Site Selection, Reservoir Definition And Estimation Of Tertiary Target Oil For The Bell Creek Unit "A" Micellar-Polymer Project
Authors: Vargo J.J. 
Published: December 2007

Abstract
Unit 'A' of Bell Creek Field in southeastern Montana is the site of a 40-acre micellar-polymer pilot which is being jointly funded by working pilot which is being jointly funded by working interest owners and the U.S. Department of Energy. This paper summarizes the work performed during the first year of the project which led to the selection of the pilot site, characterization of the reservoir at this location, and the determination of the ultimate residual oil saturation to waterflood for the sand. In addition, an estimate of the oil available for recovery at the start of this project or "Tertiary Target Oil" is provided. This target oil is that which will remain following the waterflood of the 40-acre 5-spot and the 160-acre study area in which it is embedded. The selection of the pilot site was the result of a study which included an investigation of the available geological information, analysis of log and production data, and a pressure pulse test to confirm the continuity of the reservoir across the proposed area. Determination of reservoir proposed area. Determination of reservoir characteristics was performed subsequent to the coring and logging of four pilot injectors and one observation well. Analysis of data from these sources indicates that the pay within the 40-acre 5-spot may be considered to be a single layer with an average thickness of 8 feet, porosity of 29% and permeability to air of 1050 md. An ultimate residual oil saturation to waterflood of 28% was established using a combination of data from three sources:waterflood studies on core samples from the pilot area;analysis of micro-resistivity curves from open-hole logs run on the five new pilot wells; andlog-inject-log tests run on two of the pilot injectors.
Material balance calculations indicate an average remaining oil saturation of 35% throughout the pilot study area. It is concluded that the "Tertiary Target Oil" within the 40-acre 5-spot is within the range of 214,000 to 268,000 STB. Its final determination, which will be the consequence of reservoir simulation runs, awaits the results of an ongoing waterflood tracer study and pressure transient testing.

Introduction
Bell Creek Field is located in Southeastern Montana in Powder River and Carter Counties. The field consists of six waterflood units which are shown on Fig. 1. In July 1976, Gary operating Company was awarded a Department of Energy contract to conduct a micellar-polymer pilot in Unit 'A'. According to the terms of the agreement, DOE and the working interest owners in Unit 'A' share the expected $5 million cost of the project equally.
The pilot consists of a 40-acre 5-spot and eight producers which offset it on all sides. The project producers which offset it on all sides. The project site (Fig. 2) is located in the north end of Unit 'A' in an area which has been watered-out. if the pilot is successful, future expansion of the process to the remainder of Bell Creek Field could return an additional 54 million STB of oil above that recoverable by waterflood.

UNIT 'A' GEOLOGY AND RESERVOIR CHARACTERISTICS
Production from Bell Creek is from the Muddy Sandstone which is of lower Cretaceous age. The field is located on the northeastern flank of the Powder River Basin, where the regional dip is Powder River Basin, where the regional dip is rather gentle and uniform to the west-northwest at about 100 ft. per mile. The reservoir is a stratigraphic trap located in the Upper Muddy. Regional stratigraphic studies and core studies indicate that the main producing sand is a barrier island deposit with an up-dip permeability barrier to the southeast. The permeability barrier appears to be due to finer-grained sandstones and siltstones that were deposited in a lagoon, shoreward from the barrier island. The sandstone body is elongate to the northeast, parallel to the regional strike. The reservoir is bounded down-dip by a water/oil contact.