Abstract

The Triangle "U" Unit is located in Campbell County, Wyoming, in the Powder River Basin. The field produces mainly from the Sussex "A" Sandstone, with completions and limited production from the Sussex "B" . The field recovered 12.8% OOIP on primary prior to the waterflood, which commenced in March, 1981. The Sussex "A" is relatively tight, with an average permeability of 15 md and porosity of 13.5%. The rock contains swelling and migrating clays and the initial injection water source was fresh, leading to concerns about long-term injectivity. To stabilize clays, two different processes were applied. Earlier injection wells were treated with a combination of potassium chloride and cationic polymer. Later injection wells were treated with potassium hydroxide (KOH). A recent comparison of long-term performance of the two groups of injection wells shows that the wells treated with KOH injected 556,000 BBLS / porosity-foot more water than the wells treated with cationic polymer, in 1.4 years less time. Following KOH, all injection wells were put on a low concentration of imbibition agent to maximize in-depth penetration of water into low permeable rock. Cumulative oil recovery through March, 1997, is 36.4% OOIP, compared to the original waterflood projection of 26.6% OOIP. A total of 37.7% PV water has been injected, and the water-oil-ratio is currently 0.71, for a very efficient flood in this tight, dirty sandstone.