Page 183 - Primer on Enhanced Oil Recovery

Forefront EOR 173

It should be stated that the practise shows that the reservoir response to injected
low salinity water is very complex. Few processes with different names have been
developed and used over the time: Low Salt (LoSal), Smart Water Flooding, Ion
Tuning and Advances Ion Management. From the business point of view low salin-
ity water injection speeds oil recovery by allowing oil to flow more easily through
the rock and reduces amount of the injected water. As the reservoir development
stage, the low salinity water can be injected after high salinity water flooding or
even at the exhaustion of primary oil displacement drives. Number of applications
in low salinity water injection in every of the last few years exceeds combined
number of all method applications in the previous years all together.
Water salinity is an overarching term. Precise definition of it is not so simple
and straightforward. In many cases the term is used as a synonym if the Totally
Dissolved Salts (TDS) amount. Then it indicates how much (by weight) of various
salts are dissolved in water but it does not say which kind of salts are dissolved.
Salt definition and behavior go far beyond everyday used term and properties refer-
ring to table salt or sodium chloride. In a broad definition, a salt is a chemical com-
pound in which molecules contain cationic (positively charged) and anionic (negatively
charged) parts. During dissolution in water salts can produce predominantly hydroxide
ions (alkali salts), positive ions (acidic salts), or not change water pH balance (neutral
salts). One can immediately see that salts by themselves are far from simple.
There are many salinity scales which are based on the easily applied measure-
ment methods. The following information is provided just for a guidance. Some
broadly accepted brackets for salinity are: rain water 20 mg/L; drinking water
below 500 mg/L; river water below few g/L; sea water 20 1 g/L; connate
water more than 60 g/L. One can immediately see that almost any water available
at the ground level will have lower salinity than the connate (reservoir) water. One
can immediately see, and this has been proven true in practise, that even see water
can be regarded as low salinity compared to the most of connate water and can pro-
duce low salinity water effect to a point. Care should be exercises in assessment of
divalent ions.
In many cases water salinity is measured by water electrical conductivity.
Electrical conductivity measurements are used for well logging too. The reservoir
(connate) water salinity increases with the well depth as a rule. Low water salinity
from the deep reservoir will indicate the reservoir connection to the shallow water
supply. It has been mentioned before that many chemical EOR implementations are
sensitive to the reservoir water salinity. It should be stressed again that the connate
water salt chemical composition and salt amount are very important and need to be
determined before any EOR application.
The connate water mostly contains sodium, potassium, magnesium and calcium
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cations (Na ,K ,Mg ,Ca ). On the anion side it has chloride and sulfate radi-
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cals (Cl and SO 4 ). Other anions and cations are presented too.
It is clear, that a low salinity water contains much-much less ions compared to a
connate water. All surfaces and minerals containing water-soluble ions are affected
by injected low salinity water. Low salinity water significantly affects reservoir
clay. The effect is much clay specific as it depends on the clay type. The basic