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Accurate measurement of pH is important in several diverse fields, such as process control, reaction equilibrium and kinetics, environmental research to monitor seawater chemistry and natural water quality, biomedical research and in oilfields. The International Union of Pure and Applied Chemistry (IUPAC) has issued guidelines for the standard potentiometric technique for pH measurements. These methods and calibrating standards are, however, recommended only for 278 -323°K, 0.101325 MPa and ionic strengths below 0.1 mol/kg water. 1 The reasons for these measurement constraints are the uncertainty in liquid junction potential and reference electrode stability at high temperatures, pressures and ionic strength and the lack of calibrating standards. Many chemical reactions and processes require pH monitoring and control at extreme conditions of pressure, temperature and salinity. In oilfield applications, accurate pH measurements require that they be made at downhole wellbore conditions with fluids in their native state at high temperatures (typically 323-473°K), high pressures (typically 30 to 100 MPa), and high ionic strengths (typically 0.5-5 mol/kg water). pH is a key parameter whose accurate measurement at downhole wellbore conditions is critical in understanding formation fluid water chemistry to predict corrosion and scale potential in well tubing and surface facilities.Spectroscopic measurement of pH with very high accuracy using pH-sensitive dyes is a well-established laboratory technique for ambient conditions since the early 1900's. 2,3 More recently, this technique has been shown to improve precision for seawater pH measurements. 4 Using equimolal tris buffers for total pH scales, dye equilibrium dissociation constant was characterized at 0.101325 MPa pressure as a function of temperature (293°K to 303°K) and salinity over the narrow salinity range characteristic of seawater (30 to 37 salinity range corresponding to ionic strengths of ϳ0.53 to 0.66 mol/kg water). Yao and Byrne 5 have also applied this technique for freshwater pH measurements, where potentiometric methods can prove to be problematic. Using phosphate buffers, the dye equilibrium

The spectroscopic technique for pH measurement is a well-established laboratory technique that can give high-accuracy pH values. Recent studies have shown the advantage of this technique over standard potentiometric methods for pH measurements in fresh water and seawater and also at high temperatures and pressures. However, a limitation of the spectroscopic technique is that a single pH dye is sensitive only over a narrow pH range. We have developed optimized dye mixtures that are both sensitive and accurate over a broad pH range. The measurement is robust and simple, requires a minimum of two wavelengths, and is independent of the volume of the dye mixture added. Optimization of the dye mixture formulation to maximize accuracy in a broad range of pH involves varying both the dye type and its mole fraction and also accounting for spectral noise. This technique has been successfully applied for in situ pH measurements of oilfield formation waters.

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In order to design oil production facilities and strategies, it is necessary to acquire crude oil samples from subsurface formations in oil wells in so-called openhole prior to production. In some environments, such as deepwater production of oil, decisions of huge economic importance are based on such samples. To date, there has been little quality control to verify that the crude oils collected in the sample bottles and analyzed up to a year later in the laboratory have any relation to the actual crude oils in the subsurface reservoirs. These high-pressure samples can undergo myriad deleterious alterations. Here, we introduce the chain-of-custody concept to the oilfield. The visible-near-infrared spectrum of the crude oil is measured in situ in the wellbore at the point of sample acquisition. This spectrum is compared with the spectrum measured on putatively the same fluid in the laboratory at the start of laboratory sample analysis. First, quantitative assessment is made of whether the fluid in the (high-pressure) sample bottle remains representative of formation fluids. Second, any specific changes in the spectrum of the fluid can be related to possible process control failures. Here, the entire process of chain of custody is proven. The chain of custody process can rapidly become routine in the petroleum industry, thereby significantly improving the reliability of any process that depends on fluid property determination.