Influence of Shales on Steamflood Response as Interpreted from Gas Saturation History, Kern River Field, California

Steven J. Sanford, John M. Merrell

The changing distribution of gas saturation over time, as determined by periodic neutron logging, can be used as a qualitative indication of reservoir response to steamflood. As used here, gas saturation or free gas is either a naturally occurring mixture of carbon dioxide, methane or air, or injected steam.

Comparing gas saturation distribution over time (gas saturation history) with the distribution of shales in a heavy oil steamflood project in Kern River field, California, illustrates the influence of shales on steamflood response. This comparison shows that although shales as thin as 1 ft can contribute to local vertical confinement of injected steam, shales up to 20 ft thick can allow sufficient conductive heat transfer to accelerate oil production in a sand overlying a steamflood zone.

Logs from two wireline tools proved useful in determining the presence of formation gas. Cross-over on open-hole density-neutron logs indicates gas presence in a sand when neutron porosity reads less than density porosity. Cased-hole pulsed neutron capture logs indicate gas saturation by separation of near and far detector count rates after being properly normalized, and by lower sigma values. Temperature logs run in cased holes can distinguish zones of saturated steam from naturally occurring gases in the formation by their high temperature (nominally > 212°F) and their flat temperature profiles.

Two case histories show how thin areally discontinuous shales and thick continuous shales affect the path of injected steam differently. In the first, some steam injected into a sand was confined vertically by 1-ft thick stratigraphically discontinuous shale beds. Some steam also migrated into an overlying sand through areas where the shales were missing.

In the second case history, injected steam was completely confined within sand capped by an areally continuous shale up to 20 ft thick. Gas saturation consisting of carbon dioxide, methane, and air developed in a sand overlying the steamflood zone, even though the thick shale separated the two zones. Conductive heat transfer through the shale reduced the oil viscosity in the overlying zone, accelerating oil production by gravity drainage. A gas zone formed as liquids were withdrawn.

AAPG Search and Discovery Article #91024©1989 AAPG Pacific Section, May 10-12, 1989, Palm Springs, California.