A 4D seismic survey over the Enfield oil field has proved to be a key tool for integrated reservoir management and unraveling production insights early in the field history. Enfield began oil production in July 2006. Just 7 months later, we acquired a 4D monitor survey creating the first dedicated 4D seismic survey in Australia. The results have provided us with an early understanding of injector pathways, stratigraphic and fault controlled reservoir connectivity, reservoir pressures and water front movement in the field.
Woodside acquired a dedicated baseline survey acquired in 2004 with good image quality. We designed the monitor acquisition to optimize 4D repeatability and processed baseline and monitor data in parallel to enhance production-related effects.
We conducted a series of 4D feasibility modeling studies to decide on the optimal approach for a successful 4D interpretation. We generated synthetics from the dynamic reservoir model at different time steps to determine the 4D response of production-related effects. We also modeled these effects in 4D AVO space which showed excellent separation between pressure, gas and water saturation changes. Our 4D interpretation results tie well with the feasibility study we have used both conventional and AVO-based interpretation methods integrated with the production and geological data. In this paper we show that robust 4D anomalies are clearly evident after just 7 months of oil production. We have used the 4D results to optimize the positioning of development wells and also to constrain and update the dynamic reservoir mode l.
The Enfield oil field is located in approximately 500m water depth in the North West Shelf of Australia. The field commenced oil production in July 2006. During the monitor acquisition the field contained 3 horizontal and 2 deviated oil producers, 2 gas injectors in the gas cap and 6 deviated water injectors although not all of these wells were active during the survey.
The producing reservoir consists of 2 main sand intervals, a lower channel sequence and upper debrite, separated by a major flooding event defined by a 1-2m thick shale present over most of the field. The gross reservoir ranges in thickness from 10-60m with porosity typically in the range of 20-27%. The lateral and vertical connectivity between these two main sands is important for future development of the field.
Prior to 2004, the sliver and horst blocks in the eastern part of the field were thought to be brine bearing. An AVO inversion showed that Vp/Vs ratio is required to discriminate oil here due to the presence of a different overlying shale relative to the main block. An appraisal well was drilled in 2004 confirming oil. This highlights the importance of AVO interpretation for fluid movement in this part of the field, hence it is included as part of the 4D study.
Woodside acquired the Enfield baseline survey in 2004 and the dedicated monitor survey in 2007 only 7 months after first oil production. We designed the acquisition using an innovative two-boat configuration (Ridsdill-Smith, et. al, 2007). The repeatability of the survey was high with an average NRMS below 20%.
