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 -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.
Introduction
The main objectives of formation-water sampling in exploration wells are to
obtain information regarding the scaling and corrosion potential of the water
and to establish the salinity of the water for petrophysical evaluation.
Formation-water data can also give information about compartments and
communication in the reservoir and, hence, can improve the ability to make the
right decisions early in development planning. Later in the production cycle,
formation-water data can be used to differentiate produced connate water from
aquifer- or injection-water breakthrough.
Ideally, water samples from exploration wells should consist of
representative, uncontaminated formation water, which can be difficult and
costly to obtain. The quality of formation-water data is highly dependent on
the sampling technique and the type of drilling mud used in the reservoir zone.
Oil-based drilling muds will usually provide good-quality water samples because
the mud filtrate is not miscible with water. Water-based-mud filtrate can
contaminate water samples because the filtrate is miscible with formation
water, and chemical reactions can alter the true composition.
Reservoir water samples are usually collected in open hole with a wireline
formation-testing device equipped with a probe or packer module, pumpout
module, and sample chambers. At the start of the sampling process, fluids
flowing from the formation are largely contaminated with drilling-mud filtrate
and are disposed of by pumping into the borehole in the “cleanup” phase.
Optical fluid analyzers or resistivity sensors are typically used to monitor
filtrate contamination in real time, and sample collection into sample chambers
begins once specific criteria to ensure representative water-sample quality are
satisfied. Tracers are sometimes added to drilling fluids to allow calculation
of the residual filtrate contamination in collected samples by subsequent
laboratory analysis.
Because of the lack of standard high-temperature/high-pressure laboratory
analytical techniques for water analysis, collected water samples are first
reconditioned to downhole temperature and pressure and then flashed to ambient
conditions before laboratory analysis. The gas/water ratio (GWR) is measured,
and the composition of the liberated gas is analyzed by gas chromatography. Ion
composition, pH, and low-molecular-weight organic acids are analyzed in the
water phase. The gas and water analysis and the GWR are entered into a
multiphase equilibrium model to predict the downhole pH. These data then can be
used to predict corrosion potential, scale potential, and hydrate-formation
potential.
© 2007. Society of Petroleum Engineers
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History
- Original manuscript received:
21 November 2005
- Revised manuscript received:
26 September 2006
- Manuscript approved:
20 March 2007
- Version of record:
20 June 2007
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