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OCTIO - measuring gravity, subsidence and seismic on the seabed

Friday, May 4, 2018

OCTIO of Bergen provides services for recording gravity changes, pressure changes (an indication of seafloor subsidence) and seismic from the seabed - all useful ways to get observational data on reservoirs in production.

Reservoir monitoring and subsea surveillance company OCTIO, based in Bergen, Norway, provides gravity and seafloor subsidence monitoring. The technology is currently being applied in 7 oil and gas fields in Norway. It also makes and operate subsea passive seismic recording devices.

The gravity and subsidence monitoring technology was developed internally by Statoil, and pioneered at the largest field in Norway, Troll. It was then taken over by OCTIO in 2013, with an aim to achieve a wider rollout of the technology. OCTIO's daughter company Gravitude has been surveying Norway's Ormen Lange field, the second largest gas field in Norway, since 2014.


The gravity data can be used to better understand gas production, because once gas is produced, the space it previously occupied is usually filled with water, leading to a higher gravity reading, because water has a higher density, says Martha Lien, CEO at OCTIO Environmental Monitoring.

This way, you can spot a section of the field which is not communicating with the rest of the field (the fluids are not moving, in other words), because there won't be any gravity change there.

Gravity data is gathered in dedicated surveys, in which gravimeters are deployed sequentially at a set of locations on the seafloor, situated above the producing reservoir.

By recording gravity on the seabed, the accuracy is 'orders of magnitude' better than recording it from vessels or from the air, Ms Lien says, since the recording is much closer to the ground and at stable conditions.

Oil companies use the data to improve their reservoir simulations. "In our experience, our data is used directly to improve the reservoir modelling, to enhance the confidence in the predictions of future production, and eventually to take better decisions, like placement of additional producing wells" Ms Lien says.

Gravity data and seismic data complement each other nicely for interpretation, as the former provides good quantification of mass changes while the second maps accurately the extent of the area affected.

The gravimeters used by OCTIO Gravitude are sensitive to a few microgals change, which is a billionth of the normal gravity field on the earth's surface, or the gravitational field between two people half a metre distant.

"Our clients maintain a reservoir model, and pick a few parameters within the model which determine mass changes," says Hugo Ruiz, Vice President G&G at OCTIO. "By choosing the values of this parameters that better fit the observed gravity data, they reduce significantly the space of possibilities of these parameters and hence the uncertainty in their models.'

The data can be used to monitor movement of a gas-water contact, quantify water influx from aquifers, map hydrocarbon depletion, identify compartmentalisation, map reservoir properties like compressibility away from wells. The economic value of information arises from identification of infill well planning targets, avoiding water break-in inn wells, or improved hydrocarbon reserve estimates that allow a better planning of pipelines and resources.


OCTIO Gravitude's method for measuring seafloor subsidence is based in measurements of changes of water pressure at the seafloor. As gravity monitoring, it is based on periodical surveying.

When hydrocarbons are produced, the reservoir compacts and the seafloor experiences some degree of subsidence, Ms Lien says. Changes of seafloor depth above the field are compared with measurements away from the field, to provide calibration. In this way, we obtain 2 mm accuracy in subsidence throughout the field. There is no other technology that can reach to such level of accuracy.

Subsidence data provides a map of reservoir compaction as it is being produced. You can also see lateral differences - if one part of the reservoir has more compaction than another, it tells you that there is a compartment of the reservoir that is not being depleted, and an infill well needs to be drilled there.

The subsidence measurement can be important for installation safety. In extreme cases, subsidence can damage the platforms sitting on legs on the seabed.

Gathering data

The usual workflow begins with a feasibility study, modelling the expected gravity change and subsidence to occur. The objective is to determine where these measurements will help reducing the uncertainty in the modelling, and hence provide better decisions during field development.

Typically 20-120 gravity / subsidence measurement locations are selected in a survey, marked with semi-permanent concrete platforms on the seafloor. The gravity and subsidence measurement equipment is placed sequentially over the platforms during a survey, with the help of an ROV (remote operated vehicle). This way, measurements at different surveys, which can be one to three years apart, are gathered at exactly the same position.

The instrumentation used by OCTIO Gravitude includes 3 gravity meters and 3 pressure meters. The data from the sensors is sent to the ROV, and further to the vessel, where its quality is analysed in real-time. Each reading takes about 20 minutes to take.

Case studies

On the Troll field, 4D gravity data saw a rise of 2 m in the gas water contact in the period 2002 to 2009. That couldn't be detected with time lapse seismic.

On the Mikkel field, gravity surveys have been performed since 2006 to monitor water production into the reservoir, because there were concerns about hydrate formation. The monitoring showed lower water influx than expected into the reservoir, which lead to a significant change in the estimated gas volumes in place. This, in turn, helped with long term planning of pipelines and resources.

On the Midgard field data has been gathered also since 2006, helping to monitor the reservoir draining patterns and aquifer support, the fault distribution and compartmentalisation. It was possible to see that one segment of the reservoir was underproducing, with faults acting as barriers to the flow.

The reservoir model could then be updated to include a sealing fault, and a new well could be drilled in the undrained part of the reservoir. This became the most producing well in the region.

The devices were used on the Ormen Lange field to help make decisions about installing compression facilities and infill wells, where were uncertainties about compartmentalisation in the reservoir and early water breakthrough to the production wells.

On the Statfjord field, subsidence monitoring was used to look for undrained compartments and study aquifer properties. The data was also used to calibrate the geomechanical model.

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