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 i mproved 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.

Measuring Seafloor Strain With an Optical Fiber Interferometer

“…It is possible that, once embedded in the sediment, seafloor benchmarks are quite stable. Hundreds of concrete benchmarks have been placed on the bottom of the North Sea for geodetic monitoring of deformation associated with natural gas production and, at least at the 5-mm level, their vertical positions are quite stable over many years (Eiken et al, 2008;Stenvold et al, 2006). It remains to be seen how stable the endpoints of a long-baseline strainmeter can be when attached to deep ocean sediment.…”

Practical Applications of Time-lapse Seismic Data

“…El Ouair et al (2005) and Buland and El Ouair (2006) further the concept of difference inversion. They use a Bayesian framework along with information from the expected elasticproperty changes and the measured seismic repeatability to provide uncertainty estimates for the impedance change.…”

Feasibility of time-lapse gravity and gravity gradiometry monitoring for steam-assisted gravity drainage reservoirs

“…Time-lapse gravimetry has been used extensively in oil and gas production monitoring (e.g., Alnes et al, 2008;Brady et al, 2008;Eiken et al, 2008;Ferguson et al, 2008;Alnes et al, 2011;Dodds et al, 2013) and in groundwater and aquifer studies (e.g., Cogbill et al, 2006;Davis et al, 2008;Gehman et al, 2009). A more comprehensive review can be found in Krahenbuhl and Li (2012).…”