Integrated Reservoir Study of the Who Dat Field
Authors: Gong Xinglai , Simms Gerard , Fowler Richard , Doughtie John , Cumming Earl 
Published: April 2018

The Who Dat field is located in Mississippi Canyon 503 under 3,100 ft of water, penetrating 11 stacked horizons between 10,000 – 18,000 ft, in Pliocene and Upper Miocene age. The Who Dat OPTI-EX® semi-submersible FPS was the first FPS built on speculation, and was the first privately owned FPS in the world. The field was discovered in December 2007, initial production in December 2011, and the field paid out in 2014. The field has produced more than 52 MMSTB of oil and 99 BCF of gas as of year-end 2017.

This paper will cover the integrated reservoir study of the Who Dat field, which includes geological modeling, data management, reservoir surveillance, construction of the integrated reservoir models, history match and forecast of the integrated models, depletion plan optimization, production operation optimization, sensitivity, and uncertainty analysis.

The eleven different horizons in the Who Dat field have various depositional environments, petrophysical properties, and fluid properties. As a result, the optimum depletion plan needs to be uniquely designed for each reservoir, as well as for the field as a whole.

The biggest reservoir, the 4600, exhibited steep pressure decline and great connectivity which made it a potential candidate for water flooding. However, the facilities do not have space or weight capacity for the water flood equipment, which leaves only high cost solutions such as major platform modifications (wing decks, hull blisters) or building a standalone facility on the Shelf. Based on a thermal simulation model, the injected cold water viscosity is several times higher than the in-situ water which decreases the injectivity of the water injectors. Even though the water flood project was expected to increase ultimate oil recovery significantly, the project team recommended not going forward with the project based on the integrated reservoir study and the resulting economics. If the water flood project was sanctioned, the field would not pay out for years and the net present value of the project would be significantly decreased.

The second biggest reservoir, the 4700, had a downhole sample showing undersaturated oil with a GOR of 1,280 SCF/STB; however, the well experienced an abnormally high 4,500 GOR when put on production. A later well that penetrated the updip area showed the zone to be gas bearing, which indicated the reservoir is not in equilibrium with a gas cap and undersaturated oil rim. The project team decided to decrease the rate to see whether the well was rate sensitive. With the help of multiphase meters, the team observed that with decreased rate, the GOR dropped to less than 2,500, and the productivity index increased from 4 stb/psi to 14 stb/psi. By continuing to produce the well at a reduced rate the ultimate recovery significantly increased.

The reservoir models have been very consistent after a couple of years’ production. The production and pressure forecast from the 2014 history matched models are within 10% of the historical data, which provides confidence in the integrated model. After 6 years of production, the project team is still actively updating and utilizing the integrated models to evaluate future development wells, secondary recovery opportunities, and production optimization to further increase the value of the Who Dat field.