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Spectroscopic ph measurement at high-temperature and/or high-pressure

Spectroscopic ph measurement at high-temperature and/or high-pressure description/claims

The disclosed method and apparatus relate to pH measurement of fluids using pH sensitive reagents and, more particularly, to a method and apparatus that allows accurate pH measurement at high-temperature and/or high-pressure.

Accurate measurement of pH is important in diverse fields such as process control, reaction kinetics, environmental and biomedical research and oilfield applications. Many chemical processes require pH monitoring and control at extreme conditions of temperature, pressure, and salinity. However, as will be described below, standard potentiometric techniques provide accurate measurements at moderate temperatures, pressures, and salinities. Measurements of pH of standard buffers at high-temperature and high-pressure using hydrogen and/or glass electrodes have been reported by LePeintre, Bull. Soc. Franc. Electr. 1960, 8, 584, and Kryukov, et al. as cited in pH Measurement: Fundamentals, Methods, Applications, Instrumentation VCH Publishers, 1991; however, liquid junction instability results in uncertainties in the measurement. Furthermore, pressure balancing needs and liquid junctions make it practically inconvenient to use hydrogen and/or glass electrodes for routine measurements in high-pressure, high-temperature systems. Boreng, et al. in SPE European Formation Damage Conference, May 13-14, 2003, The Hague, The Netherlands SPE 82199 describe a solid-state electrode for high-temperature and high-pressure pH measurement. While this proposed method eliminates the liquid junction uncertainty, the pH is measured relative to sodium activity that must be independently determined to determine the absolute pH.

Spectroscopic measurement of pH with very high accuracy using pH-sensitive dyes has been a well-established laboratory technique at ambient conditions since the early 1900's (Bates, Determination of pH: Theory and Practice, Chapter 6, John Wiley, 1964). More recently, this technique has been shown to improve precision for seawater and freshwater pH measurements over a range of ionic strengths where potentiometric techniques can prove to be problematic (Yao, et al., Environ. Sci. Technol., 2001, 35, 1197-1201; Martz, et al. Anal. Chem., 2003, 75, 1844-1850). These references cite the advantages of the spectroscopic technique with respect to low drift, reproducibility, and rapidness of the measurement as compared to the standard glass electrodes. Furthermore, because pH measurement depends only on the molecular properties of the indicator dyes, once the dye equilibrium dissociation constants have been characterized, the need for calibration prior to every measurement is eliminated. The methods described above allow implementation of the spectroscopic technique at close to ambient conditions and narrow ionic strength intervals corresponding to either seawater or fresh water conditions, however, the methods do not allow for implementation of the spectroscopic technique at high-temperatures and/or high-pressures.

Because of the lack of robust high-temperature high-pressure pH measurement techniques, currently high-temperature and high-pressure aqueous system equilibrium and the role of pH is characterized using chemical modeling of complex chemical equilibria to calculate the pH. In oilfields, it is important to know the pH of formation fluid to predict corrosion rates, scale formation, water compatibility, etc. Current practice involves collecting fluid samples in single-phase bottles, bringing them to surface and flashing them. The pH at high-temperature high-pressure downhole conditions is obtained by simulations that use ambient flashed gas and water phase analysis as inputs to chemical equilibrium models. This introduces errors in pH measurement due to sample handling, precipitation of ionic solids from flashed water samples and modeling uncertainties of complex ionic equilibria.

Spectroscopic measurement of pH relies on pH-sensitive dyes that can exist in an acid or base form. The optical absorbance spectra of pH-sensitive dyes change as they convert from their acid (A) to base form (B):

AB+H+  Eq. 1

The fraction of the dye present in the acid and base forms depends on the pH of the solution. The pH is calculated using the following equation:

pH =

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