Application and Research on Methods for 3D Space Rock Drillability Prediction

In the initial stages of exploration,when drilling data is scarce,it’s particularly important to predict the drillability of the target formation in a proper,efficient and low-cost manner.In this paper,common drillability prediction methods using logging,mud logging and seismic data were evaluated and determined to be inadequate.So,to meet the need for effective predicability of formation drillability,a new 3D prediction method was presented,in which a model considering both rock acoustic and density properties was constructed by combining the micro-bit drillability test and full-band and high-resolution P wave velocity cube which was generated by awell log-constrained seismic inversion technique.Prediction software was developed to create a 3D visualization of the distribution of drillability.Case studies of two wells in one block of the Tuha Basin was presented.The average error of drillability was about 10%,and drillability grade of a roller bit was 2.8-6.3 while drillability grade of a PDC bit is 2.0-5.0.The 3D visualization result indicated that an abnormally high drillability area existed in the local formation,which was in accordance with the existence of conglomerate layer shown in drilling record.The overall drillability performance of PDC bits was superior to that of the roller bits.The results showed that the model considering both rock acoustic and density properties could predict rock drillability reasonably and reveal drillability differences for different bits.The method proposed could describe the spatial distribution of rock drillability in an objective manner,providing references to speeding up drilling.