i read a paper about the rate measurement in tight gas reservoir by Agha hassan ikram. in this paper he has proposed another flow period of 2-4 hrs after main flow and then shutting the well from surface. he explains that in this way there is no water column in the tubing and it is totally filled with gas. and then he conducts calculation for rate measurement and compressibilty of gas.

Can any one explain that by introducing another flow period how it can be concluded that there is no water column in the tubing and totally gas filled???

here is the link to the paper:

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