A Drillstem Test (DST) involves a series of measurements and calculations to assess the properties of a formation. The primary focus is on determining the formation pressure, permeability, and the well’s potential productivity. Here’s a simplified example of how these calculations might be done:
- Formation Pressure: This is determined by measuring the pressure at the point of interest in the well. For example, if the gauge reads 5000 psi at a certain depth, and the hydrostatic pressure (due to the column of drilling fluid) is 3000 psi, the formation pressure would be the difference: 5000 psi – 3000 psi = 2000 psi.
- Permeability: This is a bit more complex to calculate directly from DST data, but it involves analyzing the rate at which fluids can flow into the wellbore. If, for instance, you observe a flow rate of 100 barrels per hour under a certain pressure differential, this data, along with the knowledge of the formation’s characteristics, can be used to estimate permeability.
- Productive Capacity: This is estimated by analyzing the flow rates of oil, gas, or water during the test. If the well flows 500 barrels of oil per day under the formation pressure without any artificial lift, this gives an initial estimate of the well’s productive capacity.
It’s important to note that these calculations are often more complex in practice, involving various factors like fluid properties, wellbore conditions, and formation characteristics. Advanced analysis might require specialized software and a deeper understanding of reservoir engineering principles.