An on-site computer system to control bit weight and rotary speed has been developed and successfully installed on a South Louisiana well. Early field tests indicate that the system could be invaluable in reducing drilling costs.
Previous laboratory and field experimentation has Previous laboratory and field experimentation has demonstrated the effect of several variables on drilling rate. These results have been incorporated into optimization theories for the purpose of reducing drilling cost.
Minimum-cost drilling theories rely on a combination of historical data and empirical prediction techniques for selecting optimum bit weight and rotary speed. Past solutions have required extensive use of computing equipment because of both the complexity of the mathematical formulations and the large number of parameters involved. Field attempts to apply these computed results have frequently failed because of the uncertainty of input data requirements.
Humble has developed an on-site computer system to control bit weight and rotary speed and thereby implement the concepts of minimum-cost drilling. This system can
perform short interval drilling rate tests for formation evaluation,
use the results of these tests as input information to solve minimum-cost drilling formulas, and
control bit weight and rotary speed in accordance with these computed solutions.
The purpose of this paper is to describe the computer control system and present results of initial field testing.
Minimum-cost drilling (MCD) requires a quantitative evaluation of the variables involved. Several forms of a basic mathematical model have been suggested. The relationships reported here form the basis for the solution programmed in this work. Four equations are used, expressing drilling rate, bit bearing wear, bit tooth wear, and cost.
A solution for minimum-cost drilling assuming constant bit weight and rotary speed over the entire bit life has been programmed for use in computing MCD schedules. This solution is subject to certain limiting assumptions such as those discussed by Graham et al.:
Drilling cost is the summation of bit cost, rotating cost, connection cost and hoisting cost.
Diamond bits are excluded.
Bit life is limited by either bearing failure or tooth wear, or by a combination of operational factors that make it cheaper to pull an incompletely consumed bit.
Circulating hydraulics are adequate and do not limit drilling rate.
Bit weight considerations exclude hole deviation.
Drilling rate is a function of only bit weight, rotary speed, and degree of tooth dullness; that is, the effects of pressure, lithology, fluid property, hydraulics and drill string dynamics are ignored.
JPT
P. 483
