Summary The use of recorded downhole rotational speed measurements with a bandwidth up to 9 Hz gives new insights into the conditions under which stick-slip torsional oscillations occur. Observations made while drilling two reservoir sections have shown that, out of all the stick-slip situations identified, 72% of them for one well and 64% for the other well occurred in off-bottom conditions. In these off-bottom conditions, stick-slip was systematically observed while starting the topdrive (TD) until a sufficiently high TD rotational velocity was requested. For these two sections, off-bottomstick-slip was either related to using TD speeds below 120 rev/min or to reaming down during reciprocation procedures. In on-bottom conditions, stick-slip events occurred predominantly when the TD speed was less than 120 rev/min (53 and 32% of the on-bottom cases) but also in association with downlinking to the rotary steerable system (RSS) (23 and 46% of the on-bottom cases), and this, even though the TD speed was larger than 120 rev/min. These on-bottomstick-slip situations did not necessarily occur at a very high weight on bit (WOB) because 98% of them for one well and 46% for the other well took place when the WOB was lower than 10 ton. Downhole measurements have shown that when the drillstring is subject to strong stick-slip conditions, the downhole rotational speed changes from stationary to more than 300 rev/min in just a fraction of a second. Direct observations of downhole rotational speed at high frequency help in discovering conditions that were not suspected to lead to large torsional oscillations. This new information can be used to improve drilling operational procedures and models of the drilling process, therefore enabling increased drilling efficiency.
Despite advances in Measurement-While-Drilling / Logging-While-Drilling (MWD/LWD) technologies, the oil & gas industry has until recently lacked viable technologies and tools to measure wellbore geometry in large hole sizes (>12-1/4″) while drilling and subsequently the ability to visualize and describe the borehole shape and size in an intuitive way. A direct mechanical measurement solution such as used on wireline, e.g., multi-finger tools, is not feasible to implement on MWD/LWD tools due to the nature of the drilling operation. Conventional technologies and methods, including acoustic- and density-based measurement methods have been used with reasonable results in smaller hole sizes (≤12-1/4″) when combined with low mud weights. However, many commercially available tools within the industry have low vertical and azimuthal resolution due to sparse sampling or sparse storing in its internal downhole memory of such caliper measurements, resulting in limited use of such data for borehole shape and size purposes. Such conventional technologies and methods have not been, or very seldom used in large hole sizes, primarily due to lack of available technologies and tools, resulting from challenges related to the sensor to wellbore interface standoff distance. A novel Logging-While-Drilling Caliper tool based on impulse radar technology has been developed to overcome the challenges related to mud weight, sensor to wellbore standoff in oil-based muds and at the same time addressing challenges related to sparse datasets. This tool enables the oil and gas industry to accurately image borehole shape and size with both high vertical and azimuthal resolution, including within large hole sizes where there has not been any viable solution whilst drilling. The high sampling rate together with a large downhole memory (128 GB) allows the industry to evaluate the borehole shape and size as a function of time (timelapse). An Impulse Radar Caliper tool has been pilot tested in several wells on the Norwegian Continental Shelf (NCS), in borehole sections ranging from 12-1/4″ to 17-1/2″. During the pilot testing, the Impulse Radar Caliper tool acquired wellbore shape and size measurements while drilling, and some intervals while pulling out of hole. Several wellbore features, not previously imaged in such large hole sizes, have been identified and their time-dependent development studied in detail. The results from this pilot campaign are discussed in this paper together with the 3D/4D tunnel-view visualization used to assess the processed caliper measurements.
The use of downhole rotational speed measurements made at 300Hz gives new insight into the conditions under which stick-slip torsional oscillations occur. Observations made with high-frequency magnetometers while drilling two reservoir sections have shown, that for these wellbores, using top-drive speeds below 140rpm leads to severe downhole torsional oscillations, also in off-bottom conditions. Additionally, it was observed that downlinking the rotary steerable system (RSS) initiated heavy stick-slip and that reaming downward had a negative damping effect on downhole torsional stability. These observations have been compared and partly matched with estimations made with a transient hydro-mechanical model. Some ill-defined information had to be estimated, like the amount of flow diversion during the downlinking procedure, the coefficients of static and kinetic friction along the borehole, or the bit efficiency and aggressivity. Downhole measurements have shown that when the drill-string is subject to strong stick-slip conditions, the downhole rotational speed changes from stationary to more than 400rpm in just a fraction of a second. It is therefore important to utilize sampling rates that are compatible with such very fast events. In practice, that means several hundred Hertz. A challenge associated with analysis of downhole high frequency data is time synchronization and drift of clocks between the surface and downhole measurements. After time synchronization and correction, it appears that the downhole rotational movement is delayed by several tens of seconds compared to the actual top-drive speed. This leads to question whether rotating the drill-string off-bottom, typically done in order to break gelled-up mud prior to establishing circulation, has any significant the intended effects deviated wells as torque along the drill-string must be built up. Direct observations of downhole rotational speed at high frequency help in discovering conditions that were not suspected to lead to large torsional oscillations. This new information can be used to improve drilling operational procedures and models of the drilling process, therefore enabling increased drilling efficiency.
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