During the last decade, a tremendous amount of work has been published on using surface measurement of mechanical specific energy (MSE) for enhancing drilling efficiency and maximizing rate of penetration (ROP). With an increase in directional drilling, surface measurement of MSE doesn't correlate with downhole conditions due to frictional losses along the drill string and bottom hole assembly (BHA). Attempts to measure weight on bit (WOB) and torque on bit (TOB) downhole typically involve expensive tools that are deployed 50-100 feet behind the bit. The authors present a novel in-bit strain sensor that measures WOB and TOB in the bit while drilling. This new approach is completely non-invasive and does not change the BHA. Field testing has shown that in many circumstances only 50% of the surface indicated weight reaches the bit and drilling performance suffers including low ROP and higher vibration levels. This highlights the importance of managing the directional plan and using methods to improve weight transfer. This new in-bit measurement tool has been used to drill over 100,000ft in applications around the world and provides useful insights into drilling performance.
With the ever-increasing pressure to drill wells efficiently at lower costs, the utilization of downhole sensors in the Bottom Hole Assembly (BHA) that reveal true downhole dynamics has become scarce. Surface sensors are notoriously inaccurate in translating readings to an accurate representation of downhole dynamics. The issue of 1 to 1 interpretation of surface to downhole dynamics is prevalent in all sensors and creates a paradigm of inefficient drilling practices and decision making. Intelligent mapping of downhole dynamics (IMoDD) is an analytical suite to address these inefficiencies and maximize the use of surface sensors, thus doing more with less. IMoDD features a new zeroing beyond the traditional workflows of zeroing the surface sensors related to weight and torque at the connection. A new method, Second-order Identifier of Maximum Stand-pipe-pressure: SIMS, is introduced. The method examines changes in stand-pipe pressure and identifies the point before bit-wellbore contact, using a set of conditions. The resulting calculations of weight and torque are verified with measured values of downhole weight and torque, for multiple stands of drilling in vertical, curve-lateral drilling. After the new zero, the deviation of torque-weight correlations is further examined to reveal the downhole weight changes confirmed also by the downhole sensor data. It is demonstrated that an intelligent mapping system that improves downhole characterizations would improve decision making to facilitate smoother energy transfer thus reducing Non-Productive Time (NPT) and increasing BHA life span.
Proper understanding of the strength of rocks, and its variability along the length of the well, is essential for efficient and economic drilling operation. Traditionally, the industry has used log-based strength estimates calibrated to strength measured on core samples. However, coring and core testing is costly and time consuming and downhole logs may also be left out of the program to manage costs. In comparison, drilling data is almost always available as the well is drilled. An innovative and robust method is presented which capitalizes on availability of drilling tools, which measure key drilling data downhole. As the measurements are acquired downhole, uncertainties associated with surface-to-downhole conversions are reduced. Reliable results are available over the length of the wellbore, irrespective of complexity in well trajectory. The work also reviews the development of tools measuring downhole-drilling data. This method uses downhole weight-on-bit, rotational speed, downhole torque, and rate-of-penetration to characterize the downhole mechanical specific energy (MSEDownhole) consumed in the process. The bit diameter, mud-weight, and depth of drilling are also accounted for. If the task is to optimize drilling parameters for a new formation (e.g. drill-off-test), then the parameters with the “minimum” MSEDownhole are captured. However, if the task is for stage and cluster-wise hydraulic fracture design, then “instantaneous” MSEDownhole is used to infer confined compressive strength (CCS). The CCS together with internal friction angle (IFA) provides unconfined compressive strength (UCS) using Mohr-failure envelope inversion. The MSEDownhole is compared to Drilling Strength over the same interval. Drilling Strength is defined as Weight on Bit / (Bit Diameter * Penetration per Revolution), and has been used to estimate rock strength. The comparison between MSEDownhole and Drilling Strength highlights the differences in the estimated strength from the two methods. Current work shows the results from 14 drilling simulator tests, in shale and limestone, under typical ‘drill-off-test.’ The minimum-MSE obtained was transformed to CCS using user-defined ‘efficiency factor.’ The CCS was translated to UCS using basic Mohr-failure envelope and compared with core test data. Utilization of lab tests for calibration greatly improves the trust in this conversion. The concept of ‘instantaneous MSE’ was applied in a Gulf-of-Mexico well where drilling parameters obtained from downhole sensor were maintained in a close range. Formation evaluation logs were used to compare UCS obtained. The CCS and UCS estimates benefit drilling engineers, geoscientists, and completion engineers. The less known ‘Efficiency Factor’ is also discussed and reviewed.
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