Contact

Test Your Knowledge

Quiz: Contact in Oil & Gas Exploration

Instructions: Choose the best answer for each question.

1. What does "contact" refer to in the context of oil and gas exploration?

a) The point where a well intersects a reservoir b) The depth of the interface between different fluid phases in a reservoir c) The pressure gradient within a reservoir d) The amount of hydrocarbons contained in a reservoir

2. Which of the following is NOT a type of contact commonly found in oil and gas reservoirs?

a) Oil-Water Contact (OWC) b) Gas-Oil Contact (GOC) c) Gas-Water Contact (GWC) d) Water-Sand Contact (WSC)

3. What is the primary tool used to determine contact depths in a well?

a) Seismic surveys b) Core analysis c) Well logging d) Satellite imagery

4. Why is understanding contact depths important for reservoir characterization?

a) It helps determine the amount of oil that can be extracted. b) It helps understand the distribution of oil, gas, and water in the reservoir. c) It helps identify the type of rock that makes up the reservoir. d) It helps estimate the age of the reservoir.

5. Which of the following is NOT a challenge associated with determining contact depths?

a) Dynamic nature of contacts over time b) Geological complexity of the reservoir c) Availability of accurate data d) The presence of faults and fractures

Exercise: Contact Depth Interpretation

Scenario: A well has been drilled, and the following well log data has been collected:

| Depth (m) | Gamma Ray (API units) | Resistivity (ohm-m) | Density (g/cm³) | |---|---|---|---| | 2000 | 60 | 2 | 2.4 | | 2050 | 75 | 5 | 2.3 | | 2100 | 90 | 10 | 2.2 | | 2150 | 100 | 20 | 2.1 | | 2200 | 95 | 15 | 2.0 | | 2250 | 80 | 10 | 2.1 | | 2300 | 65 | 5 | 2.2 |

Instructions:

1. Based on the well log data, identify the likely locations of the Oil-Water Contact (OWC) and Gas-Oil Contact (GOC).
2. Explain your reasoning for choosing those depths.

Techniques

Contact in Oil & Gas Exploration: A Comprehensive Guide

Chapter 1: Techniques for Contact Determination

Determining the location of oil-water contacts (OWC), gas-oil contacts (GOC), and gas-water contacts (GWC) is crucial in oil and gas exploration. Several techniques, often used in combination, provide the necessary data:

1.1 Well Logging: This is a primary method. Sensors lowered into boreholes measure various parameters, including:

• Resistivity: Measures the ability of the formation to conduct electricity. Hydrocarbons are resistive, while water is conductive. Changes in resistivity indicate fluid boundaries.
• Gamma Ray: Measures natural radioactivity. Shales typically exhibit higher gamma ray readings than hydrocarbon-bearing sands. This helps correlate lithology and fluid contacts.
• Density and Neutron Logs: These logs measure the bulk density and hydrogen index of the formation. The differences in density and hydrogen index between oil, gas, and water help identify fluid contacts.
• Sonic Logs: Measure the speed of sound through the formation. The velocity varies based on the type of fluid present.

1.2 Seismic Surveys: These provide a broader view of subsurface geology. Seismic waves are reflected and refracted at interfaces between different rock layers and fluids. Analyzing these reflections helps identify potential contacts, although the resolution might not be as precise as well logging. Different seismic techniques include:

• 2D Seismic: Provides a two-dimensional image of the subsurface.
• 3D Seismic: Provides a three-dimensional image, offering better resolution and detail.
• 4D Seismic: Incorporates time-lapse seismic data to monitor changes in reservoir properties over time, including shifts in fluid contacts.

1.3 Core Analysis: This involves retrieving physical samples of the subsurface formations (cores) from wells. Laboratory analysis of these cores provides direct information on fluid saturation, porosity, and permeability. This is a highly accurate method for determining contacts in the immediate vicinity of the wellbore but is expensive and not feasible for widespread use.

1.4 Pressure Testing: This involves measuring pressure in the wellbore at different depths. The pressure gradients can be indicative of the presence and location of different fluids.

Chapter 2: Models for Contact Representation

Understanding contact geometry is crucial for reservoir modelling. Several models are employed to represent the complexity of these interfaces:

2.1 Planar Contacts: The simplest model, assuming a horizontal or slightly dipping plane. This is often a first-order approximation.

2.2 Complex Structural Models: These models account for faulting, folding, and other geological features that influence contact geometry. They are built using geological interpretation and seismic data.

2.3 Dynamic Models: These incorporate changes in reservoir pressure and fluid saturation over time, leading to the movement of fluid contacts due to production or injection.

2.4 Stochastic Models: These incorporate uncertainty into the representation of contact geometry using statistical methods, accounting for the inherent variability in subsurface geology. They are particularly useful when data is sparse.

Chapter 3: Software for Contact Analysis

Various software packages are used for processing and interpreting data related to contact determination:

3.1 Well Log Interpretation Software: These packages process well log data, helping identify fluid contacts based on different log responses. Examples include Petrel, Kingdom, and Schlumberger's Petrel.

3.2 Seismic Interpretation Software: These packages are used to process and interpret seismic data, allowing for the identification of potential contacts based on seismic reflections. Examples include Petrel, Kingdom, and SeisSpace.

3.3 Reservoir Simulation Software: These packages use the determined contact information to simulate fluid flow in the reservoir and predict future reservoir performance. Examples include CMG, Eclipse, and Reservoir Simulation.

3.4 Geological Modelling Software: These software packages assist in creating 3D geological models of the subsurface, integrating various data sources including well logs and seismic data to refine contact geometry. Examples include Gocad, Petrel, and Leapfrog Geo.

Chapter 4: Best Practices for Contact Determination

Accurate determination of fluid contacts requires careful planning and execution:

• Data Integration: Combine data from multiple sources (well logs, seismic, cores) for a more robust understanding.
• Quality Control: Ensure the accuracy and reliability of the data through rigorous quality control measures.
• Geological Expertise: Incorporate geological understanding to interpret the data and constrain the models.
• Uncertainty Quantification: Acknowledge and quantify uncertainties associated with contact determination.
• Regular Updates: Monitor fluid contacts over time using dynamic data and update models accordingly.

Chapter 5: Case Studies

This section would include specific examples of successful (and unsuccessful) contact determination projects in different geological settings. Detailed case studies would demonstrate the application of the techniques and models described above, highlighting the challenges encountered and the solutions implemented. Examples could include:

• A case study of successful OWC determination in a clastic reservoir using a combination of well logs and seismic data.
• A case study of a complex reservoir with faulting and fracturing where advanced seismic interpretation techniques were required.
• A case study demonstrating the impact of dynamic reservoir modelling on production optimization.
• A case study comparing the results of different contact determination methods and highlighting the advantages and disadvantages of each.