POWDER SERVICE FLUIDS Field of the Invention The present invention relates to a well service fluid composition and to a method for improving the density of such fluids. Background of the Invention The general use of aqueous media, especially those containing petroleum reservoir brines, as well service fluids, such as drilling fluids, completion fluids, consolidation fluids, treatment fluids, etc., is well known. wells, fluids for underground formation tests, spacing fluids and well abandonment fluids. For all these uses, said brines suitably have a relatively high density. To achieve the latter, several components have been added to such fluids. Examples of such additives include heavy brines, such as zinc bromide or combinations thereof, for example, with hydrophilic polymeric materials, such as hydroxyethylcellulose. One of the problems that arise with such brines as currently used is that they contain zinc atoms that are not respectful with the environment. In addition, the polymeric hydroxyethylcelluloses are poorly hydrated, solvated or dispersed in aqueous systems containing water-soluble salts of multivalent metals. In order to solve the problems of toxicity, the use of non-toxic carboxylates, especially potassium format and potassium acetate, has been proposed. However, one of the problems that arise with a salt such as the potassium format is its high susceptibility to caking. This is particularly unsuitable in the case of attempting to transport solid potassium formate to the drilling point in order to prepare the well service fluid in situ, either as a new solution, with the desired density or to improve the density of the well. a service fluid in wells already used or exhausted in order to be able to recycle it. The use of conventional anti-caking agents either has an adverse effect on the density of the service fluid of the well or on the other components thereof, or it contaminates the environment or the recovered crude itself. SUMMARY OF THE INVENTION It is an object of the present invention to develop a composition which provides desired high density aqueous brine systems which are of a much lower ecotoxicity and which at the same time solve the caking problems experienced when using compounds such as potassium format as unique density enhancers. Therefore, the present invention consists of a well-serving service fluid comprising, in solution, a potassium format, characterized in that said solution also contains potassium citrate. DETAILED DESCRIPTION OF THE INVENTION Both the potassium and potassium citrate formats are readily commercially available. A feature of the present invention is that solid potassium citrate can be added to aqueous solutions of potassium formate to improve the density thereof during use as well service fluids. This is particularly important because although, theoretically, the solid potassium format itself could be added to such solutions to achieve an equivalent increase in density, the solid potassium format is much more difficult to manipulate in view of its tendency to cake . In comparison, potassium citrate is much less prone to caking and, therefore, can be easily handled to adjust the density of well service fluids at the point of use of such fluids. Thus, when a solution of potassium formate is mixed with potassium citrate, there is practically no adverse effect on the density of the potassium formate solution; on the contrary, the density of the potassium format solution is improved. This is very surprising because when an identical potassium formate solution is mixed with the same amount of potassium acetate or sodium formate, a density reduction of the potassium formate solution is achieved. The amount of potassium citrate that can be added to a solution of potassium format can vary over a wide range, since they are miscible in such broad ranges, but suitably it is from 5 to 30%, preferably from
to 25% by weight. Optionally, salts such as, for example, formate, acetate, propionate, oxalate, lactate, citrate and cesium tartarate may be added to the aqueous brines used in well service fluids. By varying the relative concentrations of the salts of carboxylic acids in the fluid it is possible to achieve the desired density of the brine or the final service fluid in which the brine is used. The formulations containing the carboxylate and hydroxycarboxylate salts described above are more convenient for improving the density since they are environmentally safe and user-friendly. Thus, using the compositions of the present invention, densities of 1.6 to 2.63 g / cm 3, preferably 1.8 to 2.5 g / cm 3 can be achieved. The well service fluids of the present invention can be prepared by mixing aqueous solutions of the individual salts. Alternatively, a mixture of the aqueous solutions d? the respective bases of the alkali metals, to which the desired carboxylic acid is added to form the respective carboxylates. The desired concentration of the respective salts in the aqueous solution thus formed can be adjusted by evaporating the water from the mixed solution. The present invention is further illustrated with reference to the following Example. EXAMPLE A control solution of 70% by weight potassium formate was prepared and to this solution (density 1.58) other salts (10% by weight each) were added to determine the effect of the incorporated salts on the density of the resulting solution. All the experiments were performed at room temperature. The results of these experiments are shown in the following table:
The above results show that the potassium format and potassium citrate are compatible and improve the density of the solution containing them. Although the addition of other aliquots of potassium formate also appears to provide a density similar to that achieved by adding potassium citrate, it is not convenient to add solid potassium formate at the well drilling point due to its tendency to cake.