Drawdown Testing

Test Objectives

The objectives of a drawdown test are to determine skin, perm and the distances to the reservoir's boundaries. We recommend doing a drawdown test to look for reservoir limits instead of running a build-up because a flowing well test does not interrupt cash flow. Since you can't see a boundary until the pressure wave hits it, there's no way to tell how long a build-up is required to see the reservoir limits. On most tests, it is possible to perform an "Integrated Volume Explored" calculation as the test progresses. This allows for a test to be run until a given amount of reserves have been proven.

Procedure

Install the SPIDR on a well that has been shut-in and stable. "Stable" is defined as the shut-in well head pressure changing at a rate of less than 1 psi per hour. Check for leaks in the system after installation. The SPIDR must be recording for at least 15 minutes prior to opening the well (check the box for the SPIDR wake-up time). Begin flowing the well on a single choke size. If the well must be "stepped-up", try to get the well up to full rate within 30 minutes. Continue flowing on a constant choke size for the duration of the test. If shut-ins or flow interruptions occur during the course of the drawdown, try to get the well back on-line as soon as possible on the same choke size.

Advantages

The main benefit of running a drawdown is that cash flow is not interrupted. Another advantage is that reservoir boundaries are easier to locate, relative to build-up tests.

Disadvantages

The drawback to running a drawdown is that the rate may not be constant. However, changing the choke periodically to maintain a constant rate will cause more problems than letting the rate fluctuate. In order to get accurate analysis on a drawdown, it is critical that no choke changes occur during the test.