A method comprises flowing a mud into a wellbore, wherein the mud has a mud composition and has a weight in a defined range. The method includes introducing a fluid pill into the mud flowing into the wellbore, wherein the fluid pill has an injection fluid with an injection composition that is different from the mud composition. A particulate has been added to the injection fluid to increase the weight of the fluid pill. After flowing the mud into the wellbore such that the fluid pill is positioned in a zone of the wellbore: filtering out the particulate from the injection fluid; injecting, after the filtering, the injection fluid into the zone; measuring a downhole parameter that changes in response to injecting the injection fluid into the zone; and determining a property of the formation of the zone based on the measured downhole parameter.