Gravimetric monitoring of gas production from the Troll field

Abstract

Four surveys of relative gravity and depth measurements have been conducted over the Troll field since gas production began in 1996. Precision in gravity (intrasurvey repeatability measured as standard deviation) has improved from 26μGal to 4-5 μGal in two surveys in 2002 and 2005. The scatter of 74 station depth differences (standard deviation) has improved to 1.0 cm (with modeled subsidence removed) for the same two surveys. Subsidence of up to 3 cm is observed above the thickest reservoir of Troll East between 2002 and 2005. Time-lapse gravity shows a general slight average increase in Troll East (95% confidence for the interval centered at 5.2 ±4.4 μGal), which is likely caused by edgewater influx. One gravity station close to Troll A shows a 9-μGal gravity increase, indicating 2.2-m water rise. Repeated logging in a nearby well indicates 2.8-m rise. These two observations agree within uncertainty bounds. Gravity decrease in Troll West is explained by oil production and a downward-moving gas-oil contact. Gravity increases in two areas in the easternmost part appear statistically significant and are likely to have been caused by edgewater influx. This has not been confirmed by other data, but aquifers located near these areas make the results plausible. The gravity and subsidence data give estimates of total mass influx and pore compaction; these data have been used in and agree with material-balance calculations. © 2008 Society of Exploration Geophysicists. All rights reserved.