Pressure Transient Analysis (PTA) is a sophisticated method for interpreting well and reservoir performance based on the analysis of pressure changes over time. Here’s a simplified overview of how a pressure transient analysis is typically conducted:
- Data Collection: The first step involves collecting pressure data from a well after a production or injection change. This can be initiated by a well shut-in, a change in production rate, or starting/stopping an injection. The pressure is measured at the wellbore, and the data collected over time reflects how the reservoir responds to the change.
- Pressure Buildup or Drawdown Test: Depending on the test, the well may be shut in after a period of production (pressure buildup test) or opened to produce at a measured rate (drawdown test). In a buildup test, the well is closed, and the pressure increase is recorded. In a drawdown test, the well is opened, and the decrease in pressure is observed.
- Data Analysis:
- Plotting: The collected pressure data and time are plotted on specific types of graphs, such as semilog plots or diagnostic plots, to identify trends and anomalies.
- Type Curves: These plots are compared against standard or generated type curves that represent theoretical reservoir responses, helping to match the observed data with known reservoir behaviors.
- Interpretation: Using various models and equations, such as the Horner plot method for buildup tests or the Agarwal-Gardner method for drawdown analysis, the data is interpreted to extract key reservoir characteristics such as:
- Permeability
- Skin factor (damage or enhancement near the wellbore)
- Reservoir boundary effects
- Boundary distances
- Reservoir pressure
- Software and Models: Specialized software and analytical/numerical models are often used to process the data, fit curves, and interpret the results. This allows for the estimation of reservoir properties and the assessment of well performance.
- Application of Results: The insights gained from PTA are used to make decisions on reservoir management, well completions, stimulation requirements, and production optimization.