Development of a high temperature and high pressure downhole string dynamic load monitor

Due to the uncertainty of the formation and the complexity of downhole tubular string loads and deformations, the existing tubular string static theory is difficult to explain and lacks support from measured data of tubular string dynamic loads. Therefore, a dynamic loads monitor for oil and gas tubular string is developed, which can synchronously measure the internal pressure, external pressure, axial force, and vibration acceleration for the string during the whole operation process. The monitor realizes simultaneous monitoring of tubular string load and vibration response parameters while meeting the requirements of various sensor placement spaces and structural strength safety, simplifying the on-site test process. By adding a power control circuit to the circuit system, the working time of the monitor is extended. The high temperature calibration test of the monitor is carried out, and the maximum calibration error is 0.10%, which meets the requirements of measurement accuracy.


Introduction
With the continuous deepening of exploration and development, the downhole working conditions are getting worse, especially during the perforation, injection and production operations of hightemperature and high-pressure wells.The vibration of the tubular string will produce drastic changes in dynamic load, which will easily cause tubular string tripping and safety accidents such as rupture [1][2].Due to the uncertainty of the formation and the complexity of the downhole tubular string load and deformation, the existing static theory of the tubular string is difficult to explain, and lacks the support of the measured data of the dynamic load of the tubular string.
During oil testing, the main loads on the tubular string are axial force, torque, internal and external pressure, etc.At present, resistive strain gauges are mostly used to measure the axial force and torque of the tubular string, and piezoelectric sensors are mostly used to measure the internal and external pressure of the tubular string [3][4].In addition, during perforation, injection and production, the tubular string vibrates due to fluid pressure changes, and the resulting dynamic load will further aggravate the failure risk of the tubular string [5][6].The response parameters describing the vibration of the system include displacement, velocity and acceleration.The vibration response of the downhole tubular string is generally measured by the acceleration sensor.This measurement method was initially applied to the drill string system, and the dynamic load of the drill string was measured by installing an acceleration sensor in the radial direction of the drill string [7,8].Liang [9] developed a storage-type downhole load measurement device that can collect parameters such as downhole temperature, pressure, and tubular string stress in real-time.Liu et al. [10] designed a storage oil well tubular string load tester based on strain detection technology, which can test oil well tubular string axial load, radial load, torque and other parameters.Hu [11] proposed a solution for measuring the tensile and compressive loads of oil well strings based on an electric bridge strain gauge structure, and used a resistance strain sensor to detect the strain stress of the downhole string packer to test its load size.Xie and Yi [12] designed a highly deviated well water injection tubular string load tester based on strain testing technology and storage and playback mode.Dou et al. [13] developed a downhole tubular string load tester for oil testing, which uses axial and strain signals measured by circumferential and 45° angle strain gauges to derive tubular string load values such as axial force, internal pressure, and torque.This article develops a dynamic load monitor (hereinafter referred to as the monitor) for oil and gas well tubular string suitable for high temperature and high pressure wells, which can simultaneously measure the internal pressure, external pressure, axial force, vibration acceleration and other parameters of the tubular string during the entire operation process.Accordingly, measured data support can be provided for the tubular string dynamics theory.

Monitoring method of tubular string dynamic load
As shown in Figure 1, before the monitor goes into the well, the acquisition strategy is preset for the monitor through the control software of the host computer, and then the monitor is connected to the tubular string and runs into the designated position downhole.During operation, the monitor measures and records parameters such as internal pressure, external pressure, axial force and vibration acceleration of the tubular string in real-time through sensors and related circuits.After the operation is over, the monitor and the tubular string are taken out together, and the test data is read and played back through the host computer control software.The host computer control software can set different collection strategies for the monitor according to the construction plan, such as sampling interval, number of sampling points and collection time.The monitor is equipped with pressure sensors, axial force sensors, acceleration sensors, temperature sensors and circuits for data processing, acquisition, communication, etc., and is powered by built-in batteries.

