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Mixing of produced water

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In situations in which two different waters are being mixed, it is desirable to measure the amounts of each in the mixed stream. If the capability exists, it is desirable to look at each constituent to see if it undergoes any phenomenon other than simple mixing. This can be a powerful technique for detecting water/rock reactions that can lead to formation damage.[1]

Mixing line technique

The fundamental concept is that mixing two waters should result in the volume-weighted average of each constituent of the two original waters, unless some chemical or biological reaction occurred. This is essentially similar in appearance to a binary phase diagram, with the endpoints of the line defined by the concentrations of the constituent in each of the water streams being mixed. For the technique to be useful, at least one species needs to be found that can act as a tracer for one of the waters. The requirements for this species are that it does not participate in chemical or other reactions under the conditions of interest; it has a relatively large difference in concentration between the two waters; and its analysis is easy and cheap with excellent accuracy.

Candidate species include boron, iodide, bromide, and chloride. The boron species is one that has been valuable in this role for reservoirs in several areas, including the North Sea and the North Slope of Alaska. Seawater is used as the injection water, with a typical boron content of 4.5 mg/L. Boron typically does not undergo any precipitation, dissolution, ion exchange, adsorption, or microbiological reactions and is stable in samples. Boron analysis by ICP is cheap, fast, accurate, and has detection limits of better than 0.05 mg/L with ≈1% relative standard deviation. The formation waters have original boron concentrations ranging from 15 to 160 mg/L; thus, boron can serve as a tracer for the formation water.

 ....................(1)

 ....................(2)

and  

 ....................(3)

With the use of this calculated index for the amount of formation (or injection) water, the concentrations of each of the species measured in the standard water analysis can be graphed. The pure formation and injection waters define the two endpoints of the mixing line. The analytical data for that species in each of the produced water samples are then plotted and compared with the theoretical mixing line.

Nomenclature

Xfm = the fraction of formation water in the mixed produced water,
[B]pw = the measured boron concentration in the produced water sample,
[B]fm = the boron concentration in the original formation water,
[B]inj = the boron concentration in the injection water,
Wi = the percentage of injection water in the mixed produced water,
Wf = the percentage of original formation water in the mixed produced water.

References

  1. Braden, J.C. and McLelland, W.G. 1993. Produced Water Chemistry Points to Damage Mechanisms Associated With Seawater Injection. Presented at the SPE Western Regional Meeting, Anchorage, Alaska, 26-28 May 1993. SPE-26045-MS. http://dx.doi.org/10.2118/26045-MS

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