An innovation in the methodology of conducting drillstem tests (DSTs) in tightgas reservoirs is presented, along with a simplification in the interpretationof the data obtained. DSTs in tight gas reservoirs are a problem because theflow rates are often too low to be measured by conventional equipment. In thesecases a normal flowing and buildup test should be followed by a closed chambertest to produce usable estimates of flow rates at various times during thepreceding normal flowing and buildup test. This estimated flow rate can then beused to interpret the buildup after the flow testing, which is produced byshutting the downhole valve.
In the interpretation of DST data from a tight gas reservoir, factors suchas the following have to be considered:
large variation of gas propertiesresulting from the large pressure range involved;
flow rate duration thatcan be many times shorter than the buildup period;
varying flow rate duringthe flowing part of the test; and
impact of boundaries andheterogeneities.
We show that a simple interpretation approach with constant flow rates andpseudo-pressure yields results that are within the intrinsic accuracy limitsexpected 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 bemeasured at surface; if it does not, a closed chamber test can be added; 2) thebenefit of the deeper radius of investigation of a test flowing at surface isretained; 3) field interpretation is simpler whether surface measurement ispossible or not.
The two major challenges that face the interpreter in tight gas DSTs are thedifficulty in measuring rate, and the selection of interpretation tools andtechniques.