Drilling-induced formation damage has played a significant role in the failure of some horizontal wells. The objective of this laboratory study is to provide insights into this complex subject by investigating the main formation damage mechanism(s) and systematically quantify the damage effect caused by the newly formulated drilling mud systems, which are designed to minimize formation damage.
Eleven commercial drilling mud systems were tested in this study and they produced a very wide range of permeability reduction on Berea sandstone cores with similar properties. Bentonite and cellulose based bridging agents appear to cause higher degrees of damage than ground calcium carbonate. High acid solubility of these muds is conducive to much improved permeability after acid stimulation.
Results of additional dynamic mud leak-off experiments show that the most important parameters which affect regained permeability are: 1) mud composition, 2) core permeability, 3) overbalance pressure, 4) filter cake rigidity, 5) pressure drawdown during backflow, and 6) mud particle versus rock pore-throat size distribution. The depth of mud solid invasion is limited to the first few centimetres of the injection end but the filtrate usually invades the whole core. Furthermore, repeated removal of the existing mud cake can cause much deeper invasion and more severe damage.