An API-sponsored work group has developed a robust field and laboratory method for determining the pH of oilfield completion brine. This method is precise, gives reliable pH values, is free from interferences, and can be easily implemented across industry. It is anticipated that this user-friendly method will be incorporated into the API Recommended Practice 13J: Testing of Heavy Brine.1
Field measurement of completion brine fluid pH and adjustments to the pH are critical. The primary pH-related concerns are formation damage, scale precipitation, corrosion of oilfield tubulars and rig equipment, rig-site filtration efficiency and brine quality. Mismanaged completion brine pH can also promote deleterious chemical reactions with the connate water or formation solids, and may promote oil emulsification. The solubility of completion fluid components and the effectiveness of additives can also be affected by pH. For example, hydrous zinc oxides can precipitate from zinc brine at pH readings above 4.
Completion fluid pH measurement has relied upon a variety of equipment and sample preparations, resulting in a wide variation in reported results. Sample pH has been measured using an assortment of pH probes and meters, or pH papers. Using pH papers for completion brine can be inaccurate because the pH paper may not be wetted sufficiently due to the hygroscopic nature of most completion brine. Completion brine and additives can contaminate, coat and dehydrate pH probes. Viscosifiers, solids and other additives can also negatively impact pH measurement accuracy by reducing ion mobility through the reference junction of pH probes. Improper handling and storage of pH probes can exacerbate these problems.
There is a perception that glass pH probes are inappropriate for brine because films and precipitates can form on the outside of the glass pH-sensing electrode. The reference electrode can become plugged, and the high ionic content of the brine can dehydrate the probes. These problems can lead to inaccurate readings, significant drift in readings and short electrode life span.
To address these problems, some procedures call for dilution of brine samples one-to-one or one-to-ten in water. The rationale is that pH is nominally -log [H+]; therefore, the effect of a 10-fold dilution can be compensated by adding or subtracting a value of one from the measured pH. However, in concentrated salt solutions, the H+ and OH- activities are often determined by their solubility products with salt ions, particularly divalent cations. Because dilution changes the salt ion activities; the pH changes in ways that are difficult to predict and correct.
As a wide variety of methods and instruments can give conflicting results for the same brine, the specification and control of brine system pH is difficult. Inaccurate pH measurements may lead to poor fluid performance. The scientific study needed to accurately measure hydrogen activity in dense brine is beyond the scope of this work. Nevertheless, an industry standard procedure is needed to provide consistent results and allow for specification and comparison of brine systems. API Subcommittee 13 charged Task Group 6 to develop such a standardized method.
This paper reports details of the research program, findings of a round robin exercise, and a description of the new method.
The theoretical pH value is a thermodynamic property equal to the negative logarithm of the hydrogen ion activity in aqueous solutions.2