CO2 Mobility Control

As early as 1955, surfactants were recognized for their effectiveness in lowering gas mobility in reservoir cores by in-situ foam generation. For commercial field application a specific surfactant must have several important characteristics. it must behighly effective with low cost,chemically stable, soluble. and surface active in oil field brines, andunaffected by contact with crude oil or reservoir minerals. A static foam generator, an adaptation of a conventional blender, was used to screen more than 150 candidate surfactants. Promising additives were then ranked in a unique dynamic test, developed at New Mexico State U., that involves sequential liquid/gas flow in a vertical tube packed with glass beads. Conventional flow tests in tight, unconsolidated sandpacks show good correlation with the dynamic and static screening tests, especially those data obtained in the dynamic experiment. Some synergism exists between additives with amine oxides and amides having the most beneficial effect on foam stability and gas mobility control. The utility of cosurfactant stabilization was demonstrated in linear, two-phase flow tests through tight. unconsolidated sandpacks involving brine and gas. A solution containing 0.45% Alipal CD-128 (TM) and 0.05% Monamid 150-AD (TM) can decrease gas mobility over 100-fold. The effect appears to be time-independent, indicative of good foam stability. Alipal CD-128 alone reduces gas mobility even more, usually by a factor of two. The moderating influence of a cosurfactant could be beneficial in avoiding "overcontrol" of mobility, especially in low-permeability reservoirs. Introduction For more than 30 years recovery experts have known that CO2 possesses a unique ability to displace crude oil from reservoir rock. Although many gases have been tested for their crude-displacing efficacy, only CO2 has the ability to reduce residual oil saturations to near zero and produce significant quantities of tertiary oil in models that have been previously waterflooded to the economic limit. Early studies provided the fundamental understanding required to explain the high efficiency of CO2, but until recently the depressed price of crude has made most, if not all, CO2 field applications unprofitable. A common failing among-as-driven oil recovery processes is the severe gas channeling that occurs in the reservoir because of excessively high gas mobility. Optimistic oil recoveries obtained in laboratory flow tests with small-diameter, linear models have never been achieved in the field. Both miscible and immiscible drive processes suffer because gas channeling causes most of the oil reservoir to be bypassed and the oil left behind. The earliest work relative to the problem of lowering the mobility of CO2 does not involve CO2 at all. Because of the high potential for miscible drives that use enriched gas mixtures, considerable study was undertaken in the late 1950's on techniques to mitigate gas channeling. A few visionary investigators considered the use of foams as a possible solution to the problem. The earliest reported work was conducted by Bond and Holbrook, whose 1958 patent describes the use of foams in gas-drive processes. Because of the high cost of CO2 relative to crude oil during this period, CO2 processes were ignored. The use of foams in conjunction with CO2, was not contemplated until much later when rising crude prices revived interest in the CO2 displacement technique. CO2 exists as a dense gas or supercritical phase under reservoir conditions: therefore, experiments on controlling gas mobility are usually applicable to CO2 even though they may have been conducted with other gases such as nitrogen, methane, or even air. Concurrent with Bond and Holbrook's work, Fried, working at the USBM laboratory in San Francisco, demonstrated the potential of foam to lower the mobility of an injected gas phase. Fried's work was followed by some excellent work reporting an experimental technique involving in-situ foam generation promoted by injecting alternate slugs of surfactant solution and gas. Their patent related to the use of foam for mobility control in CO2 injection processes is especially pertinent. Laboratory work was encouraging enough that Union Oil Co. conducted a field test in the Siggins field, IL. Foam generation by alternate-slug injection and simultaneous gas-solution injection was tested. This test indicated that at concentrations below 1% the foaming agent, a modified ammonium lauryl sulfate, did not produce an effective foam. Above 1%, reduced gas mobility was obtained; however, at least 0.06 PV of surfactant solution had to be injected to achieve lasting mobility control. Since the tests were conducted sequentially, with the higher concentrations injected last, it is possible that the required amount of surfactant may be understated. A 0.1-PV bank might be more realistic for lasting mobility control. Their results also indicated that adsorption may reduce the effectiveness of a surfactant. It was suggested that future tests might benefit by selection of agents that are less strongly absorbed than ammonium lauryl sulfate. SPEJ P. 191^