Abstract

A core hole was needed in the Willard Unit, Wasson Field, CO2 Pilot Test in order to evaluate the effectiveness of CO2 flooding. The coring program was designed to determine the residual oil saturation behind the CO2 front and the waterflood residual oil saturation in zones not yet affected by CO2. For the most definitive interpretation of the oil saturations found in the core, the presence of CO2 needed to be associated with the lowest oil saturations. Saturation measurements on conventional cores may result in incorrect values for one or both of two primary reasons:

  1. some hydrocarbon may be stripped from the coring by mud filtrate invasion,

  2. and additional fluids may be driven from the core by gas expansion as the pressure is reduced during surfacing.

If flushing by mud filtrate is controlled then valid fluid saturations can be obtained using a pressure retaining core barrel to prevent loss of fluids by gas expansion. Furthermore, if gas saturation and/or composition is desirable, then pressure coring is necessary and this was the situation at Willard. However, a major disadvantage to pressure coring is its high cost and a rather low 60% historical success ratio of getting core to surface under pressure.

This paper summarizes the work done with the pressure core barrels prior to spudding in order to increase our chances of getting core to surface under pressure. It also outlines the field procedures which were followed that resulted in obtaining 18 pressure cores out of 19 attempts. Also, discussed in this paper are the core analysis techniques which were used to determine oil, water, and gas saturations at reservoir conditions for full diameter cores.

Discussion

A pilot test of tertiary oil recovery by CO2/water miscible displacement was conducted in the Willard Unit, Wasson Field, Yoakum County, Texas. The Willard Unit pilot consisted of one injection well (32A), one logging observation well (32AO) located 100 feet to the north of the injector, and one fluid sampling and pressure monitoring well (32AS) 25 feet north of the logging well (refer to Figure 1). The test was designed to study unit displacement efficiency and in situ fluid movement and was not a producing pilot test.

The reservoir is a typical West Texas San Andres dolomite with a gross pay interval of 160 feet in the pilot area.

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