Abstract

Traditionally, CO₂ injection has been considered an inefficient method for enhancing oil recovery from naturally fractured reservoirs. Obviously, it would be useful to experimentally investigate the efficiency of waterflooding naturally fractured reservoirs, followed by CO₂ injection, before this procedure is applied to a reservoir. This issue was investigated by performing water imbibition followed by CO₂ gravity drainage experiments on artificially fractured cores at reservoir conditions. The experiments were designed to illustrate the actual process of waterflooding and CO₂ gravity drainage in a naturally fractured reservoir in the Spraberry Trend Area in west Texas, USA.

The results demonstrate that CO₂ gravity drainage could significantly increase oil recovery after a waterflood. During the experiments, we also examined the effects of different parameters such as permeability, initial water saturation, and injection schemes. We found that the efficiencies of the CO₂ gravity drainage decrease as the rock permeability decreases and the initial water saturation increases. Cyclic CO₂ injection helped improve oil recovery during the CO₂ gravity drainage process after water imbibition.

Oil samples produced in the experiment were analyzed using gas chromatography to determine the mechanism of CO₂-improved oil production from tight matrix blocks. The results show that lighter components are extracted and produced early in the test.

The results of these experiments validate the premises that CO₂ will recover oil from a tight, unconfined Spraberry matrix.