Conservative tracer tests in sandstones and carbonates using a cost-effective fluorescence method

Abstract

The use of inorganic ions as conservative tracers in porous media is limited by the need of complex and expensive analytical techniques, such as inductively coupled plasma (ICP), for tracer quantification. In this work, a cost-effective and straightforward in-house method was developed using fluorescence spectroscopy to quantify lithium in a high salinity brine, based on the formation of a complex with quinizarin (Li-Q). The method was adjusted to minimize interferences from Ca2+ and Mg2+ ions in the effluent, and to avoid extensive dilution steps in the quantification procedure. This approach yielded a sub-millimolar detection limit (8.2 × 10−5 mol L−1) in the conservative tracer tests performed using four different rock samples, namely two sandstone and two carbonate cores, at 13.8 MPa and 20 °C. The breakthrough/elution curves obtained for Berea sandstones showed distinct characteristics of homogeneous rocks, such as the sharp increase in lithium concentration, in contrast with the sluggish profile observed for more heterogeneous Indiana limestones. Moreover, the sensitivity of the method allowed to calculate the swept volumes for both cores, which resembled the rock pore volumes measured with nitrogen gas. These results compared well with those obtained using ICP-OES as a reference method (>95% agreement), which demonstrated that the proposed Li-Q fluorescence method can be used as an efficient alternative method to quantify lithium in conservative tracer tests for coreflood studies, significantly reducing time and cost of analysis, with potential to be used for field applications.