Monovalent formate brines were first introduced into the oilfield environment in the early-1990's, in response to the industry demand for better drilling and completion fluids to meet the increasingly complex technical challenges posed by modern well construction practices. After years of rigorous field-testing, in a variety of demanding well construction operations, the formate brines are now acknowledged to be probably the best foundation for any modern high performance drilling and completion fluid. The formate brines are currently having their greatest impact as the primary components of HT/HP reservoir drilling and completion fluid formulations. A number of field and laboratory tests have indicated that when formate-based formulations are used as reservoir drilling-in and completion fluids they appear to cause less formation damage than some other conventional fluid formulations, and consequently they are often seen to have a beneficial effect on well productivity. Up until now, however, little of this important information has found its way into the public domain. It has been common practice for a number of the major multinational oil companies to contract a specialist formation damage prediction company to carry out laboratory-scale formation damage tests with formate brines before using these fluids in their well constructions operations. This paper draws together the results of a significant number of these tests carried out by the specialist laboratory using formate-based fluids passed through real reservoir core materials for realistic time periods and under realistic reservoir conditions. As well as providing a unique insight into the interaction between formate brines and a range of reservoir core materials and reservoir fluid types under simulated downhole conditions, this paper also outlines the methodology developed to ensure that the formation damage tests carried out with formate brines are not influenced by laboratory artefacts. The conclusion of the paper is that a significant number of laboratory results, obtained under test conditions closely simulating reservoir conditions, tend to lend support to the growing perception that formate brines have valuable formation damage control properties that can be exploited to improve well productivity prospects in even the most demanding environments. Introduction Brines are used by the oil industry in a range of well construction operations, most commonly to create dense low-solids fluids for application in reservoir sections. A key driver for using brines in these applications has been the need to maintain well control while trying to minimise reservoir formation damage from solids invasion. Traditionally these oilfield brines have been based on halide (chloride or bromide) salts. Unfortunately the halide-based brines were never purpose-designed for oilfield applications and they have a number of performance deficiencies that become amplified as the brine density requirement is raised. Nevertheless, in the unsophisticated well construction environments that have existed in many parts of the world until relatively recently it has been possible for operators to somehow live with these deficiencies, albeit at some present or future financial cost. In recent years it has been clear that the traditional halide-based brines could no longer hope to meet the technical challenges of the increasingly complex and extreme well constructions that were being attempted to reduce field development costs and to increase production. In addition to the technical challenges that the traditional halide brines have been facing they have also been coming under pressure from the Health, Safety and Environmental lobbies. The use of halide-based brines in onshore environments has always been a concern anyway, given the toxicity of halides to aquatic life and all other non-marine organisms, and it can be expected that the legislation controlling the onshore application of halide brines will become more and more stringent.

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