Laboratory Measurement of pH of Live Waters at High Temperatures and Pressures

Abstract

This paper describes a new technique for measuring pH on live formation water samples in the laboratory at high temperature and pressure. The technique involves adding pH sensitive dyes to pressurized single phase water samples collected using a formation tester and spectroscopically determining the pH in the laboratory at reservoir conditions. Water chemistry and pH are important inputs for scale and corrosion modeling. Due to the lack of standard laboratory techniques for such measurements at high temperatures and pressures, current practice involves flashing the single phase water sample, analyzing the flashed water and gas phases, and then using water chemistry models to predict pH at reservoir conditions. Uncertainties in the thermodynamic models for formation waters at high temperature and pressure, as well as uncertainties associated with the flash process and possible precipitation of salts, can propagate as errors into scale and corrosion models. It is proposed that the direct pH measurement on live water samples described here be used as an additional input for the water chemistry models to improve confidence in their predictions. This will allow for more efficient selection of completion materials and planning for scale treatment and mitigation. In this paper, we present results of laboratory pH measurements on formation water samples from two offshore Gulf of Mexico wells for pressures to 20,000 psi and temperatures to 242°F. Results are compared to predictions from two commercial thermodynamic models that use the flashed gas and water analysis data as inputs. The setup can also measure the sensitivity of pH to pressure and temperature. Comparison of the laboratory pH measurement to real-time in situ downhole pH measurements made on the same formation water with a formation tester showed good agreement. This is an example of the implementation of the chain of custody concept, which compares a measurement made downhole with that made in the laboratory on the same sample using the same technique, to validate the representativeness of the sample as it is transported from downhole to the laboratory. Introduction Water sampling in exploration wells is usually done to obtain information regarding the scaling and corrosion potential of the water, understand reservoir connectivity, and establish the salinity of the water for petrophysical evaluation. Corrosion potential of the water is important for material selection for tubing, pipeline, and process equipment. Scaling potential is critical for the selection of an optimal development strategy to prevent scale formation by choice of operating conditions, to select and deploy scale inhibitors when needed, or both. Correct water resistivity is important for interpreting openhole wireline logs accurately. In addition, analysis related to various environmental aspects like concentration of organic compounds and heavy metals in water is performed. Dissolved organic acids also affect water chemistry because they can influence the pH of the solution. Good quality formation-water data can improve the ability to make the right decisions early in development planning. These data can give information about compartments and communication in the reservoir. Later in the production cycle, these data can be used to differentiate produced connate water from aquifer or injection water breakthrough.

Field Method for Determination of Bicarbonate Alkalinity

“…The parameters of the reservoir fluid system, such as pH, saturation indices of different minerals are vitally important to evaluate the potential corrosion or scale risks during the production process. Since in-situ pH is very expensive and difficult and pH will change with different temperature, pressure and other environmental conditions (Raghuraman et al 2007;Mathews et al 2009), pH of the fluid from reservoir to surface is usually estimated by the composition of brines, CO 2 partial pressure in gas phase and the flow rate (Kan et al 2013). The pH and calcite SI of each produced water sample at formation and wellhead were calculated by the ScaleSoftPitzer (SSP), the software developed by the author's group, to verify the accuracy of this new method (Kan and Tomson 2012).…”

Interrelationship of CO2, Weak Acids, Bases, and pH in Scale Prediction and Control

“…Mathews et al (Mathews et al 2009) compared the calculated and measured reservoir pHs of three Gulf of Mexico wells using modified technique than that of Raghuraman et al The temperature and pressure range of the three wells are 137.0-242.0 F and 9,766-19,543 psi (See Table 5). The new technique involves adding pH sensitive dyes to pressurized single phase water samples collected using a formation tester and spectroscopically determining pH in the laboratory after the formation water sample was reheated to reservoir condition.…”

“…Procedures used to measure pH of a surface sample are often of questionable reliability. In-situ measurement of live formation temperature and pressure are available (Raghuraman et al 2008;Mathews et al 2009), but can be expensive and the colorimetric method has not been calibrated to the temperature and pressure condition for ultra HTHP deep water wells. The detailed thermodynamic framework and the reason of the impossibility of measuring pH are discussed in the Appendix A.…”

“…Comparison of literature reported and ScaleSoftPitzer (SSP) calculated reservoir water pH and calcite saturation indices. The literature data were that ofMathews et al 2009.…”