Coring For In-Situ Saturations In The Willard Unit CO2 Flood Mini-Test
Authors: Bilhartz H.L. , Charlson G.S. 
Published: December 2007

Abstract
From Aug. 1972 until May 1976, Atlantic Richfield tested CO2 miscible flooding for tertiary recovery in the stratified San Andres dolomite of the Willard Unit, Wasson field. After first water-flooding the test area, CO2 and water were injected in small, alternate slugs. This was followed by continuous water injection. During all phases of the test, flood response was monitored at a logging observation well, and at the conclusion of flooding, a core was taken between the injection and observation wells with the pressure-retaining core barrel.
This paper describes our attempt to measure in-situ saturations by pressure coring. Residual oil and CO2 saturations were measured in the CO2-flooded strata, and waterflood residual oil saturations were measured for the strata not swept by CO2. Ga compositions were determined for all zones and were used to positively identify the CO2 invaded strata. The paper contains a discussion of how the cores were taken and analyzed and a discussion of the data and its significance for CO2 flooding.

Introduction
The Willard Unit tertiary recovery minitest was designed to obtain the following information for evaluating CO2 miscible flooding:reduction in waterflood residual oil saturation caused by alternate CO2/water injectionextent of gravity segregation across the entire pay interval and within porous zones, andimportance of stratification porous zones, andimportance of stratification and crossflow.
It was not a producing pilot test; instead, flood response at an observation well was made with a miscible-flood reservoir simulator. This paper describes the postflood coring phase of the test.
Fig. 1 shows the location of wells. The fluid injection well (32A) was completed using the typical fracturing technique for Willard Unit wells. A half-fracturing length of 200 ft was estimated. A logging observation well (32A0) is located 100 ft north of the injector, and a well for fluid sampling and pressure monitoring another 25 ft north of the observation well. Both these wells were cored through the pay section for reservoir information. The pressure core hole (32AC) is located 35 ft north of the injector.
At the beginning of the experiment, the test area had undergone primary depletion. From Aug. 1972 until Dec. 1973, water was injected into Well 32A to repressure the test area above minimum miscibility pressure and to waterflood the area around the observation and sampling wells. After waterflooding and until Feb. 1975, CO2 and water were injected in sm all, alternate slugs of approximately equal reservoir volume. Injection rate was decreased during this portion of the minitest to a level such that gravity segregation should be situations except those with very low vertical permeabilities. Since Feb. 1975, water has during all phases of the test, and the pressure core was drilled in April 1976.
The primary objective of coring was to measure the in situ saturation profile. We were interested particularly in accurately measuring the final oil particularly in accurately measuring the final oil saturation left in CO2 swept strata. However, since economic attractiveness of the flood depends on reduction oil saturation rather than final oil saturation alone, an estimate of waterflood residual oil saturation was an additional objective. This estimate was possible because only the high-permeability strata were flushed by the CO2 at the test conclusion. Lower-permeability strata still were responding either to the initial waterflood, waterflooded to residual oil saturation, or beginning to respond to CO2 flooding.