Laboratory Study of Effect of Overburden, Formation and Mud Column Pressures on Drilling Rate of Permeable Formations

Published in Petroleum Transactions, AIME, Volume 217, 1959, pages 9–17. Abstract One phase has been completed of a laboratory investigation of formations with relatively high permeability under conditions of overburden, formation and mud column pressures. The following statements are based on these tests within the limitations described. Drilling rate decreased when mud column pressure was greater than formation pressure. The decrease was primarily due to a layer of cuttings and mud particles held to the hole bottom by the difference in pressure. Much of the force of the bit was wasted in this layer and was unavailable for penetrating virgin formation. Adequate jet velocities helped clean the filter cake and chips away and resulted in increased drilling rate the higher the jet velocity the faster the drilling rate. Drilling rate increased slightly when formation pressure was greater than mud column pressure. The increase resulted from cleaning the hole bottom as formation fluid flowed into the borehole. Overburden pressure had practically no effect on drilling rate. Introduction Some formations are difficult to drill in a wellbore hole but are easily drilled under atmospheric conditions in the laboratory. The reasons for this apparent difference in drill ability are not fully understood. An increase in formation compressive strength causes a reduction in drilling rate. It has been shown by Griggs, Handin, and others that formation strength increases under conditions of high hydrostatic stress. Based on these results, a decrease in drilling rate would be expected with increased drilling fluid pressure in formations of low permeability having low formation fluid pressure. Such a decrease has been shown by Murray, et al and Eckel. This effect holds even when using nitrogen under high pressure as the drilling fluid (unreported tests from the Hughes laboratory). Poor bottom-hole cleaning may account for decreased drilling rates. Even though it may not be easily explained, evidence of poor cleaning can easily be seen.