Patent ID: 6009747
Filing Date: 2000-01-04
Classification: E21B

Abstract:
A method for the evaluation of drilling fluids as a test fluid to be contacted with a shale-type rock or clay-rich rocks with the aid of Thermo-Gravimetric Analysis (TGA) which comprises the following steps:collecting downhole samples of the clay-rich rock to be analyzed keeping it free from dehydration so as to preserve the downhole hydration condition and obtain preserved as collected samples known to be free from the effects of exposure to said drilling fluids;determining information regarding a qualitative and quantitative mineral composition and structure, type and amount of water in said downhole shale or clay-rich rock by submitting a piece of the preserved sample to a TGA analysis;immersing a sample of the same piece of clay-rich rock in the test fluid whose effect on the sample is to be evaluated;analyzing the sample immersed in the test fluid in terms of structure and composition with the aid of TGA at regular time intervals;comparing structure and composition of the as-collected, preserved sample as obtained by TGA analysis with the structure and composition of the sample which has been immersed in the test fluid, also obtained by TGA, with the structural modification of the shale being ascertained through the mathematical derivative curve of mass with respect to temperature.