Seawater Depleted in NaCl as a ‘Smarter’ Enhanced Oil Recovery Fluid in Carbonates
a report by
S Jafar Fathi, Skule Strand and Tor Austad University of Stavanger
It is well documented that seawater increases the water wetness of mixed wet chalk at high temperatures. Improvement in oil recovery has been reported both by spontaneous imbibition and during forced displacement.1
It is verified that the interactions between the
active components Ca2+, Mg2+ and SO42- in seawater at the rock surface are responsible for the improvement in water wetness, and
a chemical mechanism has been suggested.2 It was experimentally
documented that adsorption of SO42- onto the positively charged chalk also increased the excess of Ca2+ close to the surface, which
probably reacted with the adsorbed carboxylic material and released
it from the surface. A similar combined effect with SO42- and Mg2+ also removed organic material. The suggested mechanism is illustrated by Figure 1. The reactivity of these ions towards the carbonate surface depends on the chemical activity in the double layer close to the surface. Both the size and the nature of the double layer are related to the salinity and ion composition of the aqueous phase.
The chemical mechanism for wettability alteration in carbonates and sandstones by using ‘smart water’ is different, as discussed in a recent paper.3
where SSW was diluted with distilled water to 1,600, 10,000 and 20,000 ppm were termed Dil-SSW1600, Dil-SSW10000 and Dil-SSW20000, respectively. For the chromatographic wettability test,
the core was first flooded with SSW without SO42-, the fluid termed SW0T, to remove SO42- from the core. Then the core was flooded with the fluid termed SW1T or SW1/2T, which contained a tracer,
SCN-, and also SO42-. Oil
An acidic reservoir of stabilised crude oil was diluted with 40 vol % heptane. The mixture was centrifuged and filtered through a 5 µm Millipore filter, and no precipitation of asphaltenic material was observed during storage. The acid number (AN) of the oil was determined to be 1.90 mgKOH/g, and the density was 0.8115 g/cm3.
Core Preparation
The chalk cores were prepared according to the method described by Puntervold et al.6
In carbonates, seawater with salinity of about 33,000 ppm acts as a wettability modifier, while in sandstones a low saline water, >2,000 ppm, can act as a wettability modifier and improve oil recovery in a tertiary waterflood. In order to further document the differences in the chemical mechanisms, oil recovery from chalk by spontaneous imbibition was studied by comparing oil recovery using modified seawater and ordinary seawater. Synthetic seawater (SSW) as smart water has been changed by adjusting the concentration of NaCl (zero, one and four times SSW salinity) while keeping the concentration of the active potential determining ions – Ca2+, Mg2+
and SO42- – constant and equal to the concentration in seawater. Furthermore, seawater was diluted to reduce the salinity to a low salinity range, ~1,600 ppm. It will also be confirmed that the increased spontaneous imbibition was related to an increase in the water wetness of the rock.
Materials and Methods Chalk Core
Outcrop Stevns Klint chalk was used as the porous medium with porosity and permeability in the range of ~45 % and 1–2 mD, respectively. The surface area is approximately 2 m2/g.4,5
properties of this coccolithic material are quite similar to the North Sea chalk oil reservoirs.
Brine
Artificial formation water (FW) similar to the Valhall field was used. As displacing fluid, SSW was used as the base brine. For the fluid termed SSW0NaCl, NaCl was removed, and the fluid termed SSW4NaCl contained four times NaCl compared with SSW. The fluids
© TOUCH BRIEFINGS 2011 The
Jafar Fathi is a PhD candidate and research fellow in reservoir engineering at the University of Stavanger (UiS) with special research interest in enhanced oil recovery/improved oil recovery methods. Prior to joining UiS, he worked as a reservoir engineer in the oil industry. He holds an MSc in reservoir engineering from the University of Calgary.
In order to remove initial soluble salts, especially sulphate, which can affect the wetting properties significantly, the cores were flooded with 250 ml of distilled water at 50ºC. The porous plate technique was used to drain the cores to an initial water saturation of about 10 %. In order to establish uniform oil saturation, the cores were flooded with 2PV of oil in both directions at 50ºC. The cores were wrapped with Teflon tape to avoid unrepresentative
Tor Austad is a professor of reservoir chemistry at the University of Stavanger. He holds a PhD in physical chemistry from the University of Bergen, awarded in 1974. Dr Austad has served as chairman for the chemical and petroleum board of the Norwegian Research Council. He has also been a consultant for Statoil on reservoir fluid properties. He has published nearly 100 journal papers and review articles, mostly related to chemical enhanced oil recovery.
E:
tor.austad@
uis.no
Skule Strand is a researcher at the Department of Petroleum Engineering at the University of Stavanger, Norway. The main subject of his research is within the field of reservoir chemistry and enhanced oil recovery (EOR) from carbonate reservoirs. Dr Strand defended his PhD in petroleum technology in 2006, and has since been author or co-author of more than 10 international journal papers, mainly on chemical EOR.
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Drill & Well Technology
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