A Study on the Effect of Hydraulics on the Penetration Rate of Rotary Drilling
1968 Volume 33 Issue 3 Pages 169-174
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1968 Volume 33 Issue 3 Pages 169-174
The equations for the penetration rate reported by many researcher are applicable only under the condition of the adequate hydraulic action, i.e. clean bottom hole condition. But field drilling practices are in many case under the incomplete hydraulic condition. It is quite important to understand quantitatively the effects of hydraulics on the rock bit performance. The effects of hydraulics have been investigated from two standpoints: one is the maximum penetration rate attained under the adequate hydraulic action, other is the behavior of the penetration rate under the condition of the incomplete hydraulics. By drilling experiments by 11/4 in. two-cone microbits, three basic equations have been derived to describe the penetration trate in any hydraulic condition.
(1) The equation for a performance line. This was proposed by Bingham (1964-65). This is applicable under the complete hydraulic action.
(2) The equation for a branch-off-point penetration rate. When bit weight is increased with hydaulics maintained at the same level, the penetration rate branches off at a specific point on the performance line corresponding to the hydraulics level. Then, the penetration rate at the branch-off point is able to he used as a measure of the hydraulics effect. The relationship between them is affected by the properties of rock. When that relationship is obtained, it could be used to estimate the branch-off-point penetration rate for any level of hydraulics.
(3) The equation for a relative penetration rate. The relative penetration rate and the relative bit weight are defined as follows:
the relative penetration rate=R/R0
R=penetration rate under the incomplete hydraulic condition
R0=penetration rate calculated by the equation for the performance line at the same bit weight and rotary speed,
the relative bit weight=(W-Wc)/(Wb-Wc)
W=bit weight
Wb=bit weight at the branch-off point
Wc=performance-line intercept.
By using these quantities the relationship between the penetration rate and the bit weight is described by a single equation, i.e. the equation for the relative peneeration rate which is not affected by rotary speed, hydraulics and properties of rock. The characteristics of the bit design and wearing, if any, are only factors that affect the form of the equation. A tentative equation for the relative penetration rate has been obtained by rearranging the Thompsom's field data (1953). That is This equation would be applicable for analyzing and estimating the performance of any soft-type three-cone rock bit under the condition of the incomplete hydraulics.