Structural design of tubular string dynamic load monitor
The inner diameter of the monitor running into the casing is generally not more than 150 mm, and the inner diameter of the tubular string flow channel connected to it is not less than 46 mm.Therefore, it is determined that the maximum outer diameter of the monitor is 140 mm and the minimum inner diameter is 48 mm.According to the design requirements, the structural dimensions and strength design parameters of the monitor are shown in Table 1.
The monitor consists of a body, an inner cylinder, an outer cylinder and a lower connector, as shown in Figure 2. The main body is the backbone of the entire mechanical structure and also the carrier for transmitting the axial force of the tubular string.The monitor consists of a body, an inner cylinder, an outer cylinder and a lower connector, as shown in Figure 2. The main body is the backbone of the entire mechanical structure and also the carrier for transmitting the axial force of the tubular string.Because the axial force generated by the expansion effect needs to be offset, a gap is left between the inner cylinder and the body.This is to accurately measure the axial force, avoid stress caused by internal pressure, and improve the tensile strength of the body.To protect sensors, electronic components, batteries, etc., there is a high-temperature-resistant O-ring seal between the outer barrel and the body.In order to transmit the internal pressure and external teeth of the tubular string and install the pressure sensor, a pressure transmission hole is designed at the lower joint.The material of the monitor body, inner cylinder and outer cylinder is 40CrNiMoA, the heat treatment hardness is HRC38-42, and the yield strength is 1033 MPa.After the strength check, all parts of the monitor meet the design requirements.

Circuit system design of tubular string dynamic load monitor
The monitor circuit system is shown in Figure 3.The A/D conversion circuit is used to convert the measured signal from analog to digital; the key circuit is used for sensor zero calibration and other operations; the liquid crystal display circuit is used to display relevant parameters when debugging the system; the microprocessor is used to receive control instructions from the host computer, and controls the A/D converter to collect sensor signals; the data communication module is used to transmit data between the monitor and the host computer.In order to meet the needs of the underground high temperature environment, all sensors and electronic components have passed the high temperature resistance screening test at 150°C.

Strength check of tubular string dynamic load monitor
Figure 4 shows that when the detector is subjected to an axial force of 800 kN and internal pressure of 90 MPa, the average stress of the detector body affected by the axial force and internal pressure is 523 MPa, and the safety factor is 1.97.Because the wall thickness at the connection between the body and the lower street is small, the maximum Mises stress value is 817.5 MPa, and the safety factor is 1.25.
Because the lower joint and the inner cylinder installation sealing groove are only affected by internal pressure, the stress value is relatively small.The maximum Mises stress value is 601.8MPa, with a safety factor of 1.72, and the minimum is 451.4MPa, with a safety factor of 2.29.At the opening of the internal and external pressure transmission channel of the lower joint, the average Mises stress is 400.00MPa, and the safety factor is 2.58.The maximum stress at the thread is 830.6 MPa, which occurs at the roots of the first three threads, and the safety factor is 1.24.As shown in Figure 5.After finite element simulation, the detector strength meets the requirements.

High temperature standard inspection test of tubular string dynamic load monitor
During perforation, injection and production operations, the pressure and temperature change drastically, and the pressure sensor is extremely sensitive to temperature.Therefore, it is necessary to conduct a high temperature pressure standard inspection experiment to screen out electronic components that can work normally at 150°C and perform calibration.As shown in Figure 6, the pressure sensor is connected to the circuit board and it is installed on the High temperature pressure standard inspection test bench.The initial standard inspection pressure and temperature are 0 MPa and 20°C respectively, and the pressure and temperature gradients of each stage are 20 MPa and 40°C respectively, and each stage is stable for 2 minutes.Positive and negative stroke standard inspection test.The pressure standard test results at 150°C are selected as shown in Figure 6, and the maximum error of the standard test is 0.10%.

Ground gas internal pressure test
The ground gas internal pressure test is divided into two parts.First, the ground pressure test equipment and pressure pipelines are connected to prepare for the pressure test.Then the central tube gas is pressurized to 90 MPa and stabilized for 15 minutes.The data recorded is shown in Figure 7.
The experimental results show that the overall test air pressure on the ground is 89.0 MPa, and the pressure is maintained for 15 minutes.There are no bubbles emerging, and the test is qualified.

Conclusions
(1) This article developed a dynamic load monitor for oil and gas well tubular string suitable for hightemperature and high-pressure wells, which can measure parameters such as internal pressure, external pressure, axial force, and vibration acceleration of the tubular string during the entire operation.The simultaneous monitoring of tubular string load and vibration response parameters is realized, and the field test procedure is simplified.
(2) The inner cylinder of the monitor can isolate the influence of internal pressure on the body, which not only ensures the measurement accuracy of the axial force, but also improves the tensile strength of the body.After the high temperature standard inspection experiment, the maximum error is 0.10%, which meets the design requirements.

Figure 6 .
Figure 6.High temperature test and pressure calibration for pressure sensors.

Figure 7 .
Figure 7. Ground gas medium internal pressure test.