Research On Fiction And Wear Mechanism Of PDC Cutter And Optimal Control Of Drilling Parameters

PDC bits are widely used in engineering drilling of fields including geology, metallurgy, coal, oil and gas, civil and hydraulic engineering. Therefore, the study of PDC bit’s performance is particularly crucial to drilling. The friction and abrasion of PDC bits directly affect the efficiency and cost of drilling. Although an enormous amount of research works have been done at home and abroad in this aspect, most of them just do some statistic works by establishing empirical models with test data. Hence, on the basis of previous studies, the friction and abrasion of PDC bits and the optimal control of drilling parameters are analyzed and discussed in this paper by utilizing the methods of theoretical analysis and experimental study, and the followings are included:(1) Based on the mechanical analysis of a single PDC bit, a mathematical model for PDC bit drilling was derived by using the constants and functions which have definite physical significance. The model combined the torque, the drilling rate, cutter’s geometry and rock properties. The theoretical model is verified using data of field drilling operations and laboratory test. The results show that the theoretical model could basically match with the data of field and laboratory, which verified the rationality of the theoretical model. This model can be used to define the acting force of bit-rock interface, and study the phenomenon of heat and wear produced by friction.(2) Based on the interaction mechanism of cutter and rock, the influence of friction heat on cutter performance was study. By combining the bit wear model under temperature effect and the temperature model on wear surface. The relationship between bit operation life with weight on bit, rotary speed, bit geometry, flow rate of drilling fluid and temperature was set up, which modified and developed the bit life model and could better predict the drilling performance of bit.The predictions shows that the effect of friction dominates bit life and this effect is strongerthan that of the convective cooling.(3) Based on self-designed and developed experimental system of drilling dynamic simulator, the dynamic friction characteristics of interface between bit and rock (sandstone and granite) were experimentally studied using the electrical measuring method, and the influence of rotating speed, impacting speed and drilling fluid type on friction characteristics of bit-rock interface were analyzed. The experiments show that the key friction parameter can be accurately measured according to stress wave, and the measured friction coefficients are irrelevant with normal forces, while gradually decrease with the increasing relative sliding velocity. Dry friction coefficient is about 50% higher than that with water-base mud and oil-base mud, and there is no significant effect of drilling fluid types on friction coefficient.(4) The new drilling theory is verified with the established drilling model. The concept of maximum bit performance curve is introduced, which represents the relationship between single bit drilling depth and maximum drilling rate. This curve is a basic relation for drilling optimization of any forms, and provides a method for preparing multi-bits drilling procedure and dynamic drilling strategy. The dynamic drilling strategy provides the most optimal combination of operation bits, thereby the minimum drilling cost could be achieved within a long drilling interval. The research shows that the dynamic drilling strategy been as a new kind of drilling optimization method has a higher cost-effectiveness. The example calculations show that up to 18% cost could be saved compared to conventional drilling optimization and up to 40% saved with respect to the non-optimized drilling practices.(5) Based on the drilling model, a diagnostic model for evaluating instantaneous bit wear and detecting lithology changes was developed. According to the comparative analysis with MWD curve and bit wear measurements of a drilling engineering, the rationality and applicability of this technology was verified.Through the research above, a drilling theoretical model of PDC bit covering comprehensive parameters is established, and a dynamic strategy for drilling optimization and an in-situ method for bit condition measurement and lithological changing detection are put forward.