Several laboratory CO2-foam experiments were performed in South Cowden Unit cores to select a suitable surfactant for possible CO2-foam application in the South Cowden Unit. Four surfactants Chaser CD-1045, Chaser CD-1050, Foamer NES-25 and Rhodapex CD-128 were evaluated for their foaming ability. Chaser CD-1045 and Rhodapex CD-128 were selected for further testing after an initial screening. These surfactants were tested in co-injection as well as Surfactant Alternating with Gas (SAG) processes at various frontal velocities. The resulting foams exhibited Selective Mobility Reduction (higher resistance factor in higher permeability zones) as well as shear-thinning behavior. While average resistance factor for the foam produced in four sections of a field core was higher for the co-injection of Chaser CD-1045 than Rhodapex CD-128, the later surfactant performed better in the SAG process as well as exhibiting lower adsorption in Baker Dolomite cores. While it is difficult to select Chaser CD-1045 over Rhodapex CD-128 based on laboratory data alone, economics and calculations might select one product over the other. Two adsorption tests performed with Chaser CD-1045 in presence of 250 ppm hydroxy ethyl cellulose as a sacrificial agent did not reduce adsorption of this surfactant.
This study is a small part of a $20 MM project funded by the United States Department of Energy and the Working Interest Owners (WIO) of the South Cowden Unit. In this Class II DOE project horizontal wells have been drilled for CO2 flooding. However, as a contingency to improve sweep efficiency of CO2 in the horizontal injection wells, this study was initiated to screen four surfactants to identify the best candidate for possible CO2-foam application for mobility control in horizontal wells at the South Cowden Unit following CO2 flood. Four surfactants, Chaser CD-1045 and Chaser CD-1050 obtained from Chaser International Rhodapex CD-128 provided by Rhone-Poulenc and Foamer NES-25 obtained from Henkel Corporation, were evaluated. Since this was a comparative study, it would have been necessary to use identical cores to evaluate the foaming ability of the surfactants. However, due to a severe inhomogeneity of the South Cowden Unit cores, identical tests performed in a single field core would have been the next choice. Initial CO2-foam tests performed in a South Cowden Unit core showed that front end of the core would collapse within 2-3 core tests due to dissolution of softer parts of the core. To avoid this problem a short South Cowden Unit core was placed upstream of the test core. This provision extended the life of the test core so that all four surfactants could be tested in the same test core. The goal of these initial studies was to select the best two surfactants for further testing for selection of the best candidate for CO2-foam applications at the South Cowden Unit. The follow-up studies included evaluation of the effect of surfactant concentration, frontal velocity, comparisons of co-injection versus Surfactant Alternating with Gas (SAG) processes, and determination of surfactant adsorption in cores with no oil or cores at residual oil saturation. All surfactant solutions used in these studies were prepared in Synthetic South Cowden Unit Brine. Analysis of this brine is given in Table 1.
CO2-Foam Test Setup and Flooding Procedure All CO2-foam flooding experiments were performed at the reservoir temperature of 98 F under 2000 psi of pressure. Figure 1 shows a schematic diagram for the setup used in evaluating the foaming ability of various surfactants Core 12A, a South Cowden Unit core used to rank the four surfactants in their foaming ability, was 1" in diameter and 4.84" in length. P. 81^