Abstract

This paper presents the results of interwell tracer field work carried out in the McClesky sandstone in conjunction with a surfactant polymer flood pilot, a Dundee limestone pilot designated for an in-depth permeability modification pilot designated for an in-depth permeability modification treatment, and a Potter sand steam drive pilot. The objectives were to delineate fluid migration in the reservoirs and to compare tracer performance by co-injection. SCN was the best tracer in the sandstone with recoveries above 90%. In both the McCleskey and the Dundee, tritiated water recoveries were above 75%, with evidence of tritium - hydrogen exchange. Co and Co recoveries ranged from 35 to 55%. Tertiary butanol recoveries were near 75%, while about 50% of the injected isopropanol was recovered. The performance of ethanol, methanol and methyl ethyl ketone in the Dundee was similar to that of isopropanol. Residual oil saturations in the waterflood areas were measured using interwell tracers. The above alcohols and ionic salt tracers were used in the steam flood as vapor and water tracers, respectively. Strong signatures were observed for the alcohols, but the ionic tracer signatures were erratic.

Introduction

Secondary or tertiary oil recovery is accomplished through the injection of recovery fluids at selected points of a reservoir to mobilize and sweep residual hydrocarbons to production wells. In order to realize these recovery processes the target zone must have adequate continuity processes the target zone must have adequate continuity and uniformity in terms of fluid transmissibility and oil saturation between injection and production wells. The economic success will depend, in part, on how much of the reservoir volume can be contacted and swept by the injection fluids in a given time frame. Although seismic, geologic and depositional environment studies, as well as reservoir simulation, can provide very valuable insight into the feasibility of secondary or tertiary projects, the actual distribution of the fluid transmissibility in the reservoir must be evaluated from field data. The primary tools for in-situ studies of the transmissibility of a reservoir are in the areas of reservoir performance, pressure transient testing, and interwell tracer work.

Primary production histories will not provide much quantitative information about reservoir connectivity unless interference between producing wells is observed and analyzed. Production response to water injection may provide more definitive answers to questions about provide more definitive answers to questions about reservoir connectivity and if the waterflood is successful, most operators are satisfied with an analysis of fluid voidage, water salinity, production and pressure data. On the other hand, inspection of the literature (Ref. 1-6) shows that most tertiary recovery projects are preceded by extensive reservoir characterization which includes interwell tracer work.

Pressure transient interference testing between wells can be used to determine reservoir continuity, directional permeability trends and fractures. As shown by Mishra et. permeability trends and fractures. As shown by Mishra et. al. (Ref. 7), however, thief zones parallel to the bedding planes are difficult to identify with pressure transient planes are difficult to identify with pressure transient testing techniques as they yield an arithmetic average for the total transmissibility over the net thickness tested. Thus a pressure transient test yields no information about the range and variance of the permeability in the reservoir volume tested. Interference testing in an active field may also be difficult because of pressure transients due to infill drilling, workovers, and other field operations. The principal advantage of pressure transient work is that results are available in a relatively short time (i.e. days or weeks), while interwell tracer tests may require several months or even years to be completed.

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