Fluorinated benzoic acids (FBA) have been widely used in the oil industry as conservative tracers. However, some of these tracers have been shown to rapidly degrade when tested at temperatures above 121°C within three weeks. Naphthalene sulfonates (NSAs) have been shown to be excellent tracers in geothermal applications. However, a broader study was required to determine tracer conservation in reservoir fluids and formations typically encountered in the oil field.
In this study we compare the oil field industry standard FBA tracers to NSA tracers under dynamic test conditions in the presence of reservoir oil, sandstone, carbonates and clays. We also compare the two sets of tracers under static conditions in the presence of four crude oils and different clay mineralogy to establish tracer conservation. Seven different sodium salts of naphthalene sulfonic acids were tested to determine if the tracers were adsorbed onto natural porous media (reservoir rock) at reservoir conditions. A broad range of conditions were selected to target typical reservoirs encountered. In addition, reservoir rock and a pseudo formation containing 10 Wt.% clay in silica sand were used in sand packs saturated with surrogate brine to ensure the tracer recovery under dynamic conditions.
High pressure liquid chromatography (HPLC-FLD) separation was used for simultaneous detection of seven NSAs while FBAs were analyzed using HPLC-UV. GC analysis of isopropyl alcohol (IPA) was used as a standard against which the others were measured.
Dynamic tracer tests demonstrated that the sodium salts of naphthalene sulfonates behaved similarly to the control, IPA, with none of the tracers adsorbing on to the rock surface or partitioning into the oil phase. The naphthalene sulfonates can be successfully used as conservative tracers most specifically for high temperature applications. NSA tracers are an attractive replacement for conservative FBA tracers in the oil field due to their superior thermal stability, solubility in oil field brine, lower detection limits and cost.
Partitioning interwell tracer tests (PITTs) have been used to estimate remaining oil saturations during waterflooding. Compared to core tests, well logs and single well tracer tests, PITTs sample a much larger representative elemental volume (REV) and provide interwell estimates of remaining oil saturation. The test has historically been used to estimate residual oil saturation (Sorw) after waterflooding between injector-producer pairs when the oil is essentially immobile. During polymer flooding, especially with viscous oil, additional oil is displaced and traditional means of interpreting PITTs are not valid. In this paper we present the information gained from conducting a polymer PITT and the saturation estimated during the PITT. This paper presents mechanistic insights into tracer and polymer velocities during the PITT and hence allows for an estimate of remaining oil saturation left behind after polymer flooding and also presents a new log-normal fit which can be used to match multiple flow path responses as is seen in actual field tracer data and reduce the error in estimates of remaining oil saturation. The polymer PITT therefore allows characterization of polymer flood efficiency and is a useful tool in polymer flood evaluations in heterogeneous reservoirs.