Satellite imaging to monitor CO2 movement at Krechba, Algeria

Abstract

Abstract The Krechba field is one of several gas fields located in the Algerian Sahara desert, and was set in operation in August 2004 as part of a joint venture with BP, Sonatrach and StatoilHydro. The natural gas in the fields contains up to 10% CO 2 , which has to be reduced to 0.3% before the gas is sold, resulting in the production of around 1 million tonnes/year CO 2 . Rather than vent the CO 2 to the atmosphere (business as usual), it is re-injected into the water leg of the Krechba Carboniferous Sandstone gas producing reservoir (20 m thick) via three horizontal wells at a depth of around 1,900 metres. CO 2 injection started in August 2004 and to date nearly 2.5 million tonnes of CO 2 have been injected, amounting to approximately 25% of the gas extracted from the Krechba field over the same period. A number of key technologies to monitor the injection, and the subsurface movement and storage of CO 2 have been, and will continue to be, deployed to provide long term assurance of sequestration. Time lapse satellite images (using PSInSARTM Technology) which measure ground deformation to assess the movement of CO 2 in the subsurface have proven to be much more successful than initially thought, despite the depth of injection and the low voidage replacement rate (25%). Satellite images collected since start of injection show clear increases in ground elevation of up to 30mm around the three injectors while subsidence is also apparent in the area of maximum gas production. The images have also confirmed the CO 2 is moving in the direction of preferred fracture orientation at reservoir level. Recent downhole pressure measurements in one of the injectors also indicates that the CO 2 is being contained within the injection horizon. Work is ongoing to integrate the satellite images with geomechanical and seismic data to better understand how these images can be used for monitoring of CO 2 movement in the subsurface. A key part of the forward monitoring programme will be the acquistion of time-lapse 3D seismic, deployment of tiltmeters and GPS to confirm and calibrate the satellite imagery data, microseismic detectors in shallow boreholes to assess the degree of rock strain and use of shallow wells to monitor the water chemistry in the vadose zone. These will supplement the ongoing tracer, wellhead sampling and satellite imagery data acquistion. This paper shares the latest results of the ongoing CO 2 monitoring project.