Characteristic analysis of a new type of drilling pressure torque measurement sensor

Abstract: Near-bit engineering parameters include drilling pressure, torque, temperature and annular pressure, etc. The accurate measurement of these parameters while drilling is of great significance for safe and efficient drilling. Based on the analysis and comparison of various existing near-bit engineering parameter measurement technologies, the basic measurement characteristics of a new near-bit drilling pressure and torque while drilling sensor, such as the stress-strain relationship at different angles, the influence of drill collar internal pressure, and the coupling relationship between drilling pressure and torque, are analyzed in detail using the finite element analysis software ANSYS. This provides a theoretical basis for strain gauge pasting and internal pressure correction algorithm development, and also lays the foundation for the development of near-bit engineering parameter while drilling short-circuit measurement.
Keywords: drilling engineering; near-bit measurement; while drilling measurement; torque; drilling pressure

During oil and gas drilling, the accurate measurement of engineering parameters such as drilling pressure and torque near the drill bit while drilling is of great significance for safe and efficient drilling. With the increase of well depth, especially the development of special process wells such as branch wells, horizontal wells, and herringbone wells, various drilling accidents such as stuck drill, cutter loss, and drill bit breakage occur from time to time, which has a great impact on the safety and efficiency of drilling production. By analyzing and processing the real-time measurement values ​​of near-bit engineering parameters, we can summarize the influence of various measurement parameters on the drilling process and efficiency, timely discover and control certain drilling accidents, achieve the goal of safe and efficient drilling, and truly realize risk-free drilling.
For many years, the focus of the development of downhole measurement instruments at home and abroad has been the measurement of geological parameters such as formation resistivity, porosity, and gamma rays that are directly related to oil and gas geological reserves; the measurement and control of wellbore trajectory parameters such as well inclination, azimuth, and tool face angle related to geometric guidance; while there are few studies on the measurement technology of engineering parameters such as drilling pressure, torque, and annular pressure related to drilling safety and drilling efficiency.

