Abstract The Prosperidade field offshore Mozambique is a large deep water gas field with three main fan reservoir complexes. The field extent covers an area about fifty kilometers long and ten kilometers wide. As part of the field appraisal and development process, an extensive Drill Stem Test (DST) and Multi-Well Interference program, involving several strategically placed wells, was conducted to assist in evaluating reservoir connectivity and original gas in place. This paper presents the interference test program, First, the objectives, design methodology and criteria are discussed. Next, we present the test conduct and the methods used to retrieve interference pressure data at observation wells over time. We also discuss the challenges associated with measuring small pressure changes (< 0.5 psi) in the presence of tidal effects as well as the methods we have used to handle noise and gauge pressure drift. Finally, we present the methods used to evaluate reservoir connectivity and gas in place. In analyzing the interference pressure data, we have used both analytical methods and numerical reservoir simulation. More importantly, we have used a new material balance technique to directly calculate the original gas in place. This method uses rock and fluid data and the small changes in pressure at the observation wells. Therefore, errors and uncertainties in the input or measured data will affect the results. To account for this, the Monte Carlo simulation approach was used to provide statistics on the magnitude and range of the calculated original gas in place. The results of the work have helped us understand the connectivity of the various fan reservoirs over several kilometers. This has been an important contribution for both field development and reserve certification