Why Such a Small Low Salinity EOR – Potential from the Snorre Formation?

摘要

Two cores from the Lunde formation in the Snorre field have been tested at reservoir temperature for enhanced oil recovery by the low saline water injection technique. In both cases, the cores were cleaned by successive flooding with toluene, methanol, and finally with 10 000 ppm NaCl solution. The method to obtain initial saturation of formation water, ≈20%, was different for the two cores. In the case of Core 13, the desiccator technique, and for Core 14, the porous plate method was used. The crude oil was saturated with CO2 at 6 bars in order to mimic reservoir conditions regarding initial pH of the formation water. The oil recovery tests were conducted by flooding the cores successively with formation water, seawater, and 500 ppm NaCl solution. The oil recovery by formation water was in the range of 50% of OOIP. A small incremental oil recovery of 1-3% of OOIP was recovered by switching from formation water to seawater, and no further increase in recovery was noticed when flooding with low saline NaCl solution. The results were discussed in terms of the previously suggested chemical mechanism for the low salinity effect, which was based on a local increase in the pH close to the clay-water interface, which should promote desorption of polar components by an acid-base reaction. The results are also related briefly to previously studies performed by SINTEF/Statoil. It was suggested that the adsorption of polar basic components from the crude onto the clay was partly prevented due to the high pH, about 7.5, of the formation water during aging. The high pH was related to the presence of more than 30 wt% of Plagioclase in the core matrix.