embodiment of the invention the useful temperature range is from 120 u00b0 c to 250 u00b0 c inclusive the duration of the exposure will in practice decrease with the maximum temperature of exposure more specifically if the heat treatment temperature is 120 u00b0 c at least four hours of exposure to that temperature will be required on the other hand if the heat treatment temperature is 250 u00b0 c the duration of exposure to that temperature will not exceed 20 minutes in the most preferred embodiments of the invention the particles undergo a total exposure to temperatures in the range of 150 u00b0 c to 200 u00b0 c for a duration of 10 minutes to 90 minutes inclusive in other embodiments of the invention where heat treatment is performed u201c in situ u201d in the downhole environment the minimum downhole temperature is 80 u00b0 c and the minimum dwell time in the downhole environment is one week in practice the minimum required amount of time for adequate postcuring in the downhole environment will decrease with increasing temperature of the environment in more preferred embodiments of this class the temperature of the downhole environment is at least 100 u00b0 c in the most preferred embodiments of this type the temperature of the downhole environment is at least 120 u00b0 c the currently preferred embodiments of the invention will be understood better in the context of a specific example it is to be understood that the example is being provided without reducing the generality of the invention persons skilled in the art can readily imagine.