Drillstring vibration is one of the limiting factors maximizing drilling performance. Torsional vibrations/oscillations while drilling is one of the sever types of drillstring vibration which deteriorates the overall drilling performance, causing damaged bit, failure of bottom-hole assembly, overtorqued tool joints, torsional fatigue of drillstring, etc. It has been identified that the wellbore-drillstring interaction and well face-drill bit interaction are the sources of excitation of torsional oscillations. Predrilling analysis and real time analysis of drillstring dynamics is becoming a necessity for drilling oil/gas or geothermal wells in order to optimize surface drilling parameters and to reduce vibration related problems. It is very challenging to derive the drillstring model considering all modes of vibrations together due to the complexity of the phenomenon. This paper presents the mathematical model of a torsional drillstring based on nonlinear differential equations which are formulated considering drillpipes and bottom-hole assembly separately. The bit–rock interaction is represented by a nonlinear friction forces. Parametric study has been carried out analyzing the influence of drilling parameters such as surface rotations per minute (RPM) and weight-on-bit (WOB) on torsional oscillations. Influences of properties of drillstring like stiffness and inertia, which are most of the times either unknown or insufficiently studied during modeling, on torsional oscillation/stick-slip is also studied. The influences of different rock strength on rate of penetration (ROP) considering the drilling parameters have also been studied. The results show the same trend as observed in fields.
Formation-resistivity measurements in boreholes require accurate knowledge about the electrical properties of drilling fluid to exclude any measurement misinterpretations. For many drilling applications, oil-based mud is the preferred drilling fluid because of properties that include excellent shale stability, corrosion inhibition, lubricity, reusability, resistance to contamination, and higher rate of penetration. Although the electrical resistivity of oil-based mud is generally high, it depends on many parameters. We studied electrical resistivity and dielectric permittivity of oil-based muds as a function of frequency and temperature with varying oil and salt content and varying oil/water ratio. Measurements were performed by use of a coaxial test fixture and an impedance analyzer. The accuracy of the measurement system has been determined using standard materials such as air, deionized (DI) water, methanol, nbutanol, and n-octanol. The analysis shows that published models dealing with electrical properties of rocks can be compared well with those of oil-based muds. Resistivity and relative permittivity decrease with increasing temperature and frequency. No clear relationship is revealed between resistivity and water content, but relative dielectric permittivity does depend on water content of oil-based muds. Oil and salt content are of secondary importance. The study shows that the relative dielectric permittivity is related exponentially to the oil/water ratio of oil-based muds, which provides a means of estimating dielectric permittivity of oil-based mud if the oil/water ratio is known.
Maximizing drilling performance by pushing the limits of drilling tools is the key to reduce overall drilling cost. The interplay of complex parameters such as lithology, drilling parameters, drilling experience, downhole tool reliability, and the operational process help us to identify the limiting factors. Drilling dysfunction could deteriorate the reliability of downhole drilling tools including the overall drilling performance by causing damaged bits, failure of downhole electronic components, torsional fatigue of drillstring, over-torqued connections, etc. Precise modeling of dynamic responses of a bottomhole assembly in a given formation can help to identify operating limitations as well as post job analysis. The wellbore-drillstring interaction and well face-drill bit interaction are the sources of excitation of torsional oscillations. Stick-slip is one of the common and well-known torsional oscillation that usually occurs at low frequencies <1 Hz. High-frequency torsional oscillations (HFTO) are also recorded in the field by using high-bandwidth downhole vibration monitoring tools. HFTO occurs at much higher frequencies; >10Hz to 500 Hz. This paper summarizes the HFTO phenomenon that causes high tangential accelerations and torsional loads within the bottomhole assembly (BHA) during drilling in critical formations. HFTO modeling that is validated with high-speed measurements helps to identify tangential accelerations and torsional loads across the BHA components. This paper highlights the benefits of modeling the rotary steerable system (RSS) BHA for HFTO in order to identify the point in the BHA that is subjected to the highest tangential acceleration and torsional loads. Real time monitoring for drilling optimization of RSS BHA shows that the HFTO modeling and post-job data analysis of high-frequency data confirms the distribution of tangential acceleration and torsional loads along the BHA. The paper also highlights the accuracy of the HFTO modeling based on results of pre-job modeling and post-job high-speed data analysis from drilling runs. Based on the analysis and the study, recommendations for optimal BHA design and operational parameters are provided to yield maximum footage per run and improved BHA performance. High-frequency torsional oscillations in the BHA can be mitigated if adequate BHA design, drilling parameters, and procedures are implemented. The in-depth analysis of HFTO with accurate modeling and high-frequency measurements can be used to develop a knowledge base to further optimize the drilling process, reduce non-productive time, realize fewer trips for failures, extend the on-bottom drilling time, reduce maintenance costs, and increase the drilling efficiency and performance.
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