Capillary Pressure Curve

Test Your Knowledge

Quiz: Unlocking the Secrets of Reservoir Rocks: The Capillary Pressure Curve

Instructions: Choose the best answer for each question.

1. What is the primary focus of the capillary pressure curve in oil and gas exploration?

a) The pressure required to displace one fluid by another within a reservoir rock. b) The rate at which oil and gas flow through porous rock. c) The temperature and pressure conditions within the reservoir. d) The chemical composition of the oil and gas present.

2. What is the entry pressure on a capillary pressure curve?

a) The maximum pressure needed to displace the wetting phase. b) The pressure at which the non-wetting phase completely fills the pore space. c) The minimum pressure required for the non-wetting phase to enter a pore. d) The pressure at which the capillary pressure curve reaches its peak.

3. Which of the following is NOT a key application of the capillary pressure curve in oil and gas exploration?

a) Predicting oil and gas recovery rates. b) Evaluating the effectiveness of enhanced oil recovery (EOR) techniques. c) Determining the chemical composition of the reservoir fluids. d) Optimizing well placement strategies.

4. The phenomenon of hysteresis in a capillary pressure curve is caused by:

a) The changing temperature and pressure conditions within the reservoir. b) The presence of different types of minerals in the reservoir rock. c) The different pressures required to inject and withdraw the non-wetting phase. d) The interaction of oil and gas with the rock surface.

5. What is a common method for determining the capillary pressure curve?

a) Microscopy analysis of rock samples. b) Direct measurement of pressure within the reservoir. c) Mercury injection capillary pressure (MICP) technique. d) Chemical analysis of the reservoir fluids.

Exercise: Predicting Oil Recovery

Scenario:

You are working on an oil reservoir project. The capillary pressure curve for the reservoir rock has been determined and is shown below:

• Image of Capillary Pressure Curve: (Replace this with an actual image or diagram)

Task:

Using the capillary pressure curve, answer the following questions:

1. What is the approximate entry pressure for this reservoir rock?
2. What is the expected saturation of the non-wetting phase (oil) at a capillary pressure of 50 kPa?
3. If the reservoir is initially fully saturated with water, how much oil can be recovered by applying a pressure gradient of 80 kPa?
4. Based on the capillary pressure curve, how would you expect the recovery to differ if the reservoir rock had a higher permeability?

Exercise Correction:

Techniques

Unlocking the Secrets of Reservoir Rocks: The Capillary Pressure Curve

Chapter 1: Techniques for Measuring Capillary Pressure

The accurate determination of the capillary pressure curve is crucial for reservoir characterization and production forecasting. Several techniques exist, each with its strengths and limitations:

1.1 Mercury Injection Capillary Pressure (MICP): This is a widely used laboratory method. A dried rock sample is placed in a device, and mercury is injected under increasing pressure. The pressure at which mercury penetrates the pores is related to the pore throat size, allowing for the construction of a capillary pressure curve. Advantages include speed and the ability to measure high capillary pressures. However, it uses mercury, a hazardous material, and results may not directly translate to oil-water systems due to differences in wettability and interfacial tension. The method provides a drainage curve; imbibition is not directly measured.

1.2 Centrifuge Method: This technique uses centrifugal force to generate a pressure gradient across a rock sample saturated with two immiscible fluids (e.g., oil and water). By varying the rotational speed, different capillary pressures are achieved. The fluid saturation at each pressure is measured, yielding a capillary pressure curve. Advantages include the use of reservoir fluids, allowing for better representation of actual reservoir conditions. Limitations include challenges in accurately measuring fluid saturations at high capillary pressures and potential for sample disturbance. The method is often used for both drainage and imbibition cycles.

1.3 Porous Plate Method: A rock sample is placed in contact with a porous plate, creating a pressure difference across the sample, causing fluid flow between the sample and the plate. By controlling the pressure and measuring saturation, a capillary pressure curve can be constructed. This technique allows for both drainage and imbibition measurements and is relatively simple to perform. Limitations include difficulty in measuring very low capillary pressures and the potential for effects of the porous plate itself.

