Seismic Reservoir Monitoring of a Thermal EOR Redevelopment; Thick Heavy oil Field in Oman

Abstract

Abstract Petroleum Development Oman (PDO) is currently redeveloping a thick deep heavy oil reservoir using steam flooding. Steam has been injected in the first inverted 7-spot pattern (1P) since late 2008. An areal surveillance plan was created together with Shell International EP (SIEP) to test different monitoring techniques in order to select the most cost-efficient one for full field deployment. Ideally, the selected method would improve understanding of the thermal recovery concept and optimise further pattern development. The plan included three crosswell seismic profiles, 3DVSP and surface seismic, all in time-lapse mode. The 2006 surface seismic was used as the baseline for the surface seismic acquired in 2012. The data shows very good repeatability. However as the project construction phase took place after 2006, the 4D surface seismic was influenced by the 4D noise caused by surface changes. Detecting 4D signals is challenging particularly with the relatively small amount of injected steam. In contrast, crosswell seismic data and the 3DVSP were acquired after the construction work took place. The crosswell seismic 4D velocity profiles showed changes below and above top of reservoir, which led to the consideration of geomechanical effects. The 3DVSP data indicated reservoir changes towards the north of the injector which is in line with the temperature surveys, also indicating preferential changes towards the north. These encouraging results led to a second monitor 3DVSP acquisition followed by a pilot 3DVSP using a fibre-optic cable installed in the well for distributed acoustic sensing (DAS). The second monitor 3DVSP shows that previous 4D signals have moved further towards the north. It also indicates additional changes at the bottom perforation zone (as injection was shifted to bottom) with a preferential movement towards the north as well. Furthermore DAS VSP was found comparable enough with the conventional VSP and therefore it can detect reservoir changes. A plan is being worked to fit the existing observation wells with DAS cables and acquire a major 3DVSP baseline. The DAS cable will be combined with distributed temperature sensing (DTS) to maximize the value of the installation plan.