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GEOL 463 Oilfield Water Notes(Temporary version: to be replaced)

Oilfield waters are routinely analysed for major ions and some trace elements. The main

ions analysed are Na+, Ca2+, Mg2+, K+, Cl-, SO42-, HCO3

-, CO32-. Also commonly analysed

are Br, Li, Fe, and SiO2. Isotopic analyses (O, C, Sr) are sometimes analysed for fluidtracing and provenance studies.

Ion concentrations are variably expressed as ppm, mg l -1, meq l-1, and mmol l-1.

mg l-1 = ppm / fluid density

For dilute solutions, therefore, where water density is nearly 1.0, ppm is almost the same

as mg l-1. For saline solutions, the dissolved salts increase the density of the fluid, so the

values are not the same.

It is useful to convert the fluid concentrations to milliequivalents, to test the validity ofthe analysis:

meq l-1 = mg l-1 x valence / molecular weight

The sum of the cations expressed in meq l-1should be the same as the sum of the anions.

For fresh waters, the difference in this balance should be < 5%. For saline waters, the

difference should be < 10%. If the balance is poor, then there is probably eitheranalytical error or important ions have not been analysed.

Saline waters are commonly expressed as mg kg-1or mmol kg-1because their fluid

density can be much greater than 1.0.

Subsurface waters can be classified as freeor irreducible. Free waters are normal

waters that pass, or have passed through the pore system of the rock. They included

waters recently in circulation (meteoric) waters, and connatewaters, which are no longerin circulation and may have remained immobile in pores for millions of years. Meteoric

waters may be fresh of saline; connate waters are normally brines. Formation waters

(oilfield brines) are the fluids that were in the formation before drilling. They aremostly connate, or connate water mixed with meteoric water.

Meteoric waters generally contain < 10 g l-1total dissolved salts (TDS). They may be

oxidizing or reducing, but do not generally increase in salinity with depth.

Connate waters have salinities of 20 to > 300 g l-1TDS (for comparison, seawater salinity

is 35 g l-1TDS). The salinity of connate waters generally increases with depth. Most

subsurface waters are dominated by Na+and Cl-. Compared to seawater they show amajor decreasein SO4

2- and HCO3-, a reversalof the Mg:Ca ratio (i.e. connate brines are

relatively enriched in Ca2+), and a major increasein ionic concentration. The increasing

salinity with depth is poorly understood, but probably involves:

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Dissolution of buried evaporites Filtering of ions in shale-sandstone units (ions concentrate in sandstones) Slow sinking of brines and upward displacement of less dense fluids

Water analyses can help in both exploration and production. In exploration, variations influid composition can help to delimit reservoir boundaries and reveal connectivity of

different strata. Saline waters are generally more favorable for locating petroleum

reservoirs. If the trapped fluid is saline, then there is less chance that any associated

petroleum has been degraded by contact with meteoric water, or flushed from thereservoir.