Filtration control has a considerable impact on drilling fluid properties & performance, on drilling costs and on well productivity. The requirements for filtration control, with regard to optimisation of the various stages of drilling a well, are ambiguous. This is largely due to the lack of understanding of the functionality of the mud components and on imprecise definition of demands.

Multi-Core Dynamic Filtration equipment has been designed to study static- and dynamic filtration (including spurt loss conditions) and assess their effect on return permeability for up to 4 core samples at a time. Several core samples in one experiment will enable us to evaluate the effect of permeability variation on clean-up efficiency. In another development, equipment has been designed for continuous measurement of cake thickness during filtration.

Polymer systems have been identified that effect acceptable fluid loss without solid particle additions. Apparently, the micelles formed have the dual functionality of solids to plug the pores and polymers to reduce cake permeability. This line of investigation may have potential in the development of truly solids free drilling fluid systems based on high density brines (e.g. Formates).

Fluid loss, a key parameter in drilling fluid design

Fluid loss (control) has an effect on a number of drilling and completion parameters. It is known to have a major effect on cake properties, penetration rate and costs to name a few. It can also have a major impact on impairment of the formation. The need to evaluate the inter-dependency of the fluid loss and these parameters has long been expressed, but so far little fundamental work has been published in this field.

Research into the effects of fluid loss from drilling fluids (DF) was initiated in 1994. Initially the investigation was aimed at studying the relationship between fluid loss and impairment. It was soon realised that there were good arguments to extend the investigation to the wider field of the effect(s) of fluid loss on a number of drilling- and completion parameters. The decision was taken to design and build multifunctional experimental equipment that could be used to study a number of fluid loss related aspects of DF design. A description of the experimental equipment with some practical considerations is included.

In this paper the rationale for the fluid loss related work is given based on a review of the suggested / assumed effects of fluid loss on various drilling- and completion parameters. Further, the results of the experimental effort and the resulting conclusions are presented. This paper discusses the "mechanical" DF properties; physico-chemical properties (chemical composition), very relevant for e.g. shale stability and clay "swelling" effects, are not discussed.

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