Summary

This paper reviews the design, implementation, and performance of one of the first field-scale miscible CO2 floods to be conducted in the Rocky Mountain region of the western U.S. During the first 34 months of CO2 injection, more than 4 million bbl [636 x 10 M] of incremental oil has been recovered, clearly demonstrating that the process is displacing significant volumes of tertiary production. Several aspects of the flood's implementation and performance are discussed, including reservoir pressurization, facilities construction, impact of additional drilling pressurization, facilities construction, impact of additional drilling on production response, fluid injectivity, and operational problems.

Introduction

The Wertz Tensleep field is located in the Great Divide basin in south-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 is the Lost Soldier field (Fig. 2). Collectively, these fields are known as the Bairoil properties. The fields, which produce primarily from the Pennsylvanian Age Tensleep sandstone and Mississippian Age Madison carbonate, have a combined original oil in place (OOIP) of more than 700 million bbl [ 110 X 10 M ] in the four 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 Soldier Tensleep and Madison reservoirs in May 1989. Waterflood performance in the Wertz Madison reservoir indicates that it has limited performance in the Wertz Madison reservoir indicates that it has limited CO2-flood potential, so no firm plans have been made regarding the timing Of CO2 injection.

CO2 for the Bairoil floods is provided by Exxon Co. U.S.A.'s LaBarge project. Exxon transports the CO2 from the Shute Creek plant to a point 19 project. Exxon transports the CO2 from the Shute Creek plant to a point 19 miles [31 km] northwest of the Bairoil fields, where it is transferred to an Amoco Production Co. spur line for final delivery. The high purity injectant averages more than 98 mol% CO2, with 1 to 1.5 mol% methane and less than 0.5 mol% nitrogen.

Field History and Development

The Wertz Tensleep reservoir was discovered in 1936 by Sinclair Oil and Gas Co. Ownership passed through several companies until Amoco acquired the Bairoil properties in Dec. 1975. Primary production from Wertz Tensleep was by a combination 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 was suspended 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 10,000 B/D [480 to 1590 M/d] over an 18-month period. Waterflood performance was enhanced further by five infill- and extension-drilling performance was enhanced further by five infill- and extension-drilling pro-grams conducted from 1982 to mid-1986 that resulted in field pro-grams conducted from 1982 to mid-1986 that resulted in field development on nominal 10-acre [4-ha] well spacing. Fig. 3 presents a field production curve from Jan. 1975 through July 1989. Cumulative oil production at the start of CO2 injection was 77.6 million bbl [ 12.3 x 10 production at the start of CO2 injection was 77.6 million bbl [ 12.3 x 10 M ] , or 45.1 % of the estimated 172 million bbl [27.3 x 10 M] of OOIP.

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 [144 m] and a net thickness of 236 ft [72 m]. Net pay porosity and permeability average 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 formation is segmented into four discrete intervals (Zones 1, 2A, 2B, and 3) by dolomite sebkhas that are correlative across the field. The sebkhas have essentially no porosity and permeability, and zonal pressure data show them to be effective barriers to vertical flow. Zone 1 contains most of the pay, and pay quality generally decreases with increasing depth. Core data show that the reservoir is highly stratified, with an average Dykstra-Parsons coefficient of 0.8.

The reservoir fluid is an undersaturated black oil with a stock-tank gravity of 35 degrees API [0.85 g/cm3]. The CO2 minimum miscibility pressure (MMP) was determined experimentally to be 2,350 psig [16.2 MPa] pressure (MMP) was determined experimentally to be 2,350 psig [16.2 MPa] at the reservoir datum temperature of 165 degrees F [74 degrees C]. Table 1 lists other pertinent reservoir fluid properties.

CO2 Project Design

The Bairoil properties were identified as technically attractive CO2 flood candidates in early 1978, but lack of a CO2 source prevented further evaluation. This obstacle was removed in Jan. 1984 with Exxon's decision to proceed with development of the LaBarge reserves.

JPT

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