Gravimetric monitoring of gas production from the Troll field

Abstract

Three surveys of relative gravity and depth measurements have been carried out over the Troll field during gas production. Due to several improvements in the methodology, precision (intra‐survey repeatability measured as standard deviation) has improved from 26 μGal to 11 μGal and 4 μGal in the latest survey. This will improve the reliability of future monitoring, and also opens the range of applications to smaller and deeper fields. There is no clear subsidence of the seafloor. The estimated average depth change above the reservoir is 0.5 to 0.8 cm, but this is not statistically significant. Time‐lapse gravity results show a change, significant at the 80 % confidence level, which is likely due to water influx into the reservoir. The result is in agreement with model expectations, and with time‐lapse seismic and well log observations. It demonstrates that the method is useful for constraining dynamic models.