Wettability

Wettability: The Key to Understanding Oil & Gas Reservoirs

In the oil & gas industry, understanding the wettability of a reservoir is crucial for efficient production and recovery. Wettability refers to the tendency of a fluid to spread or adhere to a solid surface. In simpler terms, it determines which fluid (oil, water, or gas) prefers to "stick" to the rock surface of a reservoir.

Why is wettability important?

• Reservoir Performance: Wettability directly impacts the flow of fluids in the reservoir. A water-wet reservoir, where water preferentially adheres to the rock, will have higher water saturation and potentially hinder oil production. Conversely, an oil-wet reservoir will have higher oil saturation and potentially lead to higher oil recovery.
• EOR (Enhanced Oil Recovery): Understanding wettability is critical for designing and implementing successful EOR strategies. Some EOR techniques, like chemical flooding, target specific wettability alterations to improve oil recovery.
• Reservoir Modeling: Accurate reservoir simulations require reliable information on wettability to predict fluid flow and production behavior.

Measuring Wettability

While determining the exact wettability of a reservoir formation is complex, various techniques are employed to assess the dominant wetting phase:

• Contact Angle Measurement: Analyzing the contact angle between a fluid droplet and the rock surface helps determine the preferred wetting phase.
• Amott-Harvey Test: This laboratory method measures the relative wettability of a core sample by comparing the amount of oil and water displaced by a specific fluid.
• Nuclear Magnetic Resonance (NMR): NMR can provide information about the pore size distribution and fluid saturation, which can be used to infer wettability.
• Scanning Electron Microscopy (SEM): SEM allows visualization of the rock surface and can reveal the distribution of oil and water, indicating the dominant wetting phase.

Factors Influencing Wettability

Several factors can influence the wettability of a reservoir formation:

• Rock Composition: The type and composition of the rock surface play a significant role. For example, clay minerals can promote water-wet conditions, while organic matter can lead to oil-wetting.
• Fluid Composition: The presence of surfactants, salts, or other components in the fluids can alter the wettability of the rock.
• Reservoir Temperature and Pressure: These parameters can influence the interfacial tension between fluids and rock, affecting wettability.
• Reservoir History: The flow of fluids over time can change the wettability of the reservoir, especially during production.

Challenges in Wettability Determination

• Heterogeneity: Reservoirs are often heterogeneous, with different wettability characteristics in different zones. Determining average wettability can be challenging.
• Dynamic Nature: Wettability can change over time due to fluid flow, temperature variations, and other factors.
• Limited Sampling: It is not always possible to obtain representative samples of the entire reservoir, making it difficult to assess the overall wettability.

Conclusion

Wettability is a crucial parameter in oil & gas reservoir characterization and production. Understanding the dominant wetting phase allows for better reservoir modeling, enhanced oil recovery strategies, and improved production forecasts. Further research and advancements in measurement techniques are essential to better characterize wettability and optimize reservoir management for efficient and sustainable oil and gas production.