Hydrothermal Stability and Transport Properties of Optically Detectable Advanced Tracers With Carbonate Rocks in the Presence of Oil
Authors: Ow Hooisweng , Chang Sehoon , Thomas Gawain , Chen Hsieh , Al-Saleh Salah Hamad , Alotaibi Mohammed B. , Ayirala Subhash 
Published: January 2023

The use of tracer technology to illuminate reservoir characteristics such as well connectivity, volumetric sweep efficiency, and geological heterogeneity for the purpose of improving history-matching fidelity and enriching production optimization algorithms has gained momentum over the last decade. Herein, we report the stringent laboratory qualification of a novel class of fluorescent molecules, optically detectable down to ultratrace levels (&lt;ppb) in produced water, as competent crosswell water tracers for use in highly retentive carbonate reservoirs with harsh salinity and temperature requirements. Tracer molecules, with state-of-the-art fluorobenzoic acids (FBAs) as a benchmark, exhibiting requisite hydrothermal stability and nonretentive behavior in simulated reservoir conditions coreflood tests are scheduled to be field-trialed.

Our novel fluorescent tracer material systems, based on dipicolinic acid (DPA) and naphthalene sulfonates, rely on time-resolved luminescence and/or advanced chromatographic separation to eliminate the interfering fluorescent background issue in produced water for near real-time analysis. We systematically evaluated the novel tracer molecules at 95°C in high-salinity injection brine over 4 months, with periodic sampling and analysis by liquid chromatography to ascertain their hydrothermal stability. Coreflood tests at reservoir conditions were conducted to determine their interactions with carbonate rock surfaces with and without residual crude oil. All qualification tests were performed using a reference water tracer 2-FBA and/or a model partitioning tracer 4-chlorobenzyl alcohol as benchmark. Finally, reservoir simulations were performed to study both nonpartitioning and partitioning tracer transports in realistic field conditions.

Hydrothermal stability tests indicated that our novel tracers are stable for 132 days in brine under reservoir conditions. Coreflood tests without residual oil revealed that the novel fluorescent tracer materials, such as FBAs, exhibit negligible retention in carbonate rocks (almost 100% recovery of the tracers). Coreflood experiments with residual oil suggested that all tracer materials, including the FBAs, possibly reversibly interact with the rocks, resulting in lower tracer materials recovery. While the overall retention of tracer materials is minimal in the presence of residual oil, these values were found to be relatively higher to that measured without residual oil. We observed no significant change in core permeability due to tracer injection. Field-scale reservoir simulation sensitivity studies in companion with coreflood experiments indicated minimum interferences for consecutive tracer injections in the field trial settings. We believe this is the first time such direct comparative study has been performed in the existing research to evaluate the interaction of both water and partitioning tracers in carbonate rocks at reservoir conditions with and without the presence of residual crude oil.

Reducing the burden of analysis is critical in the implementation of this technology to obtain high-fidelity tracer data that can be used to improve waterflood optimization, increasing hydrocarbon recovery by a few percent per well without using additional resources for drilling or production. The ability to use presently commercialized tracer technologies, such as FBA-based molecules, in conjunction with this novel optically detectable fluorescent tracer platform will be a force multiplier to enable large tracer campaigns that provide high-fidelity tracer data for a production optimization algorithm.