Low solids drilling fluids based on formate brines (sodium, potassium and caesium salts of formic acid) were originally designed to minimise frictional pressure losses in slim hole drilling applications. In addition, their unique capability of stabilising polymers to high temperatures made them more temperature resistant than any other polymer based drilling fluids. Subsequent work has shown that these brines, because of their high densities and low corrosivity are also ideal completion and packer fluids. Formate brines have excellent HSE profiles and they are compatible with reservoir fluids, good shale stabilisers, gas hydrate inhibitors, and scale dissolvers. Also, a technique has been found for cost effective clean-up and recycling of formate based drilling fluids.
The commercialisation and introduction of these fluids into the field (especially caesium formate) has taken a long time, due to high prices and few manufacturers. This situation is now changing. as the number of manufacturers is increasing, and buy-back arrangements have been made available. Also, a number of successful drilling and completion trials have been carried out.
Recent changes in environmental legislations have driven the industry away from oil based drilling fluids. The most popular solution by far is the use of pseudo oil based muds, which are based on synthetic hydrocarbons. However, these systems are still not proven to be environmentally fully acceptable, and their future is not certain. The main concern about these fluids is biodegradation, both aerobic and anaerobic. A whole range of synthetic base fluids now exists, from those that degrade slowly and only aerobically to those that are broken down very rapidly anaerobically. 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. Major disagreement exists among the various environmental institutions and governments about what is the most acceptable solution - if any at all.
The safest solution to this problem is to avoid it in the first place. If cuttings are still to be disposed on the seabed, the use of non-toxic water based drilling fluids, disappearing from the cuttings pile during settling, is a safe way forward.
However, not only environmental requirements have to be considered when selecting a drilling fluid. Also technical requirements have to be fulfilled, such as temperature stability, good hydraulics, shale stability, tolerance to contaminations, 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 formate brines (sodium, potassium. and caesium salts of formic acid). however, have been found to fulfil all of the above mentioned requirements. These fluids were first designed for use as deep slim hole drilling fluids because of their temperature stabilising effect on polymers and their high densities. Neither bentonite nor solid weight material are needed and therefore these low solids drilling fluids have very good rheological properties. Later, they were also shown to be environmentally more acceptable than other commonly used brine systems. shale stabilising, and compatible with reservoir fluids and common drilling equipment materials. A technique has also been developed for recycling of these brines.
The formate brines also have great potential for use as completion and packer fluids, as densities up to 2.3 SG can be achieved without any solid weight material. The heaviest formate brine, caesium formate, is for the time being intended as a replacement for the highly toxic and corrosive zinc bromide brine.
Properties of Formate Brines. The formate salts of alkali metals are very soluble in water and form brines of very high densities. The three salts that have been found useful for drilling and completion fluids are sodium formate (NaCOOH), potassium formate (KCOOH). and caesium formate monohydrate (CsCOOH-H2O). Sodium formate is the least soluble of the three and can reach a density of about 1.33 SG. Potassium formate is more soluble, with a maximum brine density of about 1.59 SG, and caesium formate can reach as far as 2.3 SG. The densities of the three formate brines as a function of concentration (% weight and molar concentration) are shown in Fig 1.