Surface geophysicalmonitoring ofgas fields: someexperiences

Abstract

Gasproduction hassomedifferent reservoirtechnical challenges compared to oilproduction,due to highermobility of the hydrocarbons. Widerspatial distribution of wells and less infill drilling make estimatesof remaining reserves,and optimization of production rates and latefield life, particularly important objectives for reservoirmonitoring. Changinga water-flushed zonefrom initialtoresidual gassaturation may cause modest changes inseismic velocities,and more significant changes in densities. Changes inthe seismic response may be modest,whilemass changesmay be detectableby gravimetric monitoring. Decreasing pore pressurecausesslight velocity increase, sometimestoo small to observeintime-lapse seismic data. Surface or seafloor subsidence may be included inthe reservoirmonitoring program. 2D seismic andgravimetric monitoringhas beenselected assurface geophysicalmonitoring techniques for Troll gasproduction,for reasons ofcost and quality Aftersixyears of production,a change inresponseatthe gas–liquid contact can be observed both on seismic andgravimetric time-lapsedata, and pressuredepletion has caused a slighttime-shift. Onthe Sleipner East Field, waterrisehas beenmapped by combining4D seismic and repeated well saturation-logdata, and this information has beenused inplanning of newwells andinthe further reservoirmanagement. Geophysicalmonitor programs havefurther been initiated on the MidgardandSnøhvit gas fields. Theseareexpected to provide important input to predictions of production behaviour, to enable timely actions inthe reservoirmanagement,andhence to increase the value of the asset.