Radioactive Tracer Absorption Chromatography In Geothermal Reservoirs
Authors: Vetter O.J. , Crichlow H.B. 
Published: December 2007

Radioactive tracers, such as tritiated water, iodides, cobaltic compounds, etc., are frequently used in subterranean reservoir studies. Their great advantages over non-radioactive (chemical) tracers are often outweighed by their large losses within the reservoir matrix due to undesired adsorption. The chemical tracers can also be adsorbed at a high rate. However, their high adsorption losses do not necessarily lead to large variations of the tracer retention times in the reservoir (chromatography effects) because of their larger number of molecules or ions in the liquid (mobile) phase if the (a) adsorption is governed by Langmuir type isotherms, and (b) the concentration in the liquid phase is within the flat portion of the isotherm (asymptote).

The adsorption effects experienced with both types of tracers make very precise interpretations of tracer data obtained in field studies almost impossible. For example, material balances have shown a larger acceptable degree of tracer recovery only if very severe and extreme reservoir heterogeneities are encountered. In real matrix flow, i.e., uniform or homogenous zones, the adsorption contributes significantly to the total dispersion of the tracer, thus creating a high degree of uncertainty in the data about various reservoir characteristics obtained from the tracer test in larger reservoirs. It becomes impossible to distinguish between the three main fractions contributing to the total mass dispersion:fluid-dynamics,diffusion, andadsorption/desorption.

A new method is suggested whereby not a single tracer test but a radioactive tracer cocktail is applied. Each individual tracer contains a different radioactive element incorporated into a chemical compound. These compounds have pre-determined adsorption isotherms. These isotherms plus the material balances and the variations in the retention times for the various chemical compounds (chromatography) will allow the determination of reservoir parameters not possible by any other reservoir tracer study.

Basically, this new tracer method employs the same ideas and techniques as those used in the generally accepted analytical laboratory method of high pressure adsorption liquid chromatography (HPLC). Only the objectives are different. In HPLC, a known volume of a mixture of unknown chemical compounds flows through a known, "calibrated" porous media. Determining the "pulses" of the separated chemicals as a function of time, pulse height and pulse shape allows the analytical chemist to determine the previously unknown mixture of chemicals.

In the described reservoir tracer method, a known volume of a mixture containing known chemicals at known concentrations flows through an unknown porous media. But now, the "pulses" of the separated chemicals (function of time, pulse height and pulse shape) will allow the precise description of the porous media itself. The basic principles of both methods, HPLC and "Tracer Adsorption Chromatography Method", are the same, although the methodology (applications) and the final evaluation methods are quite different.

The described new tracer method allows the precise determination of various reservoir heterogeneities and matrix properties. In addition, it allows an experimentally determined properties. In addition, it allows an experimentally determined differentiation between the three main factors contributing to the total fluid mass dispersion (fluid-dynamics, diffusion and adsorption/desorption). This tracer chromatography method, applied in a large reservoir, cannot be duplicated with non-radioactive tracers.

Introduction

Tracer tests to evaluate the flow patterns between injection and producing wells are common practice in oil and gas field operations. Gulati described the use of tritiated water in a geothermal steam reservoir. In some recent papers we described some general aspects of the use of single tracers for reservoir verification and monitoring in geothermal steam and liquid dominated fields.