Water-oil ratio (WOR) analysis is an empirical method used in the petroleum industry to forecast future oil production performance by analyzing the historical relationship between water and oil production rates. It relies on observed trends rather than formal models or equations.
Understanding Water-Oil Ratio (WOR)
The Water-Oil Ratio (WOR) is a straightforward calculation:
- WOR = Volume of Water Produced / Volume of Oil Produced
This ratio provides insights into the reservoir's depletion stage and water breakthrough. As a reservoir matures, water production typically increases while oil production declines, leading to a higher WOR.
Forecasting with WOR Analysis
WOR analysis uses plots of WOR, and often WOR+1, against cumulative oil production to extrapolate future trends. These plots are usually semi-logarithmic, with cumulative oil production on the x-axis (logarithmic scale) and WOR on the y-axis (linear scale).
Here's a breakdown of how it works:
- Data Collection: Historical data on water and oil production rates and cumulative oil production are gathered.
- Ratio Calculation: The WOR is calculated for each data point (e.g., monthly or annual).
- Plotting: The WOR (and often WOR+1) is plotted against cumulative oil production on a semi-log graph. The x-axis represents cumulative oil production, usually on a logarithmic scale.
- Trend Identification: The analyst looks for trends in the WOR plot. Often, a linear trend emerges on the semi-log plot, especially after water breakthrough.
- Extrapolation: The identified trend is extrapolated into the future to predict future WOR values at different levels of cumulative oil production.
- Production Forecasting: The predicted WOR values are then used to forecast future oil production rates. Knowing the expected WOR and estimating the total fluid production rate, future oil and water production can be estimated.
Key Considerations and Limitations
- Empirical Nature: WOR analysis is empirical. It is based on historical trends, not fundamental reservoir engineering principles. Therefore, it is sensitive to changes in operating conditions or reservoir behavior.
- Water Breakthrough: The WOR analysis is most reliable after water breakthrough has occurred in most wells. Prior to breakthrough, the data is limited and the trends are less defined.
- Changes in Operating Conditions: Changes in well management (e.g., changes in pumping rates, well shut-ins, infill drilling) can significantly impact the WOR and invalidate the extrapolated trends.
- Reservoir Heterogeneity: WOR analysis assumes a relatively homogeneous reservoir. In highly heterogeneous reservoirs, the WOR trends may be more complex and difficult to interpret.
- No Physical Model: Lacking a formal model, the analysis cannot handle scenarios that deviate considerably from historical behavior.
Benefits of WOR Analysis
Despite its limitations, WOR analysis offers several benefits:
- Simplicity: It is a relatively simple and quick method.
- Data-Driven: It relies on readily available production data.
- Early Indication: It can provide an early indication of future production decline.
- Cost-Effective: It does not require sophisticated software or extensive reservoir simulations.
Example
Imagine a well has produced 100,000 barrels of oil and 10,000 barrels of water. The WOR is 10,000/100,000 = 0.1. Later, the well produces 10,000 barrels of oil and 50,000 barrels of water. The WOR is now 50,000/10,000 = 5. Plotting these WOR values against the cumulative oil production (100,000 and 110,000 barrels respectively) on a semi-log plot would allow for extrapolation and forecasting.
WOR analysis is a valuable tool for quickly assessing production trends and forecasting future oil production, especially when combined with other reservoir engineering techniques and geological information.
