Subsidence monitoring at offshore oilfields

Hydrocarbon production at offshore fields can cause seabed subsidence, potentially triggering dangerous geohazards including earthquakes and even fault slips. A long-term project with Norske Shell measuring seabed subsidence rates offshore Norway has now led to the latest, centimetric accuracy, long endurance autonomous instruments being developed and deployed, field-wide across the giant Ormen Lange gas field.

The challenge

The production of hydrocarbons at offshore fields causes a reduction in the pressure within the reservoir and therefore its ability to support the layers of rock above it. These layers are collectively named the overburden and they relentlessly push down on the reservoir sometimes causing structural instability surrounding the reservoir and deformation of the seabed into depressions known as a subsidence bowls.

The rate of seabed subsidence is of interest to geophysicists for two principal reasons:

Geohazard avoidance Because naturally occurring or production related subsidence has the potential to cause earthquakes and fault slips that can result in hydrocarbon leaks and/or damage to critical seabed infrastructure.
Geophysical interpretation To support identification of changes and minimise uncertainty in the reservoir, inform decision making by reservoir engineers and therefore help to maximize production and minimise risk.

Some offshore fields such as Ekofisk in the North Sea have subsided by several metres over their production lifetime, with average annual subsidence rates of 10-15 cm per year. However, many deep-water fields in regions such as the Gulf of Mexico and the pre-salt fields offshore Brazil exhibit much lower rates on the order of 2-5 cm per year, noting that accurate subsidence information is equally valuable to reservoir engineers operating at these fields.

In 2010, now retired Shell Geophysicist Dr Paul Hatchell set an industry challenge to develop a means to continuously measure seabed subsidence, over many years, that was sensitive enough to detect 1cm/year of vertical movement of the seabed and without breaking the bank in terms of the cost of operating a suitable monitoring solution.

The solution

Measuring subsidence rates to centimetric accuracies on land is comparatively easy using modern survey techniques such as GNSS and satellite altimetry. But radio waves from satellites do not propagate easily in seawater and therefore an alternative measurement approach was required.

Sonardyne’s custom engineering team therefore implemented a pressure-based subsidence monitoring technique using a seabed deployed instrument called Fetch Pressure Monitoring Transponder (PMT). Each device houses highly accurate pressure sensors and data logging electronics coupled to a wireless acoustic communications modem for data telemetry and enough internal batteries to provide more than 10 years of operational life without human intervention.

At programmable intervals ranging from minutes to hours, Fetch PMT continuously measures and records raw seabed pressure, ambient temperature, unit inclination (tilt) and sample time data which is then harvested wirelessly by periodically deploying a compatible acoustic modem from a crewed vessel or latterly from Uncrewed Surface Vehicles (USVs). The use of USVs instead of crewed vessels is rapidly becoming the norm for this operation due to the significant reduction in cost and CO2 emissions and elimination of exposure to offshore personnel.

The Fetch PMT data harvesting process can be repeated whenever the most recent data are required with typical intervals ranging from three to nine months. The resulting pressure data are converted to absolute depth and then compared with previous data to calculate the rate of seabed subsidence in cm/year.

However, it is an unfortunate truth that all marine pressure sensors suffer from drift in their output readings over time and left unchecked this drift has the potential to reduce the effectiveness by which seabed depth and subsidence rate can be monitored. To address performance this issue, Sonardyne have employed three interrelated techniques:

We routinely pre-screen and measure the drift rates of all pressure sensors over a few months prior to deployment. Any poorly performing sensors are rejected and the resulting seabed pressure data from selected sensors is adjusted to compensate for the known drift signal.

We have developed a patented method of recalibration of the pressure sensors on the seabed using a technique called Ambient-Zero-Ambient (AZA) which is incorporated into our Fetch AZA PMT instrument. This instrument enables regular in-situ drift measurement and removal without recovery of the unit.

We can measure the absolute 3D position of each Fetch PMT unit on the seafloor using a USV based acoustic ranging process called GPS Acoustic Box-in (GPS-A) and can use this information to periodically calibrate the pressure sensor data.

Each of these techniques supports pressure sensor drift removal and therefore an increase to the sensitivity of the subsidence monitoring system towards the golden target of 1cm per year.

The results

High accuracy (circa 1cm/year) continuous seabed subsidence monitoring is now an industry recognized technique for offshore hydrocarbon production.

Several discrete subsidence events (sudden changes in depth) have been detected by the instruments used and this has provided geohazard information which has proven to be extremely useful to reservoir engineers who are tasked with determining the nature and potential causes of these events. Geophysicists also routinely use the information generated to reduce uncertainty in their reservoir models and to aid their decision-making processes as part of proactive reservoir management programmes.

Over the past 15 years, these systems have been deployed many times, starting with early trials at the Norske Shell operated Ormen Lange gas field in 2007, and then field-wide deployments of Fetch PMT and AZA PMT instruments across the globe including in the North Sea, Gulf of Mexico, Norwegian Sea and offshore Malaysia.

The latest deployment at Ormen Lange saw a total of 75 instruments deployed in two phases between September 2019 and October 2020 and these units continue to provide Norske Shell with invaluable subsidence monitoring data from this field, via crewed vessel and remote data harvesting missions conducted using an XOcean USV.

Costs, CO2 emissions and human offshore exposure traditionally associated with gathering oceanographic information have also been reduced by the routine use of USVs instead of ships to collect the data from the seabed wirelessly.

The technology has also been successfully transferred into Ocean Science applications for monitoring plate tectonics, subduction zones and the submerged sides of large volcanoes that, if they break free, can cause catastrophic Tsunamis.

As a result of our efforts to unlock the secrets of our restless sea floor, Sonardyne was recently awarded the 2021 Queens Award for Enterprise in Innovation, the highest award a UK business can receive.