Plugging coal seam gas wells with bentonite

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This thesis investigates the strength of hydrated bentonite plugs when used to plug andabandon coal seam gas wells and other oil and gas wells. A set of experiments wasdesigned to measure the frictional strength between the steel casing walls and thehydrated plugs.The coefficient of friction between bentonite and steel casing was previously measuredwith Wyoming bentonite and found to be between 0.8 and 1.8, results that imply thatfriction is not the only force contributing to the plug strength (friction factor being biggerthan 1). Calculations resulting from measurements in my experiments showed thecoefficient of friction to be 1.92 for 0.0889 meter (3.5 inch) plugs and 5.5 for 0.1397 meter(5.5 inch) plugs. These coefficient of friction results suggest that the internal swellingforces should be considered as an additional term in the equation and more experimentalwork is required to evaluate internal swelling forces. Effect of swelling forces can be seenmore in the larger plug sizes as the moisture content is higher and the calculatedcoefficient of friction is higher, in fact more than one.The relationship between the height of a bentonite plug and the pressure it can stand wasinvestigated with a parabolic model, but, it has been postulated by different researchers tobe both linear and parabolic. Because of experimental equipment limitations on casingsize (plug size), the results of this research cannot lead to a clear conclusion on therelationship between the plug’s height and its dislodgment pressure as the experimentalresults fit both the linear and parabolic models. Further work is needed on several differentcasing sizes to test the true relationship.The plug failure mode was found to be frictional for these experiments but shear failure isalso possible in practice depending on the size of the hydrated plugs.The experimental results on monitoring the hydration of a plug with a middle hole of0.0317 meter (1¼ inch) show that after 46 days of hydration time the middle hole has beenfilled in due to clay expansion. However, plug cuts show that more hydration time neededfor middle of the plugs to be fully hydrated (Plugs were partially hydrated).The moisture content of the plugs after 48 days hydration were measured as 47% – 53%and 28% – 32% reducing with plug height for different plug sizes (0.0889 meter (3.5 inch)and 0.1397 meter (5.5 inch) dry plugs).