Nonuniform water encroachment is common in water-drive or peripheral waterflood reservoirs. In such circumstances, low watercut completions can still be achieved structurally low to wateredout wells, but proper risk evaluation requires understanding the preferential pathways of water influx. High-frequency genetic stratigraphy was combined with time series production maps to investigate water encroachment pathways and identify low-risk recompletion candidates in upper Miocene (Badenian) fluvial-dominated deltaic sandstones of the mature supergiant Matzen field, Vienna Basin, Austria.

Reservoir intervals were subdivided into fifth-order genetic units commonly less than 15 m thick, rarely as thick as 25 m. Facies/ gross sandstone maps for each unit document that dip-oriented channel/mouth-bar sandstone complexes are from 3 to 10 m thick and 200 m to 3 km wide, and core data indicate permeabilities commonly greater than 200 md, reaching up to 1.5 D. Bayfill splay sandstone bodies containing interbedded shale can reach 6 m in net sandstone thickness, with individual sandstone bodies less than 2 m thick. Bayfill splay facies occupy areas ranging from 200 m on a side to 1 km on a side, and permeabilities are typically less than 500 md. Strong water drive augmented by peripheral waterflood results in comparatively rapid water encroachment in wells within channel/mouth-bar facies. Later encroachment and greater cumulative oil production are encountered in wells on the margin of channel/mouth-bar complexes and within splay facies.

A reservoir revitalization project is in progress that is capitalizing on study results. Recompletion strategies include targeting untapped splay bodies and margins of channel/mouth-bar complexes, as well as selectively recompleting low-permeability facies within channel/mouth-bar sandstones where high oil saturations still exist. Similar approaches can be applied to recover large volumes of bypassed resources in other mature reservoirs ranging from fluvial/deltaic reservoirs in the Gulf Coast to deep-water clastic reservoirs in California.

AAPG Search and Discovery Article #90937©1998 AAPG Annual Convention and Exhibition, Salt Lake City, Utah