Design and Evaluation of a Nitrogen-Foam Field Trial

Abstract

Summary This paper describes the planning, implementation, and evaluation of anN2-foam field trial at the Painter reservoir in Wyoming. Foam properties of aproprietary surfactant were measured in corefloods at reservoir conditions andmodeled with an empirical foam simulator. Foam injection into a dualinjector/producer temporarily reduced injectivity but was ineffective incontrolling N2 channeling. Introduction The laboratory objectives of this project were to determine experimentallythe effectiveness of foam in controlling N2 mobility at reservoir conditionsand to develop a simulator for predicting foam behavior in a reservoir. Theobjectives of the field trial were to determine the handling characteristics ofsurface-generated foam, to provide field data for validating the simulator, andto see whether foam could control N2 channeling at a dual injector/producer. The dual well selected for the field trial had an N2 cut of 50% and was instratigraphic communication with two offset wells where the N2 cuts weregreater than 20%. Laboratory work included surfactant screening, displacementexperiments to characterize foam properties, and development of a foamsimulator. Screening procedures identified a proprietary surfactant as anappropriate foaming agent. The surfactant produced a stable, low-mobility foamin the presence of Painter reservoir fluids. A permeability reduction factormeasured in the displacement experiments was used in the simulator to predictfoam injectivity and transport behavior in the reservoir. For the field trial,20,400 res bbl [3244 res m3] of 60%-quality foam was injected over a 7-weekperiod. N2 was mixed with a brine solution containing 0.5- to 1.5-wt%surfactant before going downhole. The resulting high-density foam reduced N2injectivity by a factor of 10, as predicted by the simulator. N2 injectivityrecovered rapidly after foam injection was completed. Well performance wasessentially unchanged after the foam treatment; there was neither a reductionin N2 cut nor an increase in oil production. More N2 channeling was evident inthe postfoam injection profile, possibly from overpressuring the well. Analysisof postfoam pressure-falloff tests showed that the effects of foam diminishedgradually over several months. The systematic approach taken in this projectcorrelated laboratory and field data with an empirical foam simulator. Thisapproach yielded a better understanding of the foam process, improvedinjectivity predictions, and a more complete evaluation of the field trial. Themethods described in this paper provide the basis for designing future foamtreatments. Background Discovered in 1977, the Painter reservoir is located in the Overthrust Beltof southwestern Wyoming. Hydrocarbon introduction is from the Jurassic Nuggetsandstone, which is approximately 1.000 ft [305 m] thick. The upper two-thirdsof the reservoir is gas condensate, and the lower portion is a light, paraffinic (44 API [0.81-g/cm3]) oil. Table 1 lists average reservoirproperties. We started pressure maintenance by N2, injection in 1980. The plan was toincrease reservoir pressure from 4,200 to 4,700 psig [29 to 32 MPa] so thatcondensate would miscibly displace the oil. N2 was injected in dual-completion(injection/production) wells along the crest of the formation. N2 broke throughmore rapidly than expected in the dual injector/producers and in offsetproducers that are in stratigraphic communication with injection wells. The N2channeling at Painter is attributed to stratigraphic communication betweeninjection and production wells, coning in dual injector/producers, and complexreservoir geology. Gas flows preferentially along the bedding planes ratherthan atop down or perpendicular to the bedding planes, as postulated forefficient miscible displacement. In the dual wells, large pressure gradientscombined with high N2 mobility cause gas coning, which shuts off oilproduction. This effect is accelerated if vertical fractures exist or ifproblems with the well completion are experienced. The reservoir geologyconsists of sand dunes that have directional permeability and uncertain arealextent. Permeability also varies with the grain size of the laminae that makeup a dune set. A foam-mobility-control project was initiated to evaluate thefoam's potential for controlling N2 channeling. JPT P. 504⁁