Aqueous elements

Application

The elemental composition of formation waters can provide valuable information for reservoir management. Initial composition depends on mineralogy and diagenetic conditions, but during production infiltration by other waters, including injection water, can affect production, not least through the possibility of scale formation.

Assuming the initial composition of brines broadly reflect those in normal seawater, variations from this norm potentially reflect addition and removal processes during diagenetic reactions within the reservoir.

The following 12 species are analysed: Sr, Ba, Na, Mg, Si, S, Cl, Ca, Mn, Fe, Zn and K. Inorganic C (H2CO3, HCO3 and CO32−) is mostly driven out of solution by the acidification with nitric acid required to stabilise the other ionic species, so, along with nitrate, is not analysed.

It is essential to know details of drilling mud or coring/plugging composition throughout the reservoir section to avoid erroneous interpretation. For example, the use of barite will contribute Ba2+ and SO42–.

To investigate whether a sample is likely to be representative of formation water, the initial phase of interpretation should incorporate a charge mass balance of the principle cations and anions.

Sample requirements

Water samples of at least 1 mL are required. As well as free water, samples can also be obtained from wet core plugs by centrifugation (10000 rpm is used by APT).

Analytical procedure

Samples are initially filtered using polypropylene syringes fitted with 0.2 µm PTFE filters. Samples were diluted to an elemental concentration between 1 and 100 µg/L in a matrix of 1% HNO3 containing 10 µg/L of In as an internal standard. Elemental analysis is then performed using a Thermo Scientific Element XR high resolution ICP-MS, with the sample being introduced using a self-aspiring quartz nebulizer, a quartz cyclonic spray chamber, a quartz injector and an ESI SC autosampler system.

Linear calibrations with elemental concentrations between 0.01 to 100 µg/L were created using 6 different dilutions of certified standard solutions, traceable to NIST, containing the elements of interest in a matrix of 1% HNO3 and 10 µg/L of In as an internal standard. A calibration check solution (different from the calibration solution) and a matrix matched certified reference material were analyzed every 10 samples.

A duplicate sample and a sample spiked with 10 µg/L of the elements of interest were also analyzed every 10 samples to ascertain the variability and accuracy of the elemental analysis. Instrumental detection limits were calculated as 3 times the standard deviation of ten replicate analyses of a 0.1 µg/L solution.

Potential problems

  • samples from centrifugation of core plugs may not be representative of formation waters, particularly if small
  • drilling fluids can have a marked effect on the concentration of some species

References

Warren E.A., Smalley P.C. (1993) The chemical composition of North Sea formation waters: a review of their heterogeneity and potential applications. Petroleum Geology Conference Series 4, 1347–1352. Geological Society, London.