Completion of oil wells with sand control screens in open hole is a simple and reliable method by to complete a reservoir section.  An oil well normally penetrates formations with varying production features, which, in spite of the fact that the sand control screens are closed on the inside, may cause that undesired well fluid by-passes on the outside of these and flow into the section.  Therefore, it may be desired to control or shut off sections, which do not produce desired well fluid.  This necessitates sealing the external annulus. 
Today such seal is achieved by application of inflatable, open-hole packers (external casing packers), which are pressurised by injecting a fluid, which is confined by means of a valve system.  As soon as the packer is pressurised, it is unable to follow movements in the face of the formation.  Further it is sensitive to changes in temperature and pressure, and there are often considerable problems to achieve a complete seal.  Another disadvantage is that the installation of the packer is expensive since well operations requiring complicated equipment are requisited. 
From U.S. Pat.  No.  4,137,970 a packer is known with an element which by a chemical swelling process result in expansion of the element upon contact with water present in the well at the moment the packer is introduced to the bore hole.  The packer element is employed in mining, where water is to be drained from an aquiferous layer above a clay layer.  The sealing consists of an expanding packer element.  During such a swelling process the packer element will initially expand fast, before it expands slower.  This is impractical in an oil well, since the packer will expand before it is placed in the final operating position in the well.  This implies that the packer may be put in the wrong position in the well, if it was to be employed in an application like the present invention and cause that the completion string can not be inserted to its planned final position.  Application of a medium swelling in water will cause the element to expand upon contact with all regular applied completion fluids or drilling fluids. 
From U.S. Pat.  No.  4,633,950 polymer particles are known suspended in a special water based carrier fluid, which by circulation pumping shall be injected into a lost circulation zone.  The patent does not relate to a packer element, but to a dispersion which shall trickle into porous/fractured rock.  The features of such a dispersion implies that it can not be held in place in order to form a solid plug in the annular space of the well.  Further, the particles will upon contact with hydrocarbons expand very rapidly due to the large surface area of the small particles.  Only minor impurities of remaining oil in the system will therefore result in an undesired early expansion.  Moreover, the particles in such a system will not expand at all if they do not contact hydrocarbons before the well is flowing back.  This may lead to the polymer being produced with the produced fluids. 
Most rubbers have a larger absorption capacity and faster swelling in an aromatic and/or naphtenic hydrocarbon than in an aliphatic hydrocarbon.  Most rubbers also have considerably less swelling in water based fluid than in an oil based fluid. 
Generally base-oils used in drilling fluids have a higher portion of aliphatic (80-100%) constituents than produced hydrocarbons, normally having 35-80% aliphatic constituents.  This implies that most rubbers will have a larger and faster expansion in produced hydrocarbons than in drilling fluids.