A MIXED FORMATE AND BROMIDE BRINE USED IN DRILLING FLUIDS The present invention relates to the use of a saturated aqueous solution, comprising at least one formate salt and at least one bromide salt, in drilling fluids Well servicing fluids, such as drilling fluids, for land based as well as offshore oil drilling have normally and for many years been oil based. Changes in the environmental legislation have forced the industry to seek other types of drilling fluids and a frequently applied solution is the use of a pseudo oil based mud, which is based on synthetic hydrocarbons. However, these systems are still not proven to be environmentally fully acceptable and their future is uncertain. The main concern is biodegradation, both aerobic and anaerobic. Common to all is that they do affect the environment, either by staying on the seabed for many years or by degrading rapidly with short but more drastic effects on the environment.

However, not only environmental requirements have to be considered when selecting a drilling fluid. Also technical requirements have to be fulfilled, such as high solubility, high density, good temperature stability, good hydrolytic properties, shale stability, tolerance to contamination, material compatibility, reservoir compatibility and recycling possibilities.

Conventional water based drilling fluids can often not compete with the oil based or pseudo oil based systems in most of these areas. Drilling fluids based on brines, such as formates or bromides, however, have been found to fulfil all of the above mentioned requirements.

Bromides give in general higher water solubility and higher densities than formates, but are however more expensive and less environmentally safe.

Low solids drilling fluids based on formates were originally designed to minimise frictional pressure losses in slim hole drilling. In addition, their unique capability of stabilising polymers to high temperatures made them more temperature resistant than any other polymer based drilling fluids. Formates and bromides are, because of their high density and low corrosivity, ideal completion and packer fluids and are compatible with reservoir fluids, good shale stabilisers, gas hydrate inhibitors and scale dissolvers. Formates have for instance also been shown to be environmentally more acceptable than other commonly used brine system, shale stabilising and compatible with reservoir fluids and common drilling equipment materials. At higher densities, however, the formates are the only known brines that can increase the transition temperature of xanthan.

It has from the present invention quite unexpectedly been found that mixed brines comprising at least one formate salt and at least one bromide salt advantageously can be used in drilling fluids, such as oil well drilling fluids. The total solubility, that is the total salt content and the density of the brine, at a given temperature is for the mixed brine according to the present invention higher than the solubility for respective salt.

The brine use according to the present invention is a saturated aqueous solution comprising 40-50% by weight of water and 50-60% by weight of salts. The salts are at least one formate, preferably an alkali metal formate, and at least one bromide, likewise preferably an alkali metal bromide in a weight ratio formate to bromide of 30: 70 to 60: 40. Said saturated aqueous solution has a higher salt content and a higher density than can be obtained with respective included salt per se and most important higher than a saturated aqueous solution of included bromide salt.

The salts of the aqueous solution used according to the present invention are as previously disclosed preferably salts of alkali metals and may thus include formates and bromides of the same or different alkali metals or combinations of different alkali metal formates and/or bromides. The preferred alkali metals include potassium, sodium and/or caesium. The most preferred alkali metal is sodium.

Saturated aqueous solutions of sodium formate and sodium bromide are suitably prepared at a temperature of 0-50°C giving a brine density of for instance 1.50-1.60 g/cm3. A preferred temperature range is 5-15°C, such as 8-12°C, whereby the density of the brine at said temperature is 1.51-1.54 g/cm3. A further suitable preparation temperature is to be found within the range of 15-30°C, such as 20-25°C, whereby the density of the brine at said temperature is 1.53-1.57 g/cm3.

Advantages obtained by using the saturated aqueous salt solution of the present invention in comparison to the use of bromide salts can be summarised: -a higher density than for included bromide salt per se, -higher total salt than obtained with included bromide per se, -formates has a stabilising effect on for instance thickening agents, -formates increases the high temperature stability, -formates are less expensive than bromides.

These and other objects and the attendant advantages will be more fully understood from the following detailed description, taken in conjunction with Examples 1 and, wherein Example 1 discloses preparation at 22°C of saturated aqueous salts solutions comprising sodium formate and sodium bromide and Example 2 is Example 1 repeated at a temperature of 10°C. While particular embodiments of the invention is shown, it will be understood, of course, that the invention is not limited thereto since many modifications may be made, and it is, therefore, contemplated to cover by the appended Claims any such modifications as fall within the true spirit and scope of the invention.

Example 1 Five (5) saturated aqueous solutions, four (A-D) being inside the scope of the invention and one (E) being outside said scope, comprising sodium formate and sodium bromide were prepared by mixing in 40% by weight of water: A) 15% by weight of sodium formate and 45% by weight of sodium bromide, B) 25% by weight of sodium formate and 35% by weight of sodium bromide, C) 30% by weight of sodium formate and 30% by weight of sodium bromide, D) 35% by weight of sodium formate and 25% by weight of sodium bromide, E) 45% by weight of sodium formate and 15% by weight of sodium bromide, The mixtures were under stirring allowed to obtain a solution equilibrium at 22°C.

Undissolved salts were now removed by filtering in a tempered (22°C) filter and analysed: Solution no. Sodium Formate Sodium Bromide Total Salt Content Density Content,%-w/w Content, %-w/w %-w/w g/cm³ A 17. 7 36.8 54. 5 1. 545 B 26.2 31.9 58. 1 1. 56F-= C 26.0 30.4 56.4 1.561 C 26. 0 30. 4 56. 4 1. 561 D 29.0 27.9 56.0 1.532 E 36.2 17.4 53. 6 1.457 1 Following values are valid for respective salt at 22°C: Solubility in Water Saturated Aqueous Solution Salt %-w/w Density g/cm3 Sodium formate 45*1 l. 318*2 Sodium bromide 47.8*2 1. 520*2 *1 According to Groschuff, Ber. 36,1788 (1903) *2 According to Perstorp Polyols, 1997 Example 2 Five (5) saturated aqueous solutions, four (A-D) being inside the scope of the invention and one (E) being outside said scope, comprising sodium formate and sodium bromide were prepared by mixing in 40% by weight of water: A) 15% by weight of sodium formate and 45% by weight of sodium bromide, B) 25% by weight of sodium formate and 35% by weight of sodium bromide, C) 30% by weight of sodium formate and 30% by weight of sodium bromide, D) 35% by weight of sodium formate and 25% by weight of sodium bromide, E) 45% by weight of sodium formate and 15% by weight of sodium bromide, The mixtures were under stirring allowed to obtain a solution equilibrium at 10°C.

Undissolved salts were now removed by filtering in a tempered (10°C) filter and analysed: Solution no. Sodium Formate Sodium Bromide Total Salt Content Density Content,%-w/w Content, %-w/w %-w/w g/cm³ A 19. 1 33. 4 52. 5 1. 526 B 28.0 28. 0 56. 0 1.522 C 28. 0 30. 3 58. 3 1. 536 D 28.2 28. 6 56. 8 1. 516 E 35.4 17.8 53.21.459 Following values are valid for respective salt at 10°C: Salt Solubility in Water Saturated Aqueous Solution Density g/cm3 Sodium formate 37*1 1. 258*2 Sodium bromide 45.9*2 1. 493*2 *1 According to Groschuff, Ber. 36,1788 (1903) *2 According to Perstorp Polyols, 1997 As can be seen from solutions A-D of Examples 1 and 2, the total solubility (total salt content) at respective temperature is higher than the solubility for respective salt. It can from solutions A-D of Examples 1 and 2, furthermore, be seen that the density of the mixed solutions are higher than the density for sodium bromide as well as for sodium formate at respective temperature.