Analysis of Salem Low-Tension Waterflood Test

Abstract

A test of a low-tension waterflood process was conducted in a single five-spot pattern in the Salem field. The presence of a regional pressure gradient across the pattern is believed to have caused migration of injected chemicals and displaced oil from the pattern area, resulting in less encouraging recovery than had been anticipated. Introduction A comprehensive discussion of the results and performance of the Salem Unit low-tension waterflood pilot has performance of the Salem Unit low-tension waterflood pilot has been presented by Whiteley and Ware and Widmyer et al. Their papers present an overview of the project and the basic data relating to the execution and performance of the pilot. As is frequently true of pilot tests, conditions and performance did not conform to expectations in all respects, and Widmyer et al. conclude that inadequate or incomplete pore flushing and high surfactant retention ted to lower-than-expected oil recovery. Although the data and methodology leading to their interpretation are presented, they also recognize alternate interpretations presented, they also recognize alternate interpretations that may apply to part of the data. This paper presents alternative interpretations of those results, particularly relating to surfactant adsorption and fluid flow in the test area. Specifically, the interpretations described here indicate that surfactant retention was significantly less than the value arrived at by Widmyer et al. and that the less-than-planned oil recovery resulted from migration of injected and displaced fluids out of the test pattern. This migration was affected by a prevailing pattern. This migration was affected by a prevailing pressure gradient across the test pattern and possibly was pressure gradient across the test pattern and possibly was aggravated by operating one or more of the injectors at pressures sufficient to sustain fracture-open conditions. pressures sufficient to sustain fracture-open conditions. Test Description The Salem low-tension waterflood test is being conducted in the Salem Unit, Marion County, Ill. The test reservoir is the Benoist sandstone, which essentially is at waterflood residual oil saturation (about 30 percent). Further details of reservoir characteristics have been reviewed previously. The experimental pattern is a 5-acre, normal five-spot centered in an existing 20-acre waterflood pattern. The secondary injectors are operated as backup injectors to confine the tertiary flood chemicals. There are two observation wells located on one of the direct lines between a tertiary injector and the producer to provide data on chemical transport, stratification, and oil provide data on chemical transport, stratification, and oil cut. The producer served earlier as the secondary producer; all other wells are new completions. Fig. 1 is a plat of the test area, The low-tension waterflood process is a multislug process; the slug sizes and constituents used in the Salem process; the slug sizes and constituents used in the Salem Unit are summarized in Table 1. Slug a-1 serves to displace the saline, divalent, ion-containing formation water that is not compatible with the surfactant. The carbonate and phosphate in Slugs a-2 and a-3 precipitate any remaining divalent ions and serve as sacrificial adsorbents to minimize surfactant retention. The sodium chloride provides a salinity level that yields optimum interfacial-tension behavior by the surfactant. Slug b, the surfactant slug, mobilizes the secondary residual oil. Slug c is a mobility-control slug that precludes fingering of the Slug d drive water through the surfactant and oil banks. Nonadsorbing tracer chemicals were injected into the injection quadrant containing the observation wells during Slugs a-2 and b and into the other three quadrants during Slug b only. JPT P. 1380

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