RIASSUNTO
Summary
Recent advances in rock bit seal technology have allowed greater predictability of bearing life. Cone loss following bearing failure incurs predictability of bearing life. Cone loss following bearing failure incurs costs related to remedial activities. A risk analysis approach, utilising hearing life relationships and the inter-dependence of drilling events, is used to formulate a bit run cost optimisation method. The procedure enables a choice to be made between elastomeric and metal seals on a lowest replacement cost basis. The technique also provides a formal method for assessing the opportunity cost for employing a device for the downhole detection of bit bearing failures.
Introduction
For a given commercial environment (rig rate and bit cost), C is largely dependent on the drilling rate and life provided by the bit. Minimising bit run cost using equation l is made via manipulation of controllable operating parameters (bit type, weight on bit, rotary speed, bit hydraulics). However, inappropriate drilling parameter combinations or bit selection can lead to catastrophic bit failure and thence to expensive remedial action. Equation 1 ignores both the risk of failure and any failure costs incurred while attempting to optimise bit performance, and therefore provides an inadequate measure of operational performance, and therefore provides an inadequate measure of operational cost efficiency.
Bit run cost is a function of bit ROP and life (run length achieved), both of which are related to applied mechanical loads. Hence, in order to minimise bit run cost, models capable of describing the relationship between ROP and WOB/RPM, and between bit life and WOB/RPM, are required. Models relating WOB and RPM to drilling rate exist and are (to an extent) verifiable while drilling (drill-off tests). Models relating rock-bit bearing life to applied parameters have hitherto lacked accuracy due to the prevalence of parameters have hitherto lacked accuracy due to the prevalence of O-ring seal failure as the bearing failure mechanism, and to the wide observed variation in O-ring seal life.
Bearing failure is considered the more important life criterion for rock bits, rather than cutting structure because the former produces a catastrophic event whose consequences interrupt well progress and lead to significant remedial operations and costs. Cutting structure wear is normally non-catastrophic and it's effects on penetration rate can, to a certain extent, be overcome with increased weight on bit.
Modelling Bit Run Costs
Formulating a WOB-RPM/bit bearing life relationship has proved problematic with O-ring sealed rock bits due to the effect of ""environmental"" factors on seal life. These effects (e.g. rock particle abrasivity) are difficult to quantify from real bit runs. Lesage documented the inadequacy of time on bottom, total bit revolutions, or energy consumed as reliable indices of bearing life for elastomer seal bits, though some of the observed scatter in these parameters at bit bearing failure can be accounted for in terms of variations in bit design (e.g. cone skew angle). As a result, O-ring seal life models have tended to be entirely empirical and valid only within the drilling conditions in which the model was formulated. Other complications which introduce further variability in O-ring seal life include variation in elastomeric seal materials and geometry, and bit shirt-tail/cone backface design.
Nevertheless, there is an undoubted cost incentive for improving our understanding of the effects of applied drilling parameters on rock-bit bearing life. Uncertainty in rock bit bearing life impacts bit run cost, through conservatism in applied parameters so as to minimise the perceived risk of cone loss. The incremental reduction in drilling parameters to create a safety margin for bit integrity therefore directly reduces bit ROP.