The cost of drill bits represents only approximately 3–4% of the authorization for expenditure (AFE) of well construction, but by their overall performance, affect 15–20% of the overall well cost which can either be positive due to increased rate of penetration (ROP) or negative due to trouble time (TT) losses due to the wellbore or invisible lost time (ILT) due to connection times or other failures. When these costs are considered, the selection of the appropriate bit can be seen to be key of the overall economics of field development. Bit performance represents the achievement of preset objectives which include revolutions per minute (RPM), weight on bit (WOB) and flowrate-hydraulics inclusive. These are a function of the rate of penetration (ROP), footage, and dull grading in a given formation. Bit behavior represents the ability to achieve the required directional control within the confines of a systemized drilling environment. It is a function of the bottom hole assembly, the planned tortuosity of the well (well directional profile) and the operational environment.
In a new development, the typical procedure for the selection of drill bits is for the operator to initially approach drill bit vendors with a data package which contains information (lithology, log information(sonic, density-neutron), and bits graded from offset wells, these are used for the proposed wells as offset information. Drill bit vendors then put together a proposal based on this data and local experience; usually there is no way of validating the bit recommendation except by running the bit and evaluating the dull grade when out of hole, should the bit performance not meet the expectations then a new bit is tried. This method of trial and error exposes the operator and sometimes the cost impact is massive and has its effect on the project.
The technical limit cycle is a conceptual feedback tool for maximizing drilling efficiency, and this paper presents how proper bit selection and proper hydraulics will help to increase the ROP of a well. This in turn helps to reduce all the trouble and invisible lost times which can be created by hole cleaning, high unplanned tortuosity, vibrations, helical/sinusoidal buckling of strings, bit whirls, excessive drags which distorts the planned time of drilling a well. This increases the well cost and increases the technical limit value of the well but the reverse when the right bit is chosen and flat times eliminated.