Phenomena Affecting Drilling Rates at Depth

Abstract

Published in Petroleum Transactions, AIME, Volume 216, 1959, pages 232–239. Abstract Rock downhole is known to be less drillable than when brought to the surface. This must be ascribed mainly to the presence under downhole conditions of a pressure differential across already made chips, which hinders their being lifted. The pressure differential has partly a static and partly a dynamic origin. Balling-up of bits is another consequence of this pressure differential. Reduction in penetration rate owing to an increase in the strength of the rock is goverend by the difference between the mud pressure and the pressure of the formation pore liquid. Rotationally symmetric geostatic stresses have no effect on drillability. Fracture of rock when drilling will be brittle in most cases. The above is supported by laboratory drilling experiments with drag bits and roller bits at elevated mud, pore, and confining pressures on rocks differing in strength and permeability. Introduction In oil well drilling drillability of rock is found to decrease with increasing depth of the hole. Naturally deep rock will be more compacted and, therefore, harder to drill than shallow rock of the same type. However, apart from this the drillability of a sample of deep and compacted rock brought to the surface is generally much higher than in its original location downhole. In view of the economic implications of this reduction in drillability, it seems worthwhile to analyze its causes. The origin clearly has to be sought in the difference of environment. The only conceivable factors would seem to be the presence of mud under pressure, the pressure of the formation pore liquid, and the overburden of the rock. Down the borehole the rock is compressed triaxially by mud pressure and overburden. It is well known that the strength of rock is increased when confined by external pressure. Various authors have, therefore, ascribed the difference in drillability mainly to the strengthening of rock by triaxial compression. Another factor, mentioned by Bobo and Hoch, is that forces, including "pressure differential forces", tend to hold a dislodged particle in place. However, the conditions determining their magnitude are not clarified nor is their effect on drilling rate assessed.