Abstract

The drill string used in drilling is in a complex motion state downhole for several kilometers. The operating attitude and eccentricity of the downhole drill string play important roles in avoiding downhole risks and correcting the output of the imaging measurement sensor while drilling (IMWD). This paper proposes a method for measuring eccentricity while drilling using two sets of caliper sensors coupled with a fiber-optic gyroscope for continuous attitude measurement, which is used to solve the problem of the quantitative measurement of complex eccentricity that changes in real-time downhole. According to the measurement and calculation methods involved in this article, we performed simulations of the attitude of the drill string near where the IMWD tool is located in the wellbore under a variety of complex downhole conditions, such as centering, eccentricity, tilt, buckling, rotation, revolution, etc. The simulation and field test results prove that the distance between the imaging while drilling sensor and the borehole wall is greatly affected by the downhole attitude and revolution. The multi-sensor layout measurement scheme and the data processing based on the above-mentioned measurement involved can push the drill collar movement and eccentricity matrix specifically studied downhole from only qualitative estimation to real-time measurement and quantitative calculation. The above measurement and data processing methods can accurately measure and identify the local operating posture of the drill string where the IMWD sensor is located, and quantitatively give the eccentric distance matrix from the measuring point to the borehole wall required for environmental correction of the IMWD sensor.

Highlights

  • With the development of petroleum energy, imaging while drilling has become an important technology to improve the penetration rate of oil and gas reservoirs, accelerate drilling speed, and reduce drilling cost

  • Because the diameter of the drill string is smaller than the diameter of the wellbore, the drill string is in a complex operating state downhole and is under the conditions of gravity; the additional reaction force of the bit in contact with the wellbore wall; the friction resistance caused by the drill string in contact with the wellbore wall; and other composite factors, such as eccentric self-rotation, revolution around the shaft wall, sinusoidal bending, vortex, repeated vibration, and other operating states

  • Through the above theoretical analysis and simulation experiments, it was proven that the downhole uncentrality of drill collars in logging while drilling (LWD) leads to variation in the distance between each measuring tool face and wellbore along the axial direction of the drill collars and around the drill string

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Summary

Introduction

With the development of petroleum energy, imaging while drilling has become an important technology to improve the penetration rate of oil and gas reservoirs, accelerate drilling speed, and reduce drilling cost. During drilling, imaging measurement sensors while drilling (IMWD) mounted in special drill collars continuously scan the borehole wall and the formation behind the borehole wall several kilometers downhole to obtain changes in the wellbore surface and geological information on the formation behind the borehole wall, such as natural gamma ray, neutrons, density, and resistivity. This information is used to reveal the geology of the drilling and to analyze and find recoverable reservoirs. Because the borehole wall will expand, shrink, and collapse in the process of drilling [1,2], resulting in uneven borehole wall, etc., it is more difficult to capture the downhole running track of the drill collar in real time and obtain the eccentric position of the sensor [3,4,5,6]

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