Directional drilling utilizes the Measurement While-Drilling (MWD) to provide attitude for drilling bit. Traditional MWD system, consisting of magnetometers and accelerometers, rely on expensive and huge nonmagnetic drill collar (NMDC) to shield from magnetic interference to obtain accurate attitude angles. A fiber optic gyroscope-based MWD (FOG-based MWD) that is based on inertial navigation is an alternative survey scheme. But random vibration environment of underground introduces high amplitude random noise to the FOGs. Besides, the auxiliary measuring equipment (such as GPS) will not be available in underground environments, which will bring massive attitude error under pure inertial navigation. Therefore, combined with engineering application conditions, this paper proposes a modified attitude precision improvement approach of FOG-based MWD. Firstly, an autoregressive moving average (ARMA) error model for FOG is established to restrain the random vibration environment interference of underground. Secondly, Combined with FOG error model, a continuous backtracking navigation error model is developed to improve the azimuth accuracy. Moreover, the continuous backtracking algorithm frame is designed to estimate the attitude. Finally, Semi-physical experiment and drilling test verify that the error model has an obvious effect on reducing the amplitude of FOG random noise. The proposed backtracking algorithm make the azimuth angle achieve a high level estimation in short time. The result of drilling test show that the azimuth has an improvement of 48.79%, as compared to the traditional ZUPT.

Two key factors for calculating the trajectory of a wellbore are the attitude and interval of the survey stations. A measurement while drilling (MWD) system that relies on magnetometers and accelerometers may fail to measure the wellbore attitude in an environment with magnetic anomalies. In addition, in the case of a wellbore trajectory with large variations in the attitude, the commonly used minimum curvature method (MCM) can result in large deviations from the planned wellbore trajectory. In this study, a novel dead reckoning method was developed that is constrained by drill pipe length subdivisions for improved tracking accuracy of wellbore trajectories with a large variety of attitudes. A gyro MWD system based on fiber optic gyroscopes and accelerometers is utilized to continuously calculate the wellbore attitude. The variations in the attitude angle within the wellbore can be tracked by using the proportional relationship between increments in the attitude and drill pipe length subdivisions, which can be synchronized with the gyro MWD update frequency. Simulations and experiments were performed to verify that the proposed method could accurately track wellbore trajectories with large variations in the attitude. In the simulation, the proposed method demonstrated a mean trajectory deviation of less than 0.5 m over a distance of 30 m, which was markedly lower than the mean deviation of 2.5 m by the MCM. In the slope experiment, the proposed method demonstrated substantially better tracking accuracy of the wellbore trajectory than the MCM. Measurements from an actual wellbore with large variations in the attitude confirmed that the proposed method reduced the tracking error by up to 3 m compared to the MCM.

Vertical drilling is the first stage of petroleum exploitation and directional well technology. The near-vertical attitude at each survey station directly determines the whole direction accuracy of the borehole trajectory. However, the attitude measurement for near-vertical wells has poor azimuth accuracy because the poor signal-to-noise ratio of radial accelerometers hardly obtains the correct horizontal attitude, especially the roll angle. In this paper, a novel near-vertical attitude measurement method was proposed to address this issue. The redundant micro-electromechanical system (MEMS) accelerometers were employed to replace the original accelerometers from MEMS inertial measurement unit (IMU)/magnetometers for calculating horizontal attitude under near-vertical conditions. In addition, a simplified four-position calibration method for the redundant accelerometers was proposed to compensate for the installation and non-orthogonal error. We found that the redundant accelerometers enhanced the signal-to-noise ratio to upgrade the azimuth accuracy at the near-vertical well section. Compared with the traditional method, the experiment results show that the average azimuth errors and roll errors are reduced from 34.45° and 27.09° to 5.7° and 0.61°, respectively. The designed configuration scheme is conducive to the miniaturized design and low-cost requirements of wellbore measuring tools. The proposed attitude measurement method can effectively improve the attitude accuracy of near-vertical wells.

Inclinometers based on magnetometers are widely applied in oil and gas well measurements and have poor anti-interference ability. Affected by the drilling tools and stratum, the magnetized inclinometer will cause the output deviation of three-axis magnetometer. Therefore, periodic calibration before engineering applications is a necessary operation process. Fiber optic gyroscopes, with their characteristics of high precision and anti-interference, complement the advantages of magnetometers, which have improved the precision and reliability of the borehole attitude. In addition, a navigation grade three-axis fiber optic gyroscope provides convenience for periodic calibration of magnetometer. Consequently, a bi-vector field correction method based on a fiber-optic gyroscope and magnetometer inclinometer is presented in this study. First, the geometric relation between the seven elements of the geomagnetic field and Earth’s angular velocity was deduced with reference to the right-hand criterion. Second, a bi-vector field error compensation model was established. Finally, the simulation and experimental results demonstrated the effectiveness of the proposed calibration method.