1.4 Drainage and Imbibition Experiments: These experiments directly measure the capillary pressure during fluid injection (drainage) and withdrawal (imbibition). Specialized equipment is used to control fluid flow and saturation, generating data points for the curve. While accurate, these methods can be time-consuming and require specialized equipment. They offer the ability to model hysteresis effects directly.

1.5 Nuclear Magnetic Resonance (NMR): This technique uses NMR signals to measure pore size distribution and fluid saturation within a rock sample. By analyzing the relaxation times of the fluids, it's possible to infer capillary pressure information. Advantages include non-destructive nature and the ability to measure capillary pressure in various reservoir conditions. However, the method may be less accurate than other techniques for determining detailed capillary pressure curves.

Chapter 2: Models for Capillary Pressure

Numerous empirical and theoretical models exist to represent capillary pressure curves. The choice of model depends on the specific application and data available.

2.1 Leverett J-function: This widely used approach assumes that the capillary pressure is a function of the water saturation and a dimensionless parameter related to the pore-size distribution and interfacial tension. It allows scaling capillary pressure data from one rock to another.

2.2 Brooks-Corey Model: This model uses a power-law relationship to describe the capillary pressure curve, with parameters representing the entry pressure and pore size distribution. It's relatively simple to implement and widely used in reservoir simulation.

2.3 van Genuchten Model: This model provides a more flexible representation of the capillary pressure curve, incorporating parameters that reflect the shape of the curve and its hysteresis. It often offers a better fit to experimental data than the Brooks-Corey model.

2.4 Burdine Model: Burdine's model uses the pore-size distribution obtained from mercury injection capillary pressure to calculate the relative permeability curves. It links the capillary pressure curve directly to the fluid flow characteristics.

Chapter 3: Software for Capillary Pressure Analysis

Several software packages are available for processing, modeling, and interpreting capillary pressure data.

3.1 Reservoir Simulation Software: Commercial reservoir simulators (e.g., Eclipse, CMG) often include modules for incorporating capillary pressure data and models. These packages allow integration with other reservoir properties for comprehensive fluid flow simulation.

3.2 Specialized Capillary Pressure Software: Some software packages are specifically designed for capillary pressure data analysis, providing tools for data fitting, model selection, and visualization.

3.3 Data Processing Software: General-purpose data processing software (e.g., MATLAB, Python with SciPy) can be used for data analysis and modeling of capillary pressure curves. Customized scripts can be developed to perform specific analyses and visualization.

3.4 Spreadsheet Software: Simple analyses, such as calculating entry pressure or fitting basic models, can be performed using spreadsheet software (e.g., Excel).

Chapter 4: Best Practices for Capillary Pressure Measurement and Analysis

Obtaining reliable capillary pressure data requires careful planning and execution. Best practices include:

• Representative Sample Selection: Choosing samples that accurately represent the reservoir heterogeneity.
• Proper Sample Preparation: Cleaning and drying the samples to minimize artifacts.
• Accurate Fluid Selection: Using fluids that accurately reflect reservoir conditions.
• Rigorous Experimental Procedures: Following established protocols to minimize experimental errors.
• Data Quality Control: Thorough review and validation of the experimental data.
• Appropriate Model Selection: Choosing a model that appropriately represents the data.
• Uncertainty Quantification: Estimating the uncertainty associated with the measured and modeled capillary pressure.

Chapter 5: Case Studies

5.1 Case Study 1: Impact of Wettability on Capillary Pressure in a Carbonate Reservoir: This case study demonstrates how wettability (water-wet vs. oil-wet) significantly influences the capillary pressure curve, impacting hydrocarbon recovery.

5.2 Case Study 2: Using Capillary Pressure Data to Optimize Waterflooding in a Sandstone Reservoir: This case study shows how capillary pressure data is used to predict waterflood performance and optimize injection strategies for enhanced oil recovery.

5.3 Case Study 3: Application of Capillary Pressure Data in Tight Gas Reservoir Characterization: This case study highlights the importance of capillary pressure data in understanding the complex fluid flow behavior in tight gas reservoirs and predicting gas production. The challenges posed by low permeability and high capillary pressures are discussed.

These case studies will detail specific applications and challenges encountered in various reservoir types. Detailed data and analysis techniques will be included to illustrate the practical application of the capillary pressure curve in reservoir engineering.