1 Near-bit engineering parameter measurement technology
1.1 Force analysis of near-bit drill collar
At present, oil and gas drilling methods are mainly based on drill plate drilling and downhole power drilling tools drilling. During the different drilling processes such as drilling, drilling, and pulling out, the stress conditions and movement forms of different parts of the drill string/drill collar are very different. Mainly include: axial tension and pressure, torque, bending moment, centrifugal force, internal and external extrusion of drill collar, longitudinal vibration, torsional vibration, lateral oscillation, etc. Due to the complex movement of the drill string and drill collar, the drill bit has vortex phenomenon at the bottom of the well, the drilling pressure at the bottom of the well fluctuates greatly, and even the drill bit jumps off the bottom of the well.
In theory, the forces and moments on the drill collar can be simplified as: drilling pressure applied to the drill bit, torque transmitted to the drill string, bending moment generated by the movement of the drill string and the reaction force at the bottom of the well, and vibration of the drill bit during drilling. From the perspective of measurement technology, the forces on the drill collar can be simplified as the axial tension, compression and vibration of the thick-walled circular tube, a pair of torques around the axial direction, and the bending moment on the drill collar in the radial direction.
1.2 Principle of drilling pressure torque measurement
The measurement of tension, compression and torsional stress in material mechanics is based on the strain effect of the object under force and is realized by the principle of strain measurement. Strain gauges are pasted along the axial direction of the drill collar cylinder at 0° and 90°, and the magnitude of the tensile and compressive forces on the drill collar is obtained by measuring the resistance change of the strain gauges; strain gauges are pasted along the axial direction of the drill collar cylinder at ±45°, and the magnitude of the torsional torque on the drill collar is obtained by measuring the resistance change of the strain gauges; however, this principle is applicable to the measurement of single tensile and compressive forces and single torque forces, and cannot be directly applied to the measurement of engineering parameters under high temperature, high pressure, and composite stress in downhole.
Based on the above measurement principle and the actual working process of downhole instruments, the first drill string mechanical parameter measuring instrument was developed and patented by the French Petroleum Research Institute in 1985. Subsequently, famous oil instrument companies such as Schlumberger, Baker Hughes, APS and other companies have successively developed downhole engineering parameter measurement short circuits with different structures, and applied for relevant downhole engineering parameter measurement short circuit patents around 2000. Based on this, domestic researchers also applied for corresponding patent technologies around 2005.
1.3 Comparison of existing drilling pressure torque measurement technologies
The current representative drilling pressure torque measurement technologies are still the two types of patented technologies of the French Petroleum Institute and Schlumberger. Other technologies are more or less improved based on these two patents. The advantages and disadvantages of these measurement technologies are analyzed below.
The patents of the French Petroleum Institute and Baker Hughes are the basic tension, compression and torque measurement principles plus different structures of downhole instrument protection covers, different measurement circuits and sensor connection methods. The common disadvantage of these two patents is that it is difficult to seal the protection cover and the sensor part. Especially in the working process of the downhole drill collar, the bending moment often causes mud to invade the sensor part, resulting in the measurement circuit not working properly. For this reason, Baker Hughes has carried out a lot of work on the sealing of the protection cover, the sensor part, and the conversion circuit, which has solved this problem to a certain extent.
Schlumberger and APS have made further improvements to this technology. They drilled holes of a certain diameter and depth in the drill collar radially, pasted the strain gauge in the borehole, and then sealed the strain gauge inside with a high-pressure sealing cover. The electrode leads of the strain gauge are interconnected through the internal connection channel between the boreholes, and finally connected to the measurement circuit installed in the pressure-resistant cylinder in the middle of the drill collar or in the wall groove of the drill collar. The common point between the two is that the sealing problem of the protective sleeve is solved. The difference lies in the arrangement of the radial holes, the connection method of the strain gauge leads, and the connection method with the secondary conversion circuit. The disadvantages of this technology are also obvious: first, the internal lead holes are difficult to process, and they often need to be processed separately and then welded together, or processed with special tools; second, the diameter of the radial hole cannot be too large, which makes it very difficult to paste the strain gauge; third, the measurement characteristics of this sensor also show a certain nonlinearity, and it must be ground calibrated and calibrated before it can be applied to actual measurements.

2 Sensor structure design and characteristic analysis
2.1 Overall structure of downhole engineering parameter measurement unit
Figure 1 shows the overall structure diagram of the drilling pressure torque sensor 1 connected with the upper connecting drill collar 2, the pressure tube and the measurement circuit 5. Among them: the sensor 1 is used to paste the strain gauge for measuring drilling pressure and torque, and the strain gauge lead is introduced into the torque measurement bridge and single-chip circuit installed in the pressure tube 5 through the wire hole; the lead hole is sealed by the high-pressure sealing cover plate 4. The upper and lower cross-sectional views in Figure 1 respectively represent two mutually perpendicular overall cross-sectional views of the downhole engineering parameter measurement unit. Another cross-sectional view 3 in Figure 1 clearly shows the mud channel at the connection of the pressure tube, and 4 in Figure 1 is the lead hole sealing cover plate.

Under normal drilling conditions, the drilling pressure torque measurement value is relatively stable. The drilling pressure torque value during the drilling process is recorded and stored by the single-chip computer for drilling process analysis after drilling and playback. When the drilling pressure torque measurement value is abnormal, the real-time measurement value is transmitted to the ground monitoring system through the downhole hydraulic pulse generator for reference by drilling personnel for decision-making.
2.2 Finite element calculation model of sensor
Figure 2 (a) is the finite element calculation model of the sensor. The strain gauge is installed in three circular holes with a circumferential direction of 120 degrees. The circular holes are connected through connecting channels and wires. The connecting channels between the three circular holes refer to the radial section of the sensor 2 (b). The circular holes are sealed with the cover plate 4 and the sealing ring in Figure 1. Since the pressure in the circular hole is normal pressure during drilling, and the outside is the annular pressure of the drilling process, the sealing effect is very good. The strain gauges are distributed in 8 directions in 3 drilled holes: 0°/45°/90°/135°/180°/225°/270°/315°. The strain gauge pasting method is shown in Figure 2(c).

