Schlumberger

InSitu pH Reservoir Fluid pH Sensor

Overview Library

Measuring formation water pH downhole without contamination effects

The formation water pH is a key parameter in water chemistry, used for calculating the corrosion and scaling potential of the water, understanding reservoir connectivity and transition zones, determining the compatibility of injection water and formation water, and designating optimal salinity and pH windows for polymer and gel injections. Obtaining high-quality downhole fluid analysis (DFA) and samples of formation water relies on tracking mud filtrate contamination by distinguishing between formation water and mud filtrate in real time.

Optical pH analysis with 0.1-unit accuracy

Water pH is measured with the InSitu pH reservoir fluid pH sensor by injecting dye into the formation fluid being pumped through the flowline of the InSitu Fluid Analyzer system. The pH is calculated with 0.1-unit accuracy from the relevant visible wavelengths of the dye signal measured by an optical fluid analyzer. Making the measurement at reservoir conditions avoids the irreversible pH changes that occur when samples are brought to the surface, as acid gases and salts come out of solution with reduced temperature and pressure and routine laboratory flashing of the sample.

Robust measurement: Across the entire flowline cross section

InSitu pH sensor measures fluids across the entire flowline cross section, which makes it more robust than potentiometric methods of measurement, which are compromised when oil and mud foul electrode surfaces. Direct pH measurements with dye also avoid the limitations of resistivity measurement in monitoring contamination, which requires a sufficient resistivity contrast between the filtrate and formation water.

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Finding CO2 That Was Lost in Sample Flashing

InSitu pH measurement of pH = 3 indicated high CO2 that was missed in laboratory analysis conducted after the sample was flashed. With knowledge of the actual CO2 content, the operator could minimize subsequent corrosion and scaling problems.
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