Abstract
Enhanced oil recovery in carbonates due to wettability alteration has received much attention recently. In contrast to sandstones reservoirs, carbonates are neutral-wet or preferentially oil-wet. The use of CO2 dissolved into water has emerged as an alternative method for EOR operations in carbonates reservoirs. In the present paper, oil recovery from two carbonate outcrop rocks was evaluated with different brines containing CO2. The evaluation included a qualitative approach by spontaneous imbibition tests and a quantitative assessment by the Amott–Harvey index. The rocks tested were limestone and dolomite samples known as analogous to Brazilian pre-salt reservoirs. The testing fluids were a medium gravity crude oil, seawater concentration brine, formation equivalent brine and the carbonated version of these brines. Results shown additional oil recovery directly associated with wettability alteration driven by brine concentration switches. Oil recovery increases were observed independently if the brine concentration decreased or increased over the water replacement process. Wettability alteration took place in both carbonate types starting at oil-wet to neutral-wet conditions. Similar tests carried out with equivalent carbonated brines showed similar alterations in the wetting properties, trending no dependence on salt concentration. However, CO2 and its derived ions dissolved in the brines seem to inhibit the wettability alteration mechanism.
Highlights
The most widely used technique to obtain additional oil recovery from offshore reservoirs is the injection of seawater or a mixture between seawater and formation water
Carbonated seawater injection (CWI) as an EOR method has become more interesting in offshore fields with access to CO2 as well as in cases in which the crude oil has an important amount of CO2 in the associated gas
The second is that, for the cores and conditions used, the brine concentration does not play a significant role in oil recovery, given the similar ultimate recovery efficiency in all tests
Summary
The most widely used technique to obtain additional oil recovery from offshore reservoirs is the injection of seawater or a mixture between seawater and formation water. The technique lines up the proved practice used for pressure support with the purpose of reducing both facility requirements and environmental impacts. Such brine mixing was the field practice calling the attention to the theme of injection water composition. Carbonated seawater injection (CWI) as an EOR method has become more interesting in offshore fields with access to CO2 as well as in cases in which the crude oil has an important amount of CO2 in the associated gas. There are limited experimental data showing the advantage of use CWI as EOR method in carbonates
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