Improvement in the method for borehole caliper measurement based on azimuthal gamma-gamma density well logging.

Accurate characterization of the size and shape of the borehole is critical to the effective borehole correction for logging while drilling (LWD) measurements. It is also necessary for the real-time evaluation of geomechanical wellbore stability and for optimizing drilling operation and determining proper completion strategies. The pseudo-caliper measurement can be derived from LWD azimuthal density measurement, where a linear empirical model is employed to calculate the tool standoffs. However, we found that as the tool standoff continues to increase, the model could no longer describe the variation in borehole size accurately. In this paper, the responses of LWD azimuthal density measurement under different logging conditions are studied using the Monte Carlo modeling method. To improve the estimation of borehole geometry, a new model is proposed to determine tool standoffs and measure the borehole caliper. Simulation models with different degrees of borehole enlargement are built to investigate the performance of the model. Compared with the currently used method, the wellbore caliper values calculated using the new method are more consistent with the actual values. A field example with mechanical caliper measured from wireline logs is also presented to validate the effectiveness of the proposed method.

A method for determining density based on gamma ray and fast neutron detection using a Cs2LiYCl6 detector in neutron-gamma density logging.

With the increasing demand for radioisotope-free operations, pulsed neutron-gamma density (NGD) has become increasingly important for logging-while-drilling (LWD) development. However, current NGD tools, adopting the multiple-detector array design, are not conducive to the simplification of instrument design and measurement system. To break obstacles, based on the fast neutron-gamma coupled theory, a new density measurement method was proposed. Further, combined with the neutron-gamma simultaneous detection characteristics of the Cs2LiYCl6 (CLYC) detector, an NGD measurement system consisting of a D-T source and one CLYC detector was used. Results show that the new method is capable of determining formation density using a single CLYC detector, which can not only avoid complex instrument systems but also improve density sensitivity. Moreover, the applicability of the new density method was well verified by Monte Carlo simulation. Additionally, the method was successfully applied in a simulated well, and density results are in good agreement with the benchmarked formations. The research provides theoretical guidance for NGD instrument design.