The Wertz Tensleep CO2 Flood: Design and Initial Performance
Authors: Kleinstelber Stanley W. 
Published: December 2007

Summary
This paper reviews the design, implementation, and performance of one of thefirst field-scale miscible CO2 floods to be conducted in the Rocky Mountainregion of the western U.S. During the first 34 months of CO2 injection, morethan 4 million bbl [636 × 10 M] of incremental oil has been recovered, clearlydemonstrating that the process is displacing significant volumes of tertiaryproduction. Several aspects of the flood's implementation and performance arediscussed, including reservoir pressurization, facilities construction, impactof additional drilling pressurization, facilities construction, impact ofadditional drilling on production response, fluid injectivity, and operationalproblems.

Introduction
The Wertz Tensleep field is located in the Great Divide basin insouth-central Wyoming, about 90 miles [145 km] southwest of Casper (Fig. 1). Wertz is one of two large fields located in the immediate area; the other isthe Lost Soldier field (Fig. 2). Collectively, these fields are known as theBairoil properties. The fields, which produce primarily from the PennsylvanianAge Tensleep sandstone and Mississippian Age Madison carbonate, have a combinedoriginal oil in place (OOIP) of more than 700 million bbl [ 110 X 10 M ] in thefour reservoirs.
Three of the four major reservoirs are currently under tertiary CO2 flood.CO2, injection began in Wertz Tensleep in Oct. 1986 and in the Lost SoldierTensleep and Madison reservoirs in May 1989. Waterflood performance in theWertz Madison reservoir indicates that it has limited performance in the WertzMadison reservoir indicates that it has limited CO2-flood potential, so no firmplans have been made regarding the timing Of CO2 injection.
CO2 for the Bairoil floods is provided by Exxon Co. U.S.A.'s LaBargeproject. Exxon transports the CO2 from the Shute Creek plant to a point 19project. Exxon transports the CO2 from the Shute Creek plant to a point 19miles [31 km] northwest of the Bairoil fields, where it is transferred to anAmoco Production Co. spur line for final delivery. The high purity injectantaverages more than 98 mol% CO2, with 1 to 1.5 mol% methane and less than 0.5mol% nitrogen.

Field History and Development
The Wertz Tensleep reservoir was discovered in 1936 by Sinclair Oil and GasCo. Ownership passed through several companies until Amoco acquired the Bairoilproperties in Dec. 1975. Primary production from Wertz Tensleep was by acombination of fluid expansion and water influx. In 1941, a single-well, crestal gas injection program was started and peripheral water injection, primarily for water disposal, began in 1954.
A pilot pattern waterflood was installed in 1978, Gas injection wassuspended in 1980 and the waterflood expanded on a fieldwide five-spot pattern. Waterflood response was dramatic: oil production increased from pattern. Waterflood response was dramatic: oil production increased from 3,000 to 10,000B/D [480 to 1590 M/d] over an 18-month period. Waterflood performance wasenhanced further by five infill- and extension-drilling performance wasenhanced further by five infill- and extension-drilling pro-grams conductedfrom 1982 to mid-1986 that resulted in field pro-grams conducted from 1982 tomid-1986 that resulted in field development on nominal 10-acre [4-ha] wellspacing. Fig. 3 presents a field production curve from Jan. 1975 through July1989. Cumulative oil production at the start of CO2 injection was 77.6 millionbbl [ 12.3 × 10 production at the start of CO2 injection was 77.6 million bbl [12.3 × 10 M ], or 45.1 % of the estimated 172 million bbl [27.3 × 10 M] ofOOIP.

Reservoir Properties
The Tensleep sandstone at Wertz is at an average depth of 6,200 ft [1900 m]. An eolian deposit, the formation has an average gross thickness of 471 ft [144m] and a net thickness of 236 ft [72 m]. Net pay porosity and permeabilityaverage 9.9 % and 13 md, respectively. permeability average 9.9 % and 13 md, respectively. Fig. 4 shows a type log of the Tensleep sandstone. The formationis segmented into four discrete intervals (Zones 1, 2A, 2B, and 3) by dolomitesebkhas that are correlative across the field. The sebkhas have essentially noporosity and permeability, and zonal pressure data show them to be effectivebarriers to vertical flow. Zone 1 contains most of the pay, and pay qualitygenerally decreases with increasing depth. Core data show that the reservoir ishighly stratified, with an average Dykstra-Parsons coefficient of 0.8.
The reservoir fluid is an undersaturated black oil with a stock-tank gravityof 35 degrees API [0.85 g/cm3]. The CO2 minimum miscibility pressure (MMP) wasdetermined experimentally to be 2,350 psig [16.2 MPa] pressure (MMP) wasdetermined experimentally to be 2,350 psig [16.2 MPa] at the reservoir datumtemperature of 165 degrees F [74 degrees C]. Table 1 lists other pertinentreservoir fluid properties.

CO2 Project Design
The Bairoil properties were identified as technically attractive CO2 floodcandidates in early 1978, but lack of a CO2 source prevented furtherevaluation. This obstacle was removed in Jan. 1984 with Exxon's decision toproceed with development of the LaBarge reserves.
JPT
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