Real-Time Downhole pH Measurement Using Optical Spectroscopy

Abstract

Summary A new downhole pH sensor has been developed to provide an in-situ pH measurement of formation water at reservoir conditions, and results are presented for two wells in the Norwegian Sea. The measurement technique, for use with wireline formation-sampling tools, uses pH-sensitive dyes that change color according to the pH of the formation water. To make a real-time pH measurement, the dye is injected into the formation fluid being pumped through the tool flowline, and the relevant visible wavelengths in an optical detector are used to record the dye signal and calculate pH with 0.1-unit accuracy. The pH of a formation fluid alters as the sample is brought to surface from the high-temperature and high-pressure conditions downhole, owing to acid gases and salts coming out of solution and changes in water-chemistry equilibria. To obtain an accurate pH, the measurement must be made downhole at reservoir conditions. Unlike potentiometric methods in which fouling of electrode surfaces by oil and mud is a potential problem, the dye technique is robust because the dye is isolated from the formation fluid and is injected into the sample only when a measurement is made. The technique has been applied successfully to both oil-based and water-based drilling muds, with successful measurements even in mixed oil/water flows. Multiple measurements of pH at a single sampling station demonstrate that the method is robust and repeatable. These measurements have been compared with numerical simulations using a multiphase chemical-equilibrium model that uses laboratory analysis of collected water samples as input. pH is a key parameter in water chemistry and is critical for corrosion and scale studies. Accurate downhole pH measurement allows a more-accurate selection of appropriate completion materials and more-effective planning for scale treatment and inhibition.