Abstract

The Tensleep Formation in the Lost Soldier Field has been pattern waterflooded since 1976. Injection wells have historically exhibited large declines in injectivity which required frequent workovers. Since those declines were originally attributed to near-well damage from poor injection water quality, a cleanup program was initiated that significantly reduced the amount and size of suspended particles in the injection water. Despite the improvements, injectivity continued to decline in many of the wells. Subsequent analysis of well histories, PFOTs, and reservoir petrophysical properties suggest the reservoir damage in many instances was not only a sandface phenomena, but also caused by deep particle invasion. Workover procedures were altered to treat the formation beyond the near wellbore area resulting in increased injectivities when compared to the historical norm.

Introduction

The Lost Soldier Unit (LSU) is located in central Wyoming, approximately 100 miles southwest of Casper. Oil is produced from several formations, including the Tensleep, Flathead, and Darwin sandstones and the Madison carbonate. The Tensleep is a Pennsylvanian age sandstone located at an average depth of 5000 ft. (1524 m) in LSU. It is approximately 210 ft. (64 m) thick, with an average porosity of 9.9% and average permeability of 30 md.

Lost Soldier Tensleep first produced in October 1930. A peripheral waterflood was initiated in 1962 and was expanded to a full-field line drive waterflood in 1976. A miscible CO2 flood was started in the south half of this field during 1989.

To prepare LSU for the miscible CO2 flood, a Tensleep repressurization program was instituted to reach minimum miscibility pressure. The program consisted of shutting-in several producing wells while maintaining injection to achieve a higher injection withdrawal ratio and increased reservoir pressure.

A total of 11 wells were shut-in for the reservoir repressurization program. Prior to shut-in, these wells produced 700 bopd (111 m3 /d), and 40,000 bwpd (6360 m3 /d), yet field production declined by only 3,300 bfpd (525 m3/d) after the wells were shut-in. In addition, because higher WOR wells were shut-in, oil production stabilized and remained approximately 500 bopd (79 m3 /d) above the historical decline rate (Figure 1).

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