Twofreds Field A Tertiary Oil Recovery Project
Authors: Thrash John C. 
Published: December 2007

Abstract
The Twofreds Delaware Sand Reservoir was discovered in 1957 and developed as a water-flood unit in 1963. By 1973, after a moderately successful water-flood, the production rate had declined to near economic limit. Carbon Dioxide injection for oil recovery was instigated in 1974, therefore, oil recovery since 1974 represents tertiary oil recovery after water-flooding. This paper discusses the actual application of CO2 injection procedures and the field performance for the 5-year period since CO2 injection was started. Performance to date appears successful in recovering additional oil from reservoirs of this type.

Introduction
The Twofreds Delaware Sand Field is located in Ward, Loving, and Reeves Counties 17 miles northeast of Pecos, Texas. The reservoir is a stratigraphic sand trap Pecos, Texas. The reservoir is a stratigraphic sand trap and is found at a depth of from 4700' to 4900'. Regional dip is in an easterly direction at a rate of approximately 80' per mile. The Delaware sand is a medium to fine grained sand with thin shale laminations occurring throughout the section. Sand thickness averages 251 over the 4400 acres, productive area with a maximum thickness of 421 near the center of the northeast to southwest trending length of the field. Since discovery. the field has been subject to primary. secondary, and tertiary exploitation processes. processes. PRIMARY AND SECONDARY PERFORMANCE PRIMARY AND SECONDARY PERFORMANCE The field was discovered in 1957 and rapidly developed by a number of major and In dependent companies. In 1963, a field wide unit was formed and a pilot water-flood was started. The water-flood was expanded to a field wide program in 1964 and 1965. A maximum oil production rate of 2700 BOPD was achieved in the latter part of 1967. There are two reservoirs within the field separated by a permeability barrier (Fig. No. 1). This separation is permeability barrier (Fig. No. 1). This separation is well documented by reservoir pressure difference. The east reservoir did not respond well to water-flood and water injection ceased in this reservoir in 1969. Water injection was continued in the west reservoir until 1973 with a clear and well defined production decline history established as the field neared economic limit. (Fig. No. 2). The field produced 8,400,000 barrels by primary and secondary recovery of the estimated 52,000,000 barrels of oil in place. About 2,800,000 barrels were recovered as a result of the water-flood. The east reservoir did not respond significantly to water-flooding, however, the west reservoir performed moderately well recovering 9.5% of the estimated oil in place.

CO2 SOURCE AND SUPPLY
Carbon Dioxide is obtained from the Oasis Pipeline Co.'s MiVida Gas is Processing Plant which removes CO2 from the MiVida Ellenburger gas stream consisting of 55% hydrocarbon gas and 45% CO2. A 6500 HP, 4 stage, compressor station compresses the gas from 2.5 psig to 1800 psig. The CO2 gas is delivered to the field by an 811 high pressure pipeline approximately 12 miles in length. The capacity of the station is 15 MCF per day with a discharge pressure of 1800 psig and a discharge temperature of 125 deg. F. CO2 gas is measured by use of a orifice meter run used in connection with temperature and density measurements with back-up measurement by a standard orifice meter facility.

FIELD CO2 INJECTION
Field injection of the CO2 is obtained by use of turbine meters at the individual injection wells with wellhead pressure individually controlled to a maximum of 1400 psi to maintain a maximum bottom hole injection pressure below fracture pressure (approximately 2900 pressure below fracture pressure (approximately 2900 psi). psi).