Maximizing Drill Rates with Real-Time Surveillance of Mechanical Specific Energy

Abstract

Abstract The concept of Mechanical Specific Energy (MSE) has been used effectively in lab environments to evaluate the drilling efficiency of bits. MSE analysis has also been used in a limited manner to investigate specific inefficiencies in field operations.3,4 In early 2004, the operator initiated a pilot to determine whether the concept might be used more broadly by rig-site personnel as a real-time tool to maximize the rate of penetration. The results have exceeded expectations. The average ROP on the six rigs selected for the three-month pilot was increased by 133% and new field records were established on 10 of 11 wells. The MSE surveillance process provides the ability to detect changes in the efficiency of the drilling system, more or less continuously. This has improved performance by 1) allowing the optimum operating parameters to be identified easily, and 2) providing the quantitative data needed to cost-justify design changes to extend the current limits of the system. MSE analysis has resulted in redesign in areas as diverse as well control practices, bit selection, BHA design, makeup torque, directional target sizing and motor differential ratings. The use of MSE surveillance is a key feature in a family of well planning and operational practices that are referred to as the ExxonMobil Fast Drill Process (FDP). These are now being introduced in other operating areas throughout the global organization. Introduction Drill rate performance is difficult to assess objectively. The great majority of bits are evaluated based on their performance relative to offsets. However, drill rates are often constrained by factors that the driller does not control, and in ways that cannot be documented in a bit record. Consequently, drill rates may vary greatly between two wells running identical bits. The manner in which a bit is run is often more important than which bit is run. Drillers conduct a variety of tests to optimize performance. The most common is the "drill rate" test, which consists of simply experimenting with various WOB and RPM settings and observing the results. The parameters are then used that resulted in the highest ROP. In some sense, all optimization schemes use a similar comparative process. That is, they seek to identify the parameters that yield the best results relative to other settings. One of the earliest schemes was the "drilloff" test in which the driller applied a high WOB and locked the brake to prevent the top of the string from advancing while continuing to circulate and rotate the string.1 As the bit drilled ahead, the string elongated and the WOB declined. ROP was calculated from the change in the rate of drill string elongation as the weight declined. Figure 1 shows field data from three drilloff tests with an insert bit. The point at which the ROP stops responding linearly with increasing WOB is referred to as the "flounder" or "founder" point. This is taken to be the optimum WOB. The drilloff curve is used throughout this paper to aid in explaining the manner in which MSE is used operationally. Analysis of MSE trends show which phase of the drilloff curve a bit is being operated in. Relating MSE back to the drilloff curve helps engineers and field personnel understand its application from within an operational framework that they're already familiar with. A variety of other processes have been developed for optimizing ROP, some of which are supported by computer analysis. They are similar to drill rate and drilloff tests in that they observe trends in performance and attempt to identify the founder point, which is the point at which the ROP is maximized. These tools have enhanced performance, but they do not provide an objective assessment of the true potential drill rate, only the founder point of the current system. This is an important distinction. MSE testing has shown that in some cases the actual maximum performance that could be achieved with a given system was only 10% of what was technically possible. In the majority of cases the crew was not aware that constraints even existed because the performance was similar to that of offsets that also suffered the same inefficiencies.