological Understanding: A thorough understanding of the reservoir's geological setting, including its stratigraphy, structure, and depositional environment, is essential for accurate reservoir modeling.
• Appropriate Model Selection: Choosing the appropriate model (volumetric, material balance, decline curve, simulation) depends on the reservoir characteristics, data availability, and the level of accuracy required.
• Uncertainty Analysis: OGIP estimates always contain uncertainties due to data limitations and model assumptions. Conducting a comprehensive uncertainty analysis is vital to understand the range of possible OGIP values.
• Peer Review: Having the OGIP estimation reviewed by independent experts is a critical step to ensure accuracy and reliability.
• Transparency and Documentation: Maintaining clear documentation of the methods, assumptions, and data used in the estimation process is crucial for traceability and reproducibility.

Chapter 5: Case Studies in OGIP Estimation

Several case studies illustrate the application of different OGIP estimation techniques:

(Note: Specific case studies would require detailed information about particular oil and gas fields which is beyond the scope of this response. However, a case study would typically detail the following):

• Reservoir Description: Geological setting, reservoir characteristics (porosity, permeability, etc.), and fluid properties.
• Data Acquisition and Analysis: Methods used to acquire and analyze data (seismic surveys, well logs, core analysis, production data).
• OGIP Estimation Method: The chosen method (volumetric, material balance, etc.) and the rationale for its selection.
• Results and Uncertainty Analysis: The calculated OGIP value, its associated uncertainty, and a discussion of potential sources of error.
• Comparison with Other Estimates: Comparison of the obtained OGIP value with other independent estimates, if available.
• Lessons Learned: Insights gained from the experience, including potential improvements for future estimations.

This framework provides a comprehensive overview of OGIP estimation. Remember that each project requires a tailored approach based on its specific geological and engineering characteristics.