Surface geophysical monitoring of gas fields: some experiences

Abstract

Gas production has some different reservoir technical challenges compared to oil production, due to higher mobility of the hydrocarbons. Wider spatial distribution of wells and less infill drilling make estimates of remaining reserves, and optimization of production rates and late field life, particularly important objectives for reservoir monitoring. Changing a water-flushed zone from initial to residual gas saturation may cause modest changes in seismic velocities, and more significant changes in densities. Changes in the seismic response may be modest, while mass changes may be detectable by gravimetric monitoring. Decreasing pore pressure causes slight velocity increase, sometimes too small to observe in time-lapse seismic data. Surface or seafloor subsidence may be included in the reservoir monitoring program.2D seismic and gravimetric monitoring has been selected as surface geophysical monitoring techniques for Troll gas production, for reasons of cost and quality After six years of production, a change in response at the gas–liquid contact can be observed both on seismic and gravimetric time-lapse data, and pressure depletion has caused a slight time-shift. On the Sleipner East Field, water rise has been mapped by combining 4D seismic and repeated well saturation-log data, and this information has been used in planning of new wells and in the further reservoir management. Geophysical monitor programs have further been initiated on the Midgard and Snøhvit gas fields. These are expected to provide important input to predictions of production behaviour, to enable timely actions in the reservoir management, and hence to increase the value of the asset.