The present invention is directed toward a method and an apparatus to investigate and quantify a leakage rate for a fluid between a first pipe and a second pipe, the first pipe being surrounded by at least a portion of the second pipe, where the pipes are arranged in a well in a ground and where a measuring arrangement including a flow meter and a pressure meter is put into fluid communication with an annulus defined by the first pipe and the second pipe, where fluid in the gaseous phase is conveyed through the measuring arrangement, as the annulus is used as a separation chamber for gas and liquid.