Hannes Leetaru¹

(1) Illinois State Geological Survey, Champaign, IL

ABSTRACT: Previous Management Practices Complicate Reservoir Characterization in a Mature Field

Boyd Field, located in Jefferson County, Illinois, exemplifies the problems of improving oil recovery in mature fields. Most of the production from the field is from the Mississippian Yankeetown (Benoist) and Aux Vases Formations. The two reservoirs are separated by a 20-foot thick shale and limestone interval.

The Benoist was deposited in a deltaic depositional environment. At Boyd Field this sandstone forms a relatively continuous 40 to 60-foot thick unit characterized by abundant vertical fractures. These vertical fractures form pathways for water movement that causes premature coning of water to be a major problem in producing oil from the Benoist. In comparison, the Aux Vases sandstone was deposited in a tidally influenced coastal environment that resulted in significant lateral and vertical reservoir compartmentalization making it difficult to delineate areas that contain bypassed oil.

The practice of dually completing producing wells in both the Aux Vases and Benoist reservoirs is the greatest obstacle in recovering additional oil from Boyd Field. The Benoist, a water drive reservoir, has a higher sustainable pressure than the Aux Vases reservoir, which is predominantly a gas-depletion drive. After a few years of production, the Aux Vases production was severely inhibited by backflow from the shallower higher pressured Benoist with which it was commingled. Reservoir management of Boyd Field must focus on isolating the Aux Vases from the Benoist production. In addition, new Benoist completions must minimize the effects of water coning. An understanding of the original reservoir management practices, in conjunction with detailed reservoir characterization, may lead to recovering a greater percentage of the remaining mobile reserves.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado