Irina G.

I assume the tool used is Q3D. To answer the last question why "removing the gnd nets doesn't affect the result": Q3D calculates partial inducatnce; if Ground nets do not have Sources/Sinks defined, they don't affect inductance values and return path is not considered. 

The question "how to extract the series inductance of an open (capacitive) structure" looks strange to begin with. Please, clarify how would you use the value.