Effectiveness evaluation of fluorescent tracer in shaly-sandstone reservoir

Abstract

In petroleum exploration and production industry, tracer is a compound added to injected fluids, and can be easily detected even in a trace amount. Fluorescein sodium, a chemical tracer that can illuminate upon receiving a specific wavelength of light, was mentioned by several literatures due to its strong fluorescence at low concentration, non-toxicity, and non-biodegradability. In this study, fluorescein sodium in the form of fluorescein solution was evaluated with shaly-sandstone to observe their interactions.

The first half of this study focused on the evaluation of the chemical characteristics of fluorescein. First, the detection limit of fluorescein solution in deionized water characterized by fluorescence spectrophotometer was at 500 ppt. Fluorescein concentration at 10 ppm was a critical concentration to reverse the relationship between fluorescence intensity and fluorescein concentration. Fluorescence emission wavelength at maximum fluorescence intensity was a function of fluorescein concentration and stopped to decline as emission wavelength reached the wavelength of 512 nm at the concentration of 1 ppm. Appropriate storage for fluorescein solution was recommended to be inside an amber-colored glass bottle to prevent photodegradation at neutral to basic pH values. Fluorescein solution could be used in a petroleum reservoir with a temperature range of 30-70 degrees Celsius for 12 days without substantial thermal degradation.

The second half of this study focused on the interactions between fluorescein solution and shaly-sandstone. In the static investigation of fluorescein solution, it was found that the effects of shaly-sandstone on fluorescence dominated the presence of hydrocarbon and/or drilling fluids. Nevertheless, the effects of shaly-sandstone diminished as fluorescein concentration increased and this effect disappeared at the concentration of 100 ppm. As the concentration increased above 100 ppm, fluorescein adsorption onto shaly-sandstone occurred. In the dynamic investigation, the fluorescein breakthrough time of 1-ppb fluorescein solution was similar to the ideal breakthrough time for any liquid flowing through the movable pore volume of shaly-sandstone core sample. At higher concentrations of 100 ppb and 1 ppm, the breakthrough times were delayed possibly due to diffusion of fluorescein molecules into irreducible water portions of the core sample and fluorescein adsorption onto shaly-sandstone. Lastly, positively charged ions were found that they could amplify fluorescence intensity of fluorescein solution. Hence, at very low fluorescein concentrations in the presence of shaly-sandstone, such as at the concentration of 500 ppt, fluorescence detection would be possible. Therefore, appropriate fluorescein concentrations used in the pilot project could be in a range of 500 ppt to 1 ppb depending on the type of injected water.