Joel S. Watkins¹, Jianyong Bai¹, Carrie L. Decker¹

(1) Texas A&M University, College Station, TX

ABSTRACT: "Back Doors:" Their Role in Reservoir Charge

In all but very large reservoirs, incompressible formation water must be expelled from the reservoirs to make room for hydrocarbons. "Back doors," seals that allow formation water to escape while simultaneously trapping hydrocarbons, facilitate the escape process.

If the reservoir without a back door is being charged via a fault, the only escape path for the formation water is via the charging fault. Simulations show that a convection cell develops in the reservoir that conveys formation water to the fault entry level. Here, formation water merges with upward moving hydrocarbons and eventually escapes. This is a slow and inefficient process. In downdip reservoirs, charge is limited that part of the reservoir above the fault entry level.

Reservoirs with updip back doors charge much more quickly and efficiently than reservoirs without backdoors. However, the negative buoyancy gradient between a downdip back door and an updip charging fault impedes downdip charge. The reverse is true of reservoirs updip of the charging fault. Downdip charge occurs only when the charging pressure is greater than the negative buoyancy gradient.

A study of a South Texas field found that only reservoirs with back doors were charged. Three holes in reservoirs without back doors were dry.

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