The sensor dimensions used for finite element calculation are: drill collar outer diameter 178 mm, drill collar inner diameter 76 mm, measuring part hole diameter: 45 mm, measuring part hole depth: 39 mm, cover plate thickness: 15 mm, cover plate side length: 60 mmx60 mm. From the knowledge of material mechanics, it can be known that the strain gauges in the four directions of 0°/90°/180°/270° are sensitive to drilling pressure measurement, and the strain gauges in the four directions of 45°/135°/225°/315° are sensitive to torque measurement. In theory, connecting the strain gauges with corresponding pasting angles in different holes in series to the measurement bridge can offset the influence of the bending moment on the drilling pressure torque measurement results. If the bending moment increases the resistance of a strain gauge, it will inevitably reduce the resistance of the strain gauge in the opposite direction, so that the total resistance of the series connection remains unchanged. Therefore, the effect of the bending moment is not considered in the calculation process.
2.3 Analysis of finite element calculation results
Through ANSYS modeling, loading and calculation, the influence of drilling pressure and torque on sensor output when acting alone and in combination is analyzed, and the influence of drill collar internal pressure on these two measurement parameters is analyzed, and the correction method of the influence of drill collar internal pressure and annular pressure is sought.
The drilling pressure and torque parameters used in the calculation process are shown in Table 1. The calculation points marked in the table are the basis for the following analysis. Figures 3 and 4 respectively show the calculation results when drilling pressure and torque act alone. Considering the symmetry of the strain gauge pasting, the two figures only give the output results of the strain in the four directions of 0°/45°/90°/135°.

In Figure 3, the strain gauges at 45° and 135° increase linearly with the increase of torque, while the output of the strain gauges at 0° and 90° remains almost unchanged; in Figure 4, the strain gauges at 0° and 90° increase linearly with the drilling pressure, but the torque measurement strain gauges at 45° and 135° also change linearly with the drilling pressure.
It can be seen that the actual drilling pressure torque value can be obtained by measuring the strain of strain gauges in different directions. However, strain gauges at different angles have different sensitivities to drilling pressure and torque. Torque is only sensitive to strain gauges at 45° and 135°, while strain gauges in all four directions are sensitive to drilling pressure. In other words, drilling pressure has an impact on torque measurement, while torque has almost no impact on drilling pressure measurement. Under actual measurement conditions, the coupling degree and decoupling algorithm between the two parameters must be considered.

FIG5 and FIG6 respectively show the influence of the internal pressure of the drill collar on the measurement results. The two figures only consider the influence of the internal pressure of the drill collar on the measurement results, and do not consider the influence of the annular pressure.

The two groups of straight lines in Figure 5 represent the variation of strain under the same torque when the internal pressure is 30 and 60 MPa respectively. It can be seen from the figure that with the increase of internal pressure or the increase of the difference between the internal pressure of the drill collar and the annular pressure, the intercept of the characteristic curve changes, which will cause the zero point of the measurement circuit to change; but the slope remains basically unchanged, which requires dynamic zero point adjustment during the measurement process.
Figure 6 shows the variation of the strain of the sensor under the drilling pressure when the internal pressure is 0, 30, and 60 MPa respectively. Similar to Figure 5, the intercept of the characteristic curve changes with the change of internal pressure, but the slope hardly changes, which also requires dynamic zero point adjustment during the measurement process.

3 Conclusions
Through different strain gauge layouts, the sensor can achieve the measurement goals of drilling pressure and torque. Among them: the strain of a single working condition is linearly related to the drilling pressure and torque value. In actual use, it is necessary to consider the mutual influence of drilling pressure and torque, among which: the influence of torque on drilling pressure is small, but the influence of drilling pressure on torque is large. It also illustrates the necessity of decoupling the measurement process and calibration work before measurement. Compared with the case where the internal pressure is zero, the intercept of the characteristic curve has changed, indicating that the zero point has changed, but the slope has basically not changed, requiring dynamic zero point adjustment during the actual measurement process; the slope has not changed because within the elastic range of the material, the strain law is still the linear Hooke's law.