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Bentonite is not typically used as the primary fluid-loss agent in normal-density slurries. In low-density slurries, where higher concentrations can be used, it may provide sufficient fluid-loss control (400 to 700 cm 3 /30 min) for safe placement in noncritical well applications. Fluid-loss control, obtained through the use of bentonite, is achieved by the reduction of filter-cake permeability by pore-throat bridging. Microsilica imparts a degree of fluid-loss control to cement slurries because of its small particle size of less than 5 microns. The small particles reduce the pore-throat volume within the cement matrix through a tighter packing arrangement, resulting in a reduction of filter-cake permeability.
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Biodegradability testing of drilling fluid chemicals is becoming a requirement for use in Europe, due in part to the persistence in the environment of the oils attached to discharged cuttings from oil-based drilling fluids. This paper defines some pertinent terms, presents a brief discussion of aerobic ready biodegradability testing based on the methods published by the Organization for Economic Cooperation and Development (OECD) and presents results and experiences from using an OECD proposed seawater closed bottle test procedure to screen hydrocarbon oils and alternatives for their potential marine biodegradability.
Environmental regulatory constraints in the E and P industry have historically been based primarily on short term toxicity concerns. More recently, however, regulatory agencies both in the U.S. and in Europe have broadened their focus to include longer term effects including bioaccumulation and biodegradation .
Long range effects of chemicals discharged into the environment depend not only on the characteristics of the chemical, the amount discharged and the characteristics of the receiving environment, but also on the long term concentration in the environment which in turn is dependent on whether the chemical breaks down or persists. Oil-based drilling fluids, hydrocarbon oils and hydrocarbon alternatives in particular, have attracted attention due to the persistence of the oils attached to the discharged cuttings from oil-based drilling fluids around offshore platforms.
Demonstrable biodegradability for organic drilling fluid chemicals are currently a requirement in some North Sea countries and will be requested in others starting in 1993 . Concerns over bioaccumulation, a potential environmental problem with some chemicals (such as hydrocarbons and hydrocarbon alternatives) which fail to degrade or degrade slowly, has also prompted the EPA to require analysis for benzene, ethyl benzene, toluene and xylene (BETX) in produced waters discharged into offshore U.S waters .
Growing interest in biodegradation has often resulted in broad and sometimes conflicting claims of biodegradability for many drilling fluid chemicals without identifying the test procedure used, characteristics of the organisms, duration of the test or the extent of biodegradation obtained. As biodegradation test results can vary dramatically with the exact test procedure used , much confusion exists over which chemicals or products are biodegradable and which are not. This paper introduces some terms specific to biodegradation, gives a brief summary of the types of biodegradation tests currently in use, and presents results and observations from performing one widely used aerobic procedure.
BIODEGRADATION TERMS and CONCEPTS
Biodegradation can be defined as the destruction of chemicals by the biological action of living organisms.