A Theoretical Method For Detecting In Situ Pdc Bit Dull And Lithology Change
Authors: Kuru E. , Wojtanowicz A.K. 
Published: March 2010

A new method has been developed for in situ measurements of the PDC bit condition and lithology change detection. In this technique, a diagnostic plot is made by correlating two dimensionless groups that contain measured values of torque, weight on bit, rotary speed and penetration rate. Laboratory and field data (presented in the study) confirmed the linearity of such a plot. The diagnostic plot is a unique indicator of the bit/rock interaction and its independent of bit operational variables moreover, the instantaneous wear of a PDC bit can be computed from coordinates of the straight line points.

The technique was- verified by comparing the predicted and the measured PDC bit wear from the Measurement while Drilling (MWD) records in the Gulf Coast area. Also provided are examples of a correlation between rapid formation changes and discontinuities in the diagnostic plots.

This method is feasible for a graphical use that is supplemented with a computer program. It may contribute to the MWD software development for the purpose of the in situ rock detection and the PDC bit evaluation and control.

Introduction

Polycrystalline diamond compact (PDC) bits have proved to be long-lasting bottomhole tools because of the absence of moving parts and the high wear resistance of synthetic diamond. However, PDC bits are sensitive to formation properties and operating conditions(1). Evaluation of PUC drilling performance in harsh environments such as geothermal and hard rock drilling showed an average two-fold increase of penetration rate and bit life, resulting in a cost reduction of 10% to 15%. In other cases, however, 50% reduction of the PDC bit life was observed, which reduced cost savings by half(2,3). Therefore, a strong incentive exists to improve bit life in any potential application of PDC bits. Early detection of lithology changes followed by an adjustment of operational variables is necessary to save the bit. Such detection can be made with an instantaneous drilling data acquisition Measurement While Drilling (MWD) system. Development of the systems has been in progress for many years in the petroleum industry. However, without an appropriate data processing tool (a drilling model), some of the information is unusable.

Successful application of a drilling model is not only a result of the model's precision but is also dependent upon the quality of the input data. Accuracy of drilling measurements, specifically regarding the used bit condition, is low, Recently an effort has been made to improve bit dull evaluation(4). However, the quantitative measurement of bit dull still needs to be developed. Furthermore, there is no measurement of bit condition while drilling, which makes the verification of drilling models unreliable.

Conventional drilling models include equations that describe drilling rate and bit wear. The measured variables are weight on bit, rotary speed, and drilling rate. With the development of the MWD, bit torque has become another important measured variable for improving the verification of drilling models. The relation of torque to other drilling variables can be derived from either the energy conservation principle or the balance of forces.