The world’s first 4-D surface gravity surveillance of a waterflood is being implemented at Prudhoe Bay, Alaska. This monitoring technique is an essential component of the surveillance program for the Gas Cap Waterflood Project. A major factor in the approval process for the waterflood was to show that water movement could be economically monitored, with a very limited number of wells penetrating the waterflood area. Conventional monitoring techniques would have been cost prohibitive since they would have required the drilling of numerous surveillance wells. Modeling studies indicate that density changes associated with water replacing gas can be readily detected using high-resolution surface gravity measurements. Modeling of the gravity data determines the mass distribution, mass balance and flood front location.
This paper will focus on how to perform an extremely accurate, high-resolution surface gravity survey in the Arctic. Details on survey methodology, meter testing and calibration will be covered. The use of gravity meters in combination with the NAVSTAR Global Positioning System (GPS) for location and elevation will be described. Field tests at Prudhoe Bay indicate that gravity survey accuracy of 5-10 µGal can be achieved using relative gravity meters in combination with GPS. Model results, using reservoir simulations, have indicated that gravity surveys with this accuracy should be able to account for greater than 95% of the injected water.