The instant application relates to a xanthan concentrate, a process for preparing thereof and use thereof or a pre- concentrate - optionally supplemented with biocide - in enhanced recovery of oil from offshore subterrainian formations.

To increase the efficiency of water injection by enhanced oil recovery (EOR) polymers, e.g. the polysaccaride xanthan may be added to the injection liquid which is pumped into the sub¬ terrainian formations. At present xanthan is commercially provided as a fermentation broth, typically having 3% xanthan concentration, as concentrated fermentation broth from ultra filtration, typically having 9% xanthan contents and as xanthan powder containing 80-90% xanthan. The xanthan powder -is produced by precipitating xanthan from fermentation broth with a water miscible solvent, such as e.g. isopropanol, and then the precipited xanthan is dried and grinded to a powder.

The dried xanthan powder is suffering from a high content of micro gels or so called aggregates. These are bodies of several polymer chains. The formation of aggregates is supposed to be caused by a strong dehydration during the drying process, and that a species of cross bindings thereby are formed between the single polymer chains. For the use in polymer flooding it is critical that the xanthan which is used does not comprise substantial amounts of aggregates as these after being used for a period of time, will cause clogging of the porous reservoir and thus undesirable pressure accumulation. Such xanthan powder is therefore unsuitable for enhanced oil recovery since it is containing aggregates. Xanthan fermentation broth and fermentation broth which is concentrated by ultra filtration normally possess a low content of aggregates and are therefore more suitable for the use in enhanced oil recovery. The problem in connection with these product forms is the relatively low xanthan concentration (3-9%) involving great transport and storage problems when the xanthan is to be used offshore.

There is thus an unsatisfied demand for xanthan products which are suitable for use in enhanced oil recovery offshore.

The object of the present invention is thus to provide a product having a higher xanthan concentration (appr. 50%) than the xanthan obtainable by ultrafiltration, and which in addition does not contain substantial amounts of aggregates.

A further benefit would be if a biocide is already present in such a concentrate, such biocides always have to be used together with the xanthan in the proportion 1:1 - 1:2 to prevent microbial degradation. If the biocide is already pre- mixed in the xanthan concentrate before this is transported to a platform and the xanthan concentration at the same time is 10% or preferably higher, this would to a great extent solve the transport problem of the biocide.

The problem is solved by providing a xanthan concentrate which by dilution to use concentrations with water or brine as diluent enhances the oil recovery of oil reservoirs without forming pore clogging aggregates.

Preferably the xanthan concentrate in addition comprises a biocide. This biocide is preferably formaldehyde.

Further a processs is provided according to the invention for preparing a xanthan concentrate possessing the above mentioned properties from a pre-concentrate prepared by the precipitation of an aquaous xanthan fermentate with a water miscible solvent, whereby the water miscible solvent is stripped from the pre-concentrate in a water vapor saturated atmosphere.

Preferably the water miscible solvent is stripped by blowing through water vapour having a temperature in the range of 80-150°C.

Preferably water vapour having a temperature of 100-110°C is used. Normally the preferred water miscible solvent is isopropanol.

The pre-concentrate is prepared by the precipitation of a a xanthan fermentation broth or an already concentrated xanthan fermentation broth by adding to these liquids, dependent on the species containing 1-15% xanthan, an alchohol, e.g. isopropanol as mentioned above in a way well known per se. The precipitated

xanthan is subsequently dried by mechanically removing liquid (containing water as well as water miscible solvent, preferably isopropanol) .

The mechanical removal of the water containing liquid may in the laboratory be performed by simple manual squeezing, but can industrially be performed by careful filtration such as e.g. on a rotating drum filter scraping the filter cake off continuously, or by means of a screw press. According to the invention, the further concentration is performed by evaporation of isopropanol under a saturated water vapor atmosphere. The purpose thereof is to prevent too much water to be removed from the xanthan polymer thus forming aggregates. According to the invention the pre-concentrate or the final concentrate after the isopropanol evaporation is added a biocide solution compound, e.g. formaline, to enable a composition being stabilized against microbial degradation being transported to the site of use, e.g. platforms and being stored there until the dilution for use.

On the site of use the final concentrate may be diluted with sea water by use of shear forces to the desired xanthan and biocide concentration.

As indicated in figure 1, the xanthan concentrate of the present invention after dilution with sea water to the concen¬ tration of use after a certain amount of filtrate having passed a 5μm filter will yield a constant relative pressure accumula¬ tion, whereas redissolved xanthan powder results in a pressure accumulation increasing continuously with the amount of filtrate and thereby results in a clogging of the filter used in the test and consequently also the pores of the formation. The following examples elucidate the invention.

EXAMPLE 1

Xanthan fermentation broth comprising 2.6% xanthan was under stirring added 2-10 times the volume of isopropanol. The precipitated water/isopropanol-containing xanthan was then dried by mechanically removing liquid (e.g. by squeezing) , yielding a pre-concentrate having 26% xanthan concentration.

This pre-concentrate was then further concentrated at 100°C with a saturated water vapor stream till constant weight after 30 minutes. The product obtained had a xanthan concen¬ tration of 39%.

EXAMPLE 2

Concentrated xanthan fermentation broth (8%) was precipi¬ tated and dried as previously disclosed in example 1. Subsequent to mechanical removal of liquid, this pre-concentrate had a xanthan concentration of 36%.

The further concentration was performed according to the final part of example 1 and yielded a product having 48% xanthan.

The product was diluted with water and formaline to a 10% xanthan concentration and a 20% formaldehyde concentration.

EXAMPLE OF APPLICATION

The xanthan concentrates from exsample 1 and 2 as well as a pre-concentrate from example 2 were diluted by seawater to 0.04% xanthan concentration. Then the samples were filtered through 3 + 1.2 + 0.65μm millipore filters at high differential pressure (0.3-1.0 atmospheres) to remove cell debris. The filterabilities of these solutions were then tested by filtering through 3 x 5μm millipore filters at low differential pressures and shear velocities (4-8 s ^**1 ) and measuring the pressure increase against the filter as a function of filtered liquid volume (Fig.l). For comparison the xanthan fermentation broth was dried at 105°C to constant weight. The product was subsequently diluted to 0.04% i sea water and the filterability was tested as previously disclosed. The steep indicated curve of redissolved xanthan powder was obtained.

As previously mentioned, this test reveals that whereas the xanthan solution which was prepared by redissolving the xanthan powder yielded a continuously increasing pressure with increasing filtrate volume, the xanthan solutions being prepared by diluting the xanthan concentrate/pre-concentrate according to the present invention yielded an initially, but more slowly increasing rise of pressure, whereas a constant and

much lower pressure than that of the comparison product was obtained after a certain filtrate volume, appr. 250 pore volumes.

This test simulates the conditions of subterrainian porous formations and indicates that the products prepared by the present invention will allow passing of large filtrate volumes during displacement of oil without achieving a clogging of the formations and thus accompanying high pressure rise which again will result in interruption of the recovery.

The present inventive idea is expressed in the following claims: