Rock Strength Measurements on Cuttings as Input Data for Optimizing Drill Bit Selection

Abstract

Abstract Selecting the most appropriate bit for any hole section is a key-feature in achieving superior drilling performance. The aim, to maximize hours on bottom and rate of penetration without jeopardizing safety and bit integrity, in order to improve and optimize performance. These factors have an enormous influence on the drilling and ultimately well economics. The most important factor to correctly select, or even design "ex novo", a bit, is to understand how the bit cutting structure effects the rocks to be drilled. These interactions depend on formation physical characteristics, mainly rock structure and nature, compressive strength, abrasiveness, wettability, under-overbalance etc. The conventional approach in bit selection is to derive actual expected performances on reference data such as previous runs or information from offset bit records. In this way, the achievement of improved performances and optimization may require a long time and much expense. Analyzing the formation characteristics while drilling directly on cuttings, we may obtain substantial information that lead to define the most suitable cutting structure for any specific hole section, leading to identification of the most suitable bit and saving valuable time and money. In order to reach this goal, a methodology has been developed, based on micro indentation tests on cuttings, to mechanically characterize formations. The compressive strength index values obtained by micro-indentation are then used by a Drill Bit Optimization System to aid Operators with the bit selection decision. This system uses lithological data and compressive strength values as input and provides bit class indication, together with bit hydraulic configurations and gage protection as output. After an introduction devoted to illustrate the indentation tests, focus will be given to the actual results of field tests. We will detail the methodology and the verification process as performed during the field tests, carried out in a very demanding environment in central-southern Italy, and will point out how it can be easily implemented at the rig site, practically at no extra cost. Furthermore, discussion will cover how the planning of new drilling activities may benefit from the results derived by the Drill Bit Optimization System, supplied with data from the micro indentation test eventually validated by E-logs, in order to maximize penetration rate and drilling efficiency.