Initial Pressure (pi)

The initial pressure is the average reservoir pressure before the start of a test. For a new well (a well which has not been put on production), the initial pressure equals the virgin or original reservoir pressure, and corresponds to the drillstem test (DST) pressure. For a well that has been on production for a long time, the initial pressure may or may not be equal to the original reservoir pressure. The following example illustrates this concept.

In the above diagram, if Test 1 is being analyzed, the initial pressure is 2000. If Test 2 is being analyzed, the initial pressure is 1500, because the reservoir pressure is fully built up before this test, and the reservoir has forgotten its history prior to Test 2. Thus, it behaves like a new well with an initial pressure 1500. If Test 3 is being analyzed (which is a combination of Test 2 and some production prior to Test 2), then the initial pressure is 2000, because it must reflect the reservoir pressure before the production that is being analyzed during the test.