Oil and gas production leads to changes in the mass distribution in the subsurface. Hydrocarbon extraction, water flowing in from surrounding aquifers and injection of gas and water alter the mass of fluid present in the rock pores. Measuring changes in gravity on the seafloor provides an insight to such mass changes and hence provides a picture of the dynamical behaviour of the hydrocarbon reservoir.
Usual applications of gravity are:
- Mapping aquifer influx into the reservoir
- Monitoring the movement of the gas-water contact
- Quantifying mass changes (which is hard from time-lapse seismic)
- Mapping hydrocarbon depletion and detecting eventual compartmentalization
- Mapping reservoir properties far away from wells (e.g. permeability)
Information obtained from gravity is used to improve reservoir management:
- Infill-well planning: detection of undrained compartments, avoiding water break-through
- Update of hydrocarbon reserves: by adding a quantification of water influx to the balance between produced volumes and reservoir pressure drop
- Improved management of reservations of hydrocarbon transportation means (e.g. pipes)
- Better understanding of reservoir properties far from wells: compressibility, permeability
Gravity monitoring yields a very small amount of data: gravity changes at selected positions on the seafloor. Forward-modelling changes in gravity is an unambiguous and straight-forward procedure, which involves summing Newton's law on the volume elements in the reservoir model. In other words: gravity is easy to introduce in any history-matching workflow .
The Ormen Lange field case
Ormen Lange is the second largest gas field in Norway.
Back in 2012, installing compression facilities and drilling infill wells were considered as options to increase gass recovery at Ormen Lange. Two uncertainties needed to be addressed to decide on the optimal strategy, namely water break-through and compartmentalization.
Feasibility studies on a range of monitoring technologies, including seismic and electromagnetic methods, concluded that monitoring 4D gravity and seafloor subsidence would provide the required answers in the most timely manner.
Gravity surveys have been performed in Ormen Lange in 2012, 2014 and 2016.
The conclusions from the operator company are:
- Gravity and subsidence surveys are key for understanding aquifer influx and reservoir compaction
- Gravity provides valuable input to understanding mass changes in the reservoir, with immediate insight into the energy balance of the field
- Gravity can be quickly integrated into history matching workflows