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SPE Member Abstract 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. General Introduction 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. Introduction to the Formate Brines 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. P. 483

Xanthan gum is the staple viscosifier and fluid loss control agent used in reservoir drilling and completion fluids. In over 40 years of use in this application xanthan has built up a deserved reputation for reliable performance in drilling and completion fluids, and for generally minimizing damage to reservoir formations. Its only peculiarity is its low transition or melting temperature (Tm) in low salinity fluids and in bromide brines, meaning that its viscosity can collapse very suddenly at 70-120°C. This can be a nuisance in applications where viscosity maintenance at high temperatures is important, but a benefit if the application requires a self-breaking polymer. Previous research has shown that formate brines are capable of increasing the thermal stability of xanthan gum by increasing its Tm and by providing anti-oxidant protection. The extent to which the thermal stability of xanthan may be increased depends primarily on the type and concentration of the alkali metal formate salt in solution. The most effective brine from this perspective is a very concentrated potassium formate brine, which can increase the Tm of xanthan to over 200°C (396°F) and raise the 16-hour thermal stability to almost 180°C (356°F). The objective of the study described in this paper was to investigate if it was possible to use combinations of commercial polymer stabilizing additives to further increase the temperature ceiling of xanthan gum dissolved in formate brines. The opportunity was also taken to look at the high-temperature behavior of xanthan in rubidium formate brine and cesium acetate brine. Before starting on the additive screening program the thermal stability of xanthan in formate brine was compared against other well-known natural biopolymers used in drilling and completion fluids. The tests confirmed that xanthan really was the best performing viscosifier in buffered and alkaline formate brines, although welan showed some promise in unbuffered formate brines at neutral pH. Tests also showed that there was no significant difference in thermal stability between six different commercial xanthan products in formate brines. The transition temperature of xanthan in cesium formate brine was found to reach a maximum of almost 180°C (356°F) at a brine density of 2.20 g/cm3 (18.4 lb/gal) compared with almost 200°C (396°F) in 1.57 g/cm3 (13.1 lb/gal) potassium formate brine, and blends of the two brines produced Tm values in the range 180-200°C (356-396°F). High-density rubidium formate brines raised the xanthan Tm to around 185°C (365°F) at a density of 2.11 g/cm3 (17.6 lb/gal). Hot-rolling tests of cesium acetate brines viscosified with xanthan have shown that cesium acetate is able to stabilize xanthan to similar temperatures as cesium formate. The best additive package for increasing the thermal stability of xanthan in potassium formate brine was found to be a blend of magnesium oxide and 5% v/v of a polyethylene glycol. Potassium formate brine containing xanthan and this additive package was found to retain some viscosity after hot rolling for 16 hours at at least 194°C (381°F). It seems likely that the glycol acted as a sacrificial scavenger, mopping up free radicals before they could attack the xanthan. This finding validates a suggestion from work carried out in 1980’s that the best thermal stabilizer package for xanthan would include a concentrated salt brine containing a glycol and an oxygen scavenger Deployment of the stabilizer package identified in this study should raise the thermal stability ceiling of xanthan in buffered formate-based non-damaging drilling and completion fluids by more than 20°C (36°F).

