| dc.description.abstract | In recent years, the focus on enhancing the oil recovery from oil fields has increased. This is because of falling oil reserves and the increased oil price. Because of this there is a big focus on enhancing oil recovery (EOR) mechanisms in the petroleum industry. In this thesis, the EOR mechanism low salinity waterflooding has been investigated both in a literature review and in experiments on sandstone cores from the Frøy field in the North Sea.
Usually injecting of saltwater into the reservoir is the main oil recovery technique. By doing this, the pressure in the field is kept high and the water can displace the oil. Freshwater is not used because the clay will start to swell and ruin the permeability in sandstones. In more recent years, laboratory experiments have shown that by decreasing the salinity of the injection water from 30.000ppm, which is typical saltwater salinity, to below 5.000ppm, oil droplets are released from the clay surface. The low salinity water affects the wettability and the interfacial tension leading to a reduction of the residual oil saturation. Pressure buildup is often observed during low salinity waterflooding.
In this project, experiments have been carried out on seven cores from the Frøy field in the North Sea. Most of the cores were high permeable in the range of 3-4D and had low clay minerals around 1,5-5%. The cores were prepared with crude oil from the Frøy field. During the waterflooding, the cores were flooded first with normal 100% seawater then followed by low salinity water. Five of the seven cores which were flooded with low salinity water showed increased oil recovery between 5,9percentage points (pp) and 13,9pp, giving a mean of 7,9pp after reducing the salinity of the injection water from 43.350ppm to 435ppm. The residual oil saturation was reduced with 6,2pp on average. Most of the increased oil production during low salinity waterflooding occurred after injecting 5-7 pore volume of low salinity water. It is therefore recommended to start low salinity waterflooding some time before oil production with normal seawater has ceased. After flooding, new air permeability tests were conducted. These tests showed that most of the cores had a reduction down to 30-40% of the initial air permeability.
Coreflooding by going directly to low salinity water showed the same recovery as flooding with normal seawater first. This shows that flooding with normal seawater does not affect the formation in a negative way before low salinity waterflooding.
The conclusion of this project is that low salinity waterflooding enhances the oil recovery on most of the sandstone cores from the Frøy field.
Since it varies which sandstone zones give an increased oil recovery by low salinity it is recommended to perform laboratory experiments on cores before considering low salinity waterflooding on a field scale. Further analysis of the effect of smectite in low salinity waterflooding is also recommended. | nb_NO |
