B.P. intends to use a desalination plant to reduce the salt content of seawater so that it can more effectively flush oil from the surfaces of the rock reservoirs that contain it. If the process can increase the amount of oil recovered by another 4%, from a current average of 35%, this will mean a substantial increase in output across the industry. Applied to the Clair Ridge field, which is in the North Sea, it is thought that, over the lifetime of the well, 640 million barrels of crude oil might be recovered using untreated sea water, but that 42 million barrels, over and above this, could be made available using the low-salt water. The cost of the desalination plant is $120 million, which will add $3 to the cost of a barrel of oil.
Crude oil contains a wide range of molecules, many of which are charged or polar. In the B.P. research laboratories at Sunbury on Thames, in the south of England, it has been discovered that the oil molecules form chemical bridges with doubly charged cations, such as Ca2+ and Mg2+ that are present on the surfaces of clay particles in sandstone. At the normal salt concentrations in seawater (3,500 ppm), the oil molecules are compressed close to the mineral surface, so preventing access by free cations that are necessary to displace the Ca2+ and Mg2+ cations and thus free the oil from the surface. As the salt concentration is reduced, the thickness of the thin film of water between the oil molecules and the surface increases, through an effect known as “expansion of the electrical double layer”, and permits access of free cations from the seawater. This releases the oil molecules from the rock surface.
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For the method to work, two criteria must pertain: (1) the total salinity of the water must be low enough to relax the electrical double layer compression, and (2) the dipositive cation concentration needs to be lower than that of the reservoir water. Together, these factors allow most of the oil to be released that is bound to the reservoir surface by this mechanism. B.P. uses water flooding to recover 60% of its oil, and it is predicted that the low-salt (LoSal®)method might increase production over the company’s holdings by 700 million barrels. It is salutary to note, however, that this amount is only enough to meet world demand for petroleum by some 8 days.
By. Professor Chris Rhodes