TX 75083-3836 U.S.A., fax 1.972.952.9435. AbstractDrilling and completion fluids based on cesium formate brines were selected by Statoil for use in the development of the high pressure high temperature Kvitebjørn field. Cesium formate brine was selected primarily to minimize well control problems and maximize well productivity. These important benefits had been recognized by Statoil in previous HPHT drilling and completion operations over the past 5 years. The use of the same fluid system for both drilling and completion gives the additional benefits of simplified operations, reduced waste, and elimination of fluid incompatibility problems.The challenge on the Kvitebjørn field was to drill long deviated well paths through significant sequences of shales into reservoirs with pressures of up to 81 MPa (11,700 psi) and temperatures up to 155°C (311°F). So far the cesium formate brine has enabled the successful drilling, completion, and logging of 7 high angle HPHT production wells on Kvitebjørn, two completed with a cemented perforated liner and five with sand screens.Additionally, an extended-reach exploration well was drilled from the Kvitebjørn platform to the Valemon structure. The 705 m (2,313 ft) long reservoir section of this 7,380 m (24,213 ft) long well with an inclination of 69°, was successfully drilled with the same cesium formate fluid system.In all these wells the cesium formate brine system once again demonstrated clear performance benefits such as very low ECDs, moderate to high ROPs, good hole-cleaning, and excellent wellbore stability while logging. Quick, trouble-free, safe, and robust completion operations were also accomplished, and the wells that have been put on production show high production rates with low skin.Full open-hole formation evaluation of the Kvitebjørn reservoir has been carried out with LWD tools. The evaluation has been aided by the development of a novel logging interpretation solution for a LWD density tool, in which the extremely high photoelectric effect of cesium-rich filtrate plays a vital role. Using photoelectric factor and bulk density data, combined with resistivity measurements from both the LWD drill pass and the ream pass, produces a very reliable and consistent net reservoir definition. The final interpretation result matches the core porosity from different lithologies in 3 different wells.Cesium formate brine has helped Statoil to achieve a remarkable record of zero well control incidents in all 15 HPHT drilling operations and 20 HPHT completion operations in the Kvitebjørn, Kristin, and Huldra fields over a period of 5 years.

This paper describes new research into the role and importance of the carbonate/bicarbonate pH-buffer added to formate based well construction fluids along with a newly developed field method for determining buffer concentrations. For many years it has been recommended practice to buffer formate brines with carbonate and bicarbonate for corrosion control. Bicarbonate is also the product of the dominant thermal decomposition reaction in formate brine. Since this is an equilibrium reaction, according to Le Chateliers Principle, this additive should cause the equilibrium to establish sooner. Past attempts to demonstrate this have been unsuccessful because it has been difficult to accurately reproduce downhole hydrothermal conditions in laboratory autoclaves. This challenge has now been overcome with specialist equipment that has made it possible to accurately simulate downhole conditions and prove that chemical equilibria do indeed establish downhole. Test results have shown that the equilibrium favors high formate concentrations, meaning that the presence of only relatively small amounts of carbonate/bicarbonate is required for the equilibrium to establish. Along with the growing understanding of the importance of the carbonate/bicarbonate buffer, there has been a growing demand for a field method to accurately measure carbonate and bicarbonate concentrations as the standard API method for determination of alkalinity does not work in formate brines. The newly developed, simple field method consists of an accurate pH-determination combined with a standard phenolphthalein titration. The method has been extensively tested in the laboratory, and should be reviewed by API for possible inclusion in the API recommended practices. This new understanding of the roles of the carbonate/bicarbonate buffer has provided us with a better basis for making recommendations as to how to buffer the brine and maintain the appropriate buffer levels for different applications. Combined with the new field method for determining buffer levels, these new insights provide the oil industry with a robust technology to meet the well challenges of the future. Introduction Formate fluids have unique physico-chemical properties that make them the ideal well-construction fluids for challenging well construction projects where extraordinary fluid performance is critical for economic success. They have been used in thousands of wells across the world since their commercial introduction in 1993. The natural attributes of the formate brines (high density, anti-oxidant, alkaline pH, lubricious, biostatic, low water activity, low TCT) make it possible to formulate them into high-performance well construction fluids with a minimum number of additives. Corrosion inhibitors, biocides, lubricants, antioxidants, oxygen scavengers, and solid weighting material are typically not required in formate fluid formulations. There is, however, one additive package that is essential in all formate-based well construction fluids, namely: a carbonate/bicarbonate pH buffer. The need for a carbonate additive package for maintaining fluid pH control and reducing corrosion was understood from the earliest stages of formate brine development (Downs 1992, Howard 1995). Some of the other benefits of adding a carbonate/bicarbonate blend to formate brines have only become apparent after many years of use and testing. The purpose of this paper is to explain the full functionality and benefits provided by these two additives.