Standoff calculation and application of azimuthal density logging based on broad- beam γ-ray attenuation

Abstract

BackgroundAccurately calculating the borehole size and standoff between the tool and borehole wall is the premise for correcting the effect of the borehole environment and improving measurement precision in logging while drilling (LWD).PurposeThis study aims to obtain an accurate calculation method for the standoff size of LWD.MethodsFirstly, the theoretical relationship of the standoff was derived based on the broad-beam γ-ray attenuation model for azimuth density LWD. Then, Monte Carlo N-Particle transport code (MCNP) was employed for simulation, and results were benchmarked according to experimental tool data. By analyzing the influence of standoff, mud and formation density, and other factors on the detector response, an accurate standoff calculation formula was derived. Finally, logging curves were drawn using the CIFLog platform and the calculated standoff results were compared with the measured ultrasonic results, hence to verify the accuracy of the proposed standoff calculation method.ResultsThe calculated standoff results in the simulation are fundamentally consistent with the theoretical value. The calculation result using measured data processing is in approximate agreement with the ultrasonic standoff, and the calculated calipers have good correspondence with the wireline well diameter. The proposed method provides accurate measurement even when the standoff is small and ultrasonic standoff measurement is abnormal. The applicable range of standoff calculation based on azimuthal density LWD is 0~3.81 cm.ConclusionsThe broad-beam γ-ray attenuation based standoff calculation method for density LWD complements to larger range ultrasonic measurements to provide key parameters for borehole correction for LWD tools.

Keywords

• azimuthal density logging-while-drilling
• broad-beam attenuation
• standoff
• caliper
• monte carlo simulation