LEETARU, HANNES E., Illinois State Geological Survey, Champaign, IL

ABSTRACT: Improved Oil Recovery in Mature Fields through Reservoir Characterization and Management

The Illinois basin is mature with respect to hydrocarbon exploitation in the Pennsylvanian and Mississippian strata. Available subsurface data for the basin commonly are 30 to 50 yr old and of lower quality than today's state-of-the-art data. Recent evaluation of two geologically similar Illinois oil fields shows how the application of new concepts and technologies to the old data can be used to improve oil recovery.

Boyd and King fields, located in Jefferson County, Illinois, produce from the Mississippian Aux Vases formation, a unit that was deposited in nearshore mixed siliciclastic-carbonate environments. Prospective areas for further development were delineated by conventional reservoir-characterization methods. Three-dimensional modeling was used to enhance visualization of the lateral and vertical heterogeneity of these reservoirs.

At King field, mixing of intercalated siliciclastic-carbonate facies causes significant reservoir heterogeneity; numerous compartments have been bypassed by the existing waterflood. Targeted infill drilling of additional producing and injector wells should recover 1-2 million bbl of additional hydrocarbons.

At Boyd field, delineation of areas that contain bypassed oil is more difficult because many of the wells have not penetrated the entire reservoir. An additional problem is that almost all of the production from the original Aux Vases wells was severely inhibited by backflow from a higher pressured, shallower reservoir with which it is commingled. In this type of field, reservoir management must focus on isolating the Aux Vases, producing intervals and deepening individual wells through the entire reservoir.

Although the Aux Vases reservoirs at Boyd and King fields are geologically similar, reservoir management practices in the two fields have been distinctly different. The study of these two fields suggests that detailed geologic characterization of the internal reservoir architecture is not enough. Effective reservoir characterization for improved oil recovery must include both reservoir geology and an understanding of previous reservoir management techniques.

AAPG Search and Discovery Article #90990©1993 AAPG International Conference and Exhibition, The Hague, Netherlands, October 17-20, 1993.