It has long been known that only a portion of the oil can be recovered from a permeable oil-bearing subterranean formation as a result of the natural pressure of the reservoir. So-called secondary recovery techniques are used to force the oil out of the reservoir. The simplest method of forcing the oil out of the reservoir rock is by direct replacement with another fluid. Water-flooding is one of the most successful and extensively used secondary recovery methods. Water is injected, under pressure, into reservoir rocks via injection wells, driving the oil through the rock toward the production wells.
It has been reported that the salinity of an injection water can have a major impact on the recovery of hydrocarbons during waterfloods, with increased recovery resulting from the use of diluted brines (see, for example, “Labs Spin Out Oilfield Technologies”, American Oil & Gas Reporter, Vol 41, No. 7, July 1988, 105-108; “Effect of brine composition on recovery of Moutray crude oil by waterflooding”, Journal of Petroleum Science and Engineering 14 (1996), 159-168; and “Prospects of improved oil recovery related to wettability and brine composition”, Journal of Petroleum Science and Engineering 20 (1998) 267-276.
It is also known that the injection water used in a waterflood should be compatible with the formation water. Thus, underground formation waters can contain resident ions such as barium (e.g. at a level of up to 3000 ppm, for example 50-500 ppm) and usually also calcium (e.g. at a level of up to 30,000 ppm, for example 1000-5000 ppm) both in the form of soluble chlorides, but also in the presence of sulphate ions, so the water is saturated with barium sulphate, and usually also calcium sulphate. This formation water can meet seawater water, which can contain precipitate precursor ions such as soluble carbonate (e.g. at 100-5000 ppm) and sulphate (e.g. at 1000-3500 ppm). Mixing the two waters produces an aqueous supersaturated solution of barium sulphate and/or barium carbonate, and/or calcium sulphate and/or calcium carbonate, from which scale comprising these compounds deposits on surfaces. The meeting of the two waters can be in the formation, when seawater containing precipitate precursor ions is injected into the formation through an injection well at a distance from a production well to enhance oil recovery (i.e. a water flood treatment). The scaling may occur in the production well or downstream thereof e.g. in flow lines, or gas/liquid separators (for separating oil/water from gas) or in transportation pipelines leaving the gas/liquid separators. Carbonate scale may particularly form in the gas/liquid separator or downstream thereof, due to reduction in gas pressure causing soluble calcium bicarbonate to form insoluble calcium carbonate.
U.S. Pat. No. 4,723,603 relates to a process for reducing or preventing plugging in fluid passageways of hydrocarbon-bearing formations and in production wells which is caused by the accumulation of insoluble salt precipitates therein. This objective is achieved by removing most or all of the precursor ions of the insoluble salt precipitates from an injection water at the surface before the water is injected into the formation. Thus, insufficient precursor ions are available to react with ions already present in the formation to form significant amounts of the insoluble salt precipitates. The precursor ions of the insoluble salt precipitates are removed by means of a reverse osmosis membrane.
Pacenti et al describe a submarine seawater reverse osmosis desalination system in “Desalination” 126 (1999) 213-218. It is stated that conventional reverse osmosis (RO) systems have the disadvantage that they have to pressurize large amounts of water in the feed. The main operative differences of the submarine system concerns the fact that a high-pressure pump is required for pumping the desalinated water produced at great depth up to the sea surface while only a low-head circulation pump is needed for feeding seawater to the RO modules and for discharging the produced brine away from them. It is said that a prototype desalination unit will be immersed at a depth of approximately 600 m below sea level. The RO process will then be driven by seawater hydrostatic head pressure, and the produced desalinated water will be pumped through a tube from the submarine desalination unit to the sea surface via a specially designed pumping device. The technology is said to be particularly suitable for islands and remote coastal areas. However, there is no suggestion that this technology may be used for enhancing hydrocarbon production from a hydrocarbon bearing formation; and/or to provide desalinated injection water to prevent deposits of insoluble mineral salts in a hydrocarbon bearing formation and in an associated production well. Submerged desalination plants for producing potable water are also described in European Patent Application Number 0 968 755 and U.S. Pat. No. 5,366,635.
US Patent Application Publication Number US 2003/0230535 describes a method for desalinating saline aquifer water, the method comprising the steps of: providing a well extending from the surface into a saline aquifer, the well comprising a downhole membrane effective to desalinate or purify the saline aquifer water; allowing saline aquifer water to flow into the well from the saline aquifer; separating the saline aquifer water into a primary desalinated water stream and a secondary concentrated brine reject stream; and producing the primary desalinated water stream to the surface.
This method relies on the aquifer water flowing from the subsurface aquifer layer into a well, either by a natural pressure gradient between the aquifer and the well, or supported by a downhole pump. Thus, the method does not rely on hydrostatic head pressure to provide at least part of the pressure to overcome the osmotic pressure over the downhole membrane. Also, there is no suggestion that the primary desalinated water stream may be injected into a subterranean hydrocarbon bearing formation.
UK Patent Application GB 2 068 774 relates to an apparatus for desalinating seawater or brackish water by reverse osmosis where the osmosis cell or cells are located at a level sufficiently below the saline water supply and the brine discharge point so that the hydrostatic pressure resulting from the head provides the major component of the pressure at the saline side of the osmosis cell or cells needed to bring about reverse osmosis. There is no suggestion that the treated water may be injection into a hydrocarbon bearing formation to bring about enhanced hydrocarbon recovery.
Similarly, U.S. Pat. No. 4,125,463 describes a well system for desalination of salt water by reverse osmosis which takes advantage of the hydrostatic head of salt water therein for providing the differential pressure necessary for osmotic separation. Thus, a permeator assembly which includes one or more units having osmotic membranes therein is placed in a well at a depth such that the static head of salt water is great enough to create the necessary differential pressure for effecting the necessary osmotic separation. The concentrated salt water left over from the osmotic separation is allowed to exit the well. Again, there is no suggestion of injecting the salt-free water into a subsurface zone. Instead, the salt-free water is pumped to the surface of the well.