Oil reservoirs in the Uintah Basin are extremely complex and have to be considered in the ‘Unconventional’ category. This is partially because of the lacustrine depositional environment which resulted in numerous self-sourcing reservoirs that vary greatly in wettability and mineralogy (very clastic to very carbonate). Analysis of these reservoirs is compounded because of the low porosity/permeability, high paraffin content, and natural fracture distribution. Standard log analysis to determine OIP has proved problematic. As a result, a more unconventional logging suite to examine fluid distribution and properties is being employed.
This paper presents some of our findings in the understanding of the liquid hydrocarbon distribution. Only then are we able to determine efficient primary production techniques as well as flow characteristics for secondary water-flood projects. Because of the discontinuous nature of the reservoirs, we have found that some intervals are most economically produced with vertical wells while others are best produced with horizontals.
The viscosity of the oil is highly temperature dependent because of the high paraffin content. The viscosity is also dependent upon asphaltine content which is present in the ‘black’ wax oil component. Fortunately, with new NMR techniques and continuous logging, the in-situ viscosity of the oil can be estimated. The NMR log is also used to differentiate between solid hydrocarbon (kerogen) and the liquid component. Dielectric measurements provide us with a water volume estimate that is essentially independent of the water salinity. This is particularly valuable in the water-flood area of the reservoir to determine remaining oil and residual oil saturation.
Petrophysical results from the open-hole logging that are calibrated to core are used in combination with core flooding experiments to feed into reservoir models. Fluid flow characteristics in this complex environment are measured at in-situ conditions through proper petrophysical analysis of core and logs.