Downhole seismic monitoring has become an alternative to more conventional surface seismic in cases where the latter has low signal-to-noise ratio or poor repeatability, or when frequent monitoring of a local target is required. Downhole 3D VSP monitoring has been applied to both onshore (O'Brien et al., 2004; Kiyashchenko et al., 2011) and offshore environments (Wu et al., 2011). In onshore environments, surface seismic repeatability often suffers from surface noise and near-surface variations caused by water-table changes or surface facilities construction. For VSP surveys, the wavefield separation into downgoing and upgoing fields presents an opportunity to discriminate between near-surface and reservoir changes, thus making time-lapse applications more effective.
A steam flood re-development of a mature field in the South of Oman is well underway with the first pattern of steam injection active since late 2008. The areal surveillance programme designed by a joint effort between Petroleum Development Oman (PDO) and Shell International Exploration and Production (SIEP) includes the simultaneous recording of a variety of seismic methods of different resolution. The main purpose of the time-lapse work is to monitor the steam conformance and sweep efficiency as the steam is injected in this heavy oil reservoir over 1,000 m below surface. Key aspects of the data acquisition campaigns, state-of-the-art processing and time-lapse interpretation will be discussed. The ultimate objective of the programme is to evaluate the effectiveness of the techniques used in order to select the most appropriate and economically viable reservoir surveillance tools for full field deployment.
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.
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