Improved tracer techniques for georeservoir applications

Abstract

For an efficient and sustainable use of georeservoirs, optimal reservoir management procedures are required. Such procedures often rely on tracer tests. Due to in-situ interactions between the tracer and the reservoir, recorded tracer signals contain an integral signal of the reservoir properties. For this reason, tracer test application offers a powerful technique for the characterization and observation of georeservoirs. This is especially true when reactive tracers are used, because the dynamic approach involved is unlike the established routines that are used with conservative tracers. Yet, the analysis and interpretation of reactive tracer signals is often biased, misguided or downright impossible due to the application of inappropriate physicochemical models, or false assumptions regarding tracer-behavior in the reservoir. The use of a selective and specific reaction pattern allows the diagnostic capacity of reactive tracers to be expanded to include accurate estimates for such significant reservoir metrics as (for example), heat exchange surfaces and prevailing temperatures. In this thesis, hydrolyzing tracers, which are characterized by their reaction with water, were examined. As thermo-sensitive tracers, they provide information regarding in-situ temperatures and cooling fractions, as expressed by means of the observed concentrations in the recorded response curves, for which, the well-known Arrhenius parameters for the employed pseudo first-order reaction are necessary prerequisites. Theoretical and experimental investigations with regard to this group of tracer-compounds are elaborated here, in order to mitigate interpretation uncertainties with their application in the field. Controlled laboratory experiments are conducted, in order to investigate the sensitivity and practical limitations of thermo-sensitive tracers. The designed experimental setup consists of two consecutively connected columns: both of which are sand-packed and heated to a specified temperature using a rapid-flow water bath, enabling the assessment of various thermal setups. Different experimental schemes are applied to mimic such various field scenarios as: Flow-through, Moving Thermal Front and Push-pull. The tracers are either injected continuously, or pulsed. Furthermore, the employed compounds have fluorescent properties which allow online measurement. Not only do the results of the lab experiments confirm the inherent expectations of the underlying theory, but when the pH dependency of the hydrolysis reaction is considered in the analysis, reservoir temperatures can be estimated with a precision and accuracy of up to 1 K. Such estimates are not influenced by variability in residence time or measured concentration. Furthermore, it is also possible to derive an estimate for the fraction of cooling when different temperatures are applied to the columns. Finally, under controlled and well-defined laboratory conditions, the effective application of thermo-sensitive tracers is, reliably achieved for the first time since their introduction. An additional application of hydrolysis tracers is also proposed. How effectively CO2 is trapped in water, by solubility trapping for carbon capture and storage applications, is determined by the interface area between CO2 and the formation brine in deep reservoirs. With the employment of target-designed kinetic interface sensitive tracers (KIS-tracers), the respective dissolution properties of the tracer and the reaction product, in both the organic phase and in water can be combined with the hydrolysis reaction at the interface to potentially estimate the interface area. In addition to the basic concept of, and requirements for KIS-tracers, an initial laboratory experiment is presented which demonstrates the successful molecular target design and provides an experimental basis for the development of a macroscopic numerical model. Then, the related numerical simulations are implemented to examine the interplay of KIS-tracers with a dynamic interface area in various hypothetical scenarios. Due to the temperature dependent reaction-speed of hydrolysis tracers, thermal transients are typically observed. Hence, the last part of this thesis endeavors to further exploit the available information from such recorded temperature signals. For an idealized single-fracture system, a set of analytical solutions is discussed; spatial and temporal profiles are derived for thermal single well injection/withdrawal experiments. With the application of a mathematically efficient inversion method known as “Iterated Laplace Transform”, computationally efficient real-space solutions can be obtained. With the introduction of three dimensionless numbers, the ‘return profiles’ can be analyzed for fracture-width or heat-transport rate, variable pumping/injection rates and nearby spatial observations. Finally, an application of the aforementioned inversion method is used to derive a set of kernel functions for nonlinear optimization algorithms. The presented work narrows the existing gap of tracer choice and field application: helping to mitigate future planning and analysis uncertainties, while demonstrating a sensitivity to resolve temperatures, cooling fractions, liquid/liquid interface, fracture-width and heat transport rate. The capability to estimate an expanded set of reservoir metrics, as made possible by the presented tracer concept/methods, promises to enhance reservoir management procedures. The experimental results, in combination with the new analytical models, afford a more comprehensive insight into the collective role of the parameters controlling hydrolysis reaction and heat transport in georeservoirs.