Effect of Preconditioning and Ageing on Rheological Properties of Model Drilling Fluids
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Many drilling fluids are thixotropic, meaning that viscosity decreases with time when subjected to shear. Other fluids may show anti-thixotropic (rheopectic) behavior, i.e. viscosity increase with time when subjected to shear. In both cases it is important to have a consistent procedure for how to treat the fluids prior to rheological measurements. This is vital in order to be able to compare measurements of different samples, with different instruments and between different laboratories. In the oil industry, the ISO 10416/ISO 10414-1/2 standards are used for determination of viscosity and gel strength of fluids by use of direct-indicating viscometers (Fann 35 viscometers). However, these standards do not specify in detail how the fluids should be pre-treated before measurements. Further, if results from Fann 35 viscometers are to be compared to measurements done on a rheometer, it is even more important to have a consistent pretreatment of the fluids before the measurements. A systematic study is performed in which the effects of pre-shearing and ageing history on the rheological measurements of model drilling fluids are investigated by using a Fann 35 viscometer and an Anton Paar MCR 302 rheometer. The results demonstrate the importance of consistent preconditioning of fluids before measurements. A test-procedure standard is proposed, enabling higher measurement precision and comparability of rheological measurements.