During of drilling practice, force is transmitted to bit by mechanical devices. Bit is the major element for applying stress on rock. Under this stress, not only the rock is crushed, but also the bit is worn. In accord with high price of drilling bits, especially in deep drillings of oil and gas wells, wearing phenomenon of bits imposes high costs on drilling operations. Therefore, optimization of bit wear will have significant effect in reduction of drilling costs. In this study, relationship between drilling parameters is investigated. By means of these relations, conditions of high penetration rates can be determined and according to these conditions, drilling parameters are optimized. Results of this investigation denote the pairs of parameters which have direct or inverse relations.
Estimation of cutting advancement and tool wearing under a given condition of rock can lead to significant reduction of costs and selection of suitable drilling methods in design stage. However, investigation on these parameters is seldom done. This is partly due to the variety of engaged parameters and the difficulty of measuring them [1, 2 and 3]. Drillability can be defined from colloquial or operative standpoints. Colloquial definition is the rate of bit penetration in rock (meter per minute) , while operative definition is used to describe the effects of various parameters on performance of drilling operations and wearing of tools . Assessment of drillability, in a given condition of rock can lead to reduction of undesirable costs. For proper forecasting of drilling rate and bit wear, effective parameters must be recognized and investigated . These parameters can be categorized in three sets: drilling equipments, geotechnical conditions and performance of operations (Fig 1). It should be noted that performance of operations means periodic and continual maintenance of equipments and is used to monitor human factors on the drilling process . Fig. 1. Major effecting parameters on drillability . Geotechnical parameters or strict variants are dictated by the drilling environment. However, drilling equipments are modifiable variants that can be predicted or optimized according to conditions such as bit type, applied force on bit, rate of rotation and volume of drilling fluid . Among conducted studies on drillability, Wilbur's classification system (1982) is the most famous method which classifies rock mass by means of geotechnical parameters. This method was established according to the rating of some geotechnical parameters and can predict quality of drilling. Rating of parameters in this system is qualitative and descriptive terms are used to determine the weight of parameters. Due to complexity of rock mass, such system may confuse the user and yield inconsistent results. Many efforts have been done in order to alleviate the problems associated with such classification systems. These efforts mainly determine some quantitative parameters, weigh them and finally determine a drillability index [8, 9, 10, 11 and 12]. Major difference between various methods is due to the selected parameters and their weights, which are determined by empirical observations and engineering judgments.