This paper describes first results of an extensive laboratory research program aiming at identifying potential additives for bentonite suspensions which would extend their use at high temperatures by maintaining or even improving the rheological and the filtration characteristics of these fluids.

Five different Grrek lignites have been tested, at 3% wt. concentration, together with a commercial lignite product, used as reference. API filter press data and rheological data showed that thermal aging of bentonite suspensions results in deterioration of rheological and filtration properties while lignite addition was very beneficial.

Static filtrations tests against Berea sandstones have been performed using a core holder and the various mud formulations. Pressure ports along the core holder allowed determination of permeability evolution during testing of the formed cake as well as of the core itself. A backflash arrangement allowed for determination of permeability recovery.

Thermally aged muds reduced significantly core permeability and gave recovery rates around 50%. Three of the lignite samples, have maintained original core permeability giving also very high recovery rates. Some of the lignite samples, including the commercial one, did not perform as well and also gave permeability recoveries between 10 to 80%. Thus, static filtration against cores allows for differentiation of well performing lignite samples not only in API filter press tests but also in core permeability experiments.

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