ABSTRACT

ABSTRACT

Pore-fluid/rock chemical interactions on both short and long time scales can produce significant pore structure and mineralic changes that can markedly alter transport properties, such as fluid permeability. The introduction of the chemical factor requires modification of experimental systems and methodologies developed for measurements in which chemical effects are minimal. Particular attention must be given to temperature effects, fluid chemistry, fluid/rock reactions and kinetics, fluid flow rate, and fluid/experimental-system reactions. The ability to obtain very low, steady flow rates at constant fluid pressure is important for study of some fluid/rock systems. Minimization of metal corrosion is of paramount importance and requires choice of proper metals, such as titanium, and certain Ti-alloys and Ni-alloys. In this paper, we briefly discuss the relevant chemical factors affecting system and experimental design. The primary focus, however, is on the characteristics of highly corrosion resistant metals, thermal and chemical resistant jacketing materials, and the attributes and limitations of various flow-rate and back-pressure control systems and flow-rate measurement systems.

INTRODUCTION

There is an ever increasing need to develop models that predict variations of transport properties, such as permeability, associated with natural or man-induced pore-fluid/rock chemical interactions, particularly at elevated temperature and stress states. The introduction of the chemical factor, however, requires modification of measurement systems and experiment methodology developed for property measurements in which chemical effects are minimal. Particular attention must be given to factors related to fluid chemistry, fluid/rock reactions and kinetics, fluid flow rate, and fluid/experimental-system reactions. The elevated temperature state enhances reaction rates and modifies reactions, but it also introduces additional experimental difficulties, such as enhanced metal corrosion and changes or possible degradation of system-component materials, thus imposing physical limitations to their use or system capabilities. In terms of system design, special attention must be given to choice of corrosion resistant metals, sample jacketing materials, fluid flow-rate control and measure- ment, fluid-sampling systems, and back-pressure regulation systems. Specifics of experiment design also are influenced by reaction kinetics and scientific objectives. In this paper, we discuss many of these important aspects, with emphasis on details associated with fluid permeability systems and measurements.

CHEMICAL FACTORS RELEVANT TO SYSTEM AND EXPERIMENT DESIGN

The nature of the alteration of pore-structure and associated transport properties depends, in part, on the initial pore-fluid chemical composition, rock mineralogy and the P-T regime. In general, the choice of a working fluid is dictated by the specific geologic or man-induced conditions or process(es) being studied. System design requires anticipation of the probable range of fluid chemistries and temperatures to be used, which largely determine the choice of corrosion resistant metal(s) to be utilized in the system. The more chemically reactive natural pore fluids typically are saline, containing significant concentrations of chlorides and metal ions, such as Na, Ca, K, Mg. Halide ions, especially in the presence of certain metal ions, low pH and elevated temperature are particularly conducive to metal corrosion. Fluid corrosivity depends equally upon the pH and Eh conditions. Extremes in pH generally enhance corrosion of many metals. The role of Eh is metal specific.

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