As E&P companies turn their attention to heavy oil to bolster their production capacity, CGGVeritas offers a unique seismic monitoring solution which helps actively manage SAGD production and optimise recovery in these difficult fields.
Time-lapse seismic plays an increasingly important role in reservoir monitoring. It can be used in conjunction with the well data to improve understanding of reservoir behaviour and to optimise hydrocarbon recovery. Typical oil and gas reservoirs have a lifespan of many years and evolve slowly over their productive life, so repeat surveys every few years are appropriate for 4D seismic monitoring.
Heavy oil reservoirs produced using steam-assisted gravity drainage (SAGD) have a much shorter life-cycle. The state of the reservoir can vary significantly over the course of weeks so it is important to monitor production effects on a much more frequent basis. In fact it would be ideal to have access to real-time data, which creates a challenge for seismic observation.
To have a seismic crew on call to perform weekly or even monthly surveys is economically unrealistic. Amongst the other problems faced in this scenario is the extensive surface infrastructure around the fields which will interfere with acquisition, both in terms of surface coverage and high levels of background noise. Crucially, SAGD production effects may be rapid but they can also be small, requiring highly sensitive acquisition in this noisy environment.
The answer to this problem is a seismic monitoring system originally devised for an equally unconventional and challenging application: underground gas storage monitoring. SeisMovie was developed by CGGVeritas, Gaz de France and IFP to provide high-resolution continuous seismic monitoring of a natural reservoir used for the seasonal storage of methane. The solution had to be economic, highly reliable and have the ability to accurately measure gas saturation and the extent of the gas bubble.
To fulfil these criteria it was decided to design a completely autonomous system utilising buried receiver arrays and a novel piezoelectric vibrator source. Using a permanent buried installation ensures excellent 4D repeatability and coupling. By going a step further and placing the source and sensors below the weathering layer, SeisMovie eliminates near-surface variations to further improve repeatability over extended periods.
The “exotic” source has some unique characteristics which are critical to the SeisMovie system. Firstly it is maintenance-free and reliable, which suits permanent installation. Secondly it has a very stable source signature which contributes further to the excellent repeatability of the system. Several of these sources can be distributed around the field, and by using a patented discrete frequency sweep they can operate simultaneously.
SeisMovie is flexible and versatile and can use a variety of receiver configurations to achieve different monitoring objectives. For areal coverage over a large target, long arrays or grids of buried geophones can be installed. To increase the vertical sensitivity of the system, arrays of sensors (2, 3 or 4 component) can be installed vertically in boreholes to provide VSP-style datasets.
With this combination of exceptional repeatability and low-noise recording environment SeisMovie systems routinely achieve a sensitivity of 0.1 milliseconds to vertical transit time variations, and a usable signal bandwidth of up to 250 Hz. This enables the monitoring of subtle and rapid reservoir variations which conventional 4D acquisition techniques fail to discern.
What makes SeisMovie particularly attractive is its minimal environmental footprint and economic benefits. These both stem from the fact that after the initial installation, the operation is fully autonomous and can be controlled remotely. This includes data acquisition, on-site pre-processing and data recovery, which eliminates the need for on-site personnel. Furthermore, the maintenance-free buried installation is well protected from vehicles, animals and the weather, reducing the chance of damage to the system or the environment. Consequently on-site intervention should be unnecessary and on-going operational costs are minimal.
After success with three gas storage monitoring projects in France, the first heavy oil pilot project was conducted in 2005 in Canada. A single buried source and a grid of 397 receivers were used to monitor a shallow, unconsolidated tar sand reservoir for a one-month period of steam injection. Recording was near-continuous, with 240 records per hour being acquired. These were pre-processed automatically on-site and transferred to the office via an internet link for analysis.
As the data streamed in from the field, the seamless “movie” of the reservoir showed the progressive effects of steam injection in unprecedented detail. In particular the transit time maps revealed the irregular nature of the steam propagation into the reservoir and the corresponding effectiveness of the individual injection wells. Despite high levels of noise generated by the adjacent steam plant and nearby drilling operations the SeisMovie results showed good signal content and bandwidth. There was also good correlation with the available well data and detailed QC of the continuous seismic record ensured anomalies could be easily identified and the results validated.
Having proved its worth in this specialist role, there is growing interest in SeisMovie as a heavy oil production monitoring tool with similar projects ongoing in their early phases in Russia, Brazil and Venezuela.
SeisMovie is a trademark of CGGVeritas
For more information, visit CGGVeritas on booth 7330 or go to www.cggveritas.com/seismovie.
