With the majority of the world’s oceans having now been surveyed for new oil and gas reserves, operators are shifting their focus towards maximising recovery from existing fields. Shaun Dunn, Global Business Manager for Exploration reports on the role Sonardyne’s reservoir surveillance technologies are playing in enabling asset teams to develop enhanced recovery strategies.
For many years, vessels have explored the oceans looking for new fields and reservoirs. The majority of geographical regions and sedimentary basins have now been explored with reducing volumes of untapped conventional resources available to be discovered, most notably for giant and super giant fields in excess of 500 Mmbbls (million barrels) reserves. There are a few exceptions in places such as the Arctic and the very deep water regions of the Gulf of Mexico and the South Atlantic margin, but accessing these reserves is fraught with technical uncertainty, risk and high cost per-barrel extraction; these regions are increasingly commercially challenged and are currently rendered unappealing.
For this reason, the continuing trend for upstream oil and gas companies is to extract the maximum benefit from their existing fields and spend less time searching for new prospects. This is driving a steady increase in the development and deployment of reservoir surveillance techniques for a more detailed understanding of the distribution of hydrocarbons in producing reservoirs, leading to the development of recovery improvement programmes.
A variety of surveillance tools and techniques are available but one of the primary methods remains the application and use of repeated seismic acquisition across a reservoir throughout its production lifecycle. The resulting data from these time-lapse surveys is used to monitor the reservoir and fluid movements over production time. To detect what are often quite subtle changes, it is imperative that repeated surveys are conducted with minimal variation. Minimising geometric differences between source and receiver positions is one way to reduce variability; another is the careful monitoring of environmental variables including water velocity and tidal height. This can be conducted using Sonardyne’s Pressure Inverted Echo Sounder (PIES) which monitors two-way travel time through the water column whilst simultaneously measuring pressure at the seabed.
The vast majority of exploration projects are still conducted using streamer-based acquisition but geophysicists generally agree that the most reliable reservoir surveillance requires seabed recording equipment to minimise variation between surveys and enable repeatability of data acquisition. With this method, seabed receivers are stationary and provide the highest possible definition time-lapse reservoir imagery. It is therefore unsurprising that there is a steady increase in seabed seismic acquisition activity with service providers competing to offer high definition imagery using seismic nodes deployed on the seabed.
For many years, Sonardyne has supplied acoustic and inertial equipment that supports streamer, node and source positioning operations. This technology continuously evolves to match the ever increasing demand for higher position accuracy and reliability so that seismic operators can achieve lower time-lapse variability and generate more detailed reservoir information.
Seismic source positioning
In non-permanently deployed seismic systems the receivers are deployed in one of two ways. Norwegian seismic acquisition expert Magseis uses thousands of small autonomous nodes attached to long steel cables. This node and cable combination is deployed from the surface vessel and forms a grid pattern of receivers on the seabed, ready for the survey to begin. It is vitally important that the cables are laid in the correct positions to avoid becoming entangled with subsea infrastructure and to ensure good repeatability between surveys.
For these projects, Magseis uses Sonardyne’s Small Seismic Transponders (SSTs) attached near the nodes at regular intervals along the cable and positioned by a Ranger 2 system with a Lodestar GyroUSBL transceiver. With this, it is possible to track the cable’s position as it descends through the water column to the seabed. Using Ranger 2 GyroUSBL, it is possible to simultaneously track hundreds of SSTs, providing highly accurate real-time information to ensure the cables meet their target locations.
One remaining challenge of surface vessel based positioning is the ability to accurately track the laydown position of the cable when it is landing far behind the vessel – such is the case when deploying at high speed in shallow water. To overcome this, an Autonomous Surface Vehicle (ASV) equipped with the same Ranger 2 GyroUSBL system will, in future, be able to track the touchdown point during deployment, resulting in reduced operational time whilst improving cable position accuracy.
In deeper water or when surveying in close proximity to subsea infrastructure, several seismic nodes are lowered to the seabed at a time, where ROVs are then used to deploy them in their final position. In contrast to cables, these nodes are not positioned directly but instead by tracking the ROV position as it uses its manipulator to plant the node on the seabed. Position accuracy is still vitally important but with hundreds of seismic nodes in a typical array and each having a finite battery life before recharging is required, speed of deployment is also critical.
In this case, the Sonardyne Ranger 2 GyroUSBL system installed on the surface vessel is coupled with SPRINT technology on the ROV along with external inputs from Doppler Velocity Logs (DVL), depth and sound speed sensors. When used in combination, this system provides extremely high accuracy position information with low noise and a fast update rate, allowing the ROV to deploy nodes quickly and accurately, ready for surveying.
Reservoir Surveillance settlement monitoring
Another reservoir surveillance tool quite separate from seismic acquisition is settlement monitoring. Years of oil extraction without appropriate and effective recovery mechanisms can cause the pore pressure of the oil bearing rock to decrease, often reducing its ability to support the layer of rock above it (known as the overburden). The result is that the overburden layer can sink, causing a small but detectable settlement of the seabed by a few centimetres per year. Examples of this are the chalk fields in the Norwegian and Danish sectors of the North Sea where sea floor foundering as a result of high production rates has required costly retrofitting and remediation of seabed tethered production facilities.
Sonardyne’s settlement monitoring technology consists of large arrays of Autonomous Monitoring Transponders (AMTs) and Fetch longlife sensor logging nodes deployed on the seabed for several years. They take highly accurate pressure (depth) and acoustic range measurements at pre-determined intervals. These measurements are repeatedly collected over a set time period (up to several years) and the results uploaded acoustically to a passing surface vessel or ASV/USV and forwarded to geophysicists for analysis.
One such system was deployed in 2011 at Shell’s Ormen Lange field in the North Sea. The array consists of over 200 AMTs spread over a wide area where every few hours, each transponder wakes up and takes pressure measurements and acoustic ranges between other transponders in the array before going back to sleep. To date, this array has made over 250 million observations resulting in over half a Gigabyte of sensor data uploaded to either a conventional surface vessel of opportunity or unmanned surface vehicle for onward transmission to Shell analysts for processing. With this system it is possible to detect movement in offshore fields of just a few centimetres per year or better, providing highly insightful surveillance data about the status of the underlying reservoir.
The way forward
It is generally accepted that geophysicists will spend more of their time interpreting reservoir surveillance data as oil and gas companies strive to maximise recovery from producing fields to offset poorer than expected exploration successes.
Increasing the quality and amount of this information is a never ending quest for offshore exploration and production companies. Sonardyne is constantly striving to improve its product portfolio to help with this seismic shift in reservoir surveillance and stay at the forefront of this critical area of subsea technology.