An innovation in the methodology of conducting drillstem tests (DSTs) in tight gas reservoirs is presented, along with a simplification in the interpretation of the data obtained. DSTs in tight gas reservoirs are a problem because the flow rates are often too low to be measured by conventional equipment. In these cases a normal flowing and buildup test should be followed by a closed chamber test to produce usable estimates of flow rates at various times during the preceding normal flowing and buildup test. This estimated flow rate can then be used to interpret the buildup after the flow testing, which is produced by shutting the downhole valve.

In the interpretation of DST data from a tight gas reservoir, factors such as the following have to be considered: 1) large variation of gas properties resulting from the large pressure range involved1,2 ; 2) flow rate duration that can be many times shorter than the buildup period3,4,5,6 ; 3) varying flow rate during the flowing part of the test; and 4) impact of boundaries and heterogeneities.

We show that a simple interpretation approach with constant flow rates and pseudo-pressure yields results that are within the intrinsic accuracy limits expected from such a test.

The benefits to the field interpreter of using the methods presented are 1) operational flexibility: if the well flows strongly enough the rate may be measured at surface; if it does not, a closed chamber test can be added; 2) the benefit of the deeper radius of investigation of a test flowing at surface is retained; 3) field interpretation is simpler whether surface measurement is possible or not.

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