Relative Permeability Studies of Gas-Water Flow Following Solvent Injection in Carbonate Rocks

Flow studies were conducted of 19 preserved cores from four oil-wetcarbonate reservoirs to provide data for evaluating the water-rich, gas-injection improved recovery process. Results indicate that these cores werewater repellent following displacement of oil by a solvent similar to thereservoir solvent. Restored-state tests of some of the same cores followingcleaning by a polar solvent yielded water-wet flow behavior. These resultsindicate that tests of preserved cores are required if water-gas flow dataapplicable to oil-wet reservoirs are to be obtained. Water-gas relativepermeability data also were obtained from preserved cores following bothcomplete and preserved cores following both complete and incompletedisplacement of oil by solvent. The presence of a small "bypassed" oilsaturation presence of a small "bypassed" oil saturation significantlyincreased the trapped gas saturation and reduced water permeability atflood-out. Use of these data in a mathematical model of the reservoir processgave reduced water injectivities (compared process gave reduced waterinjectivities (compared with those attained during water preinjection) similarto those experienced in the field. Introduction Increased emphasis is being placed on the application of secondary andtertiary recovery processes for increasing oil recoveries from known fields.One of these processes involves the injection of gas that is rich either inlight hydrocarbons or in carbon dioxide. Under appropriate reservoir pressureand temperature conditions, rich-gas injections can develop a solvent zone thatwill miscibly displace the oil it contacts in the reservoir. A dry scavenginggas usually follows the injection of some predetermined volume of the rich gas.Water also may be injected alternately with the rich or dry gas to decrease thegas mobility and, thus, improve reservoir sweep efficiency. Mathematical modelstudies of these recovery processes are made frequently. In the planning stagesof the improved recovery program, the model studies assist in sizing therich-gas volume to be injected and the subsequent dry-gas and water volumes.Once the program has been put into operation, the model may be used to matchfield performance (such as injectivity, pressure, or productivity), diagnoseunexpected problems, and extrapolate the future performance. Relativepermeability data applicable to a reservoir recovery process are a prerequisiteto reliable performance predictions with a mathematical mode. This means thatthe predictions with a mathematical mode. This means that the displacementsequence and mechanisms that occur in the reservoir must be known andreproduced accurately in the laboratory flow test. One important additionalrequirement is the need for reservoir core samples that adequately representthe producing horizon. This paper is concerned with the development of relativepermeability data for application to a recovery process of permeability datafor application to a recovery process of the type just described. Theparticular reservoirs of interest are West Texas carbonates. In thesereservoirs, rich-gas injection is being used as a secondary recovery process.However, water preinjection was necessary to process. However, waterpreinjection was necessary to increase reservoir pressure to a level such thatmiscibility could be achieved between the reservoir oil and the subsequentlyinjected rich gas. Because of this water preinjection, the displacement processis essentially the same as would occur in a tertiary application (that is, after waterflooding). Thus, relative permeability data were obtained forseveral different displacement conditions, selected specifically to model thereservoir process shown schematically in Fig. 1. The stepwise simulation ofthis recovery process in laboratory flow tests is discussed later. Ofparticular interest in this study were the relative permeabilities applicableto the water-displacing-gas process, permeabilities applicable to thewater-displacing-gas process, permeabilities that are needed to model thegas-water injection permeabilities that are needed to model the gas-waterinjection process following solvent displacement of oil. process followingsolvent displacement of oil. SPEJ p. 23