A fast forward algorithm of azimuthal gamma imaging logging while drilling.

Logging while drilling (LWD) technology can evaluate the formation interface and stratigraphic properties around the borehole in real time, so as to adjust the drilling trajectory and effectively improve the reservoir encounter rate. Fast forward algorithm can be used with LWD, by comparing the fast forward results with the actual gamma LWD results, it can distinguish the formation interface more accurately. Fast forward algorithm also can provide a theoretical basis for geosteering and azimuthal gamma logging interpretation. In this paper, based on the spatial distribution law of gamma rays and The Monte Carlo N-particle code (MCNP)-driven spatial sensitivity function, a 3D fast forward method of gamma ray LWD is proposed. Compared with the traditional algorithms, the new method can realize the fast simulations of gamma ray logging in four sectors, which enables fast azimuthal imaging. The comparison shows that whether in vertical wells or high-angle wells, the results of the proposed method are in high agreement with the MCNP fine simulations. In addition, the new method improves the calculation speed by tens of thousands of times with comparable accuracy. Finally, the gamma logging-while-drilling data of a highly deviated well in a field case verifies the accuracy and applicability of the method. The algorithm can meet the requirements of LWD for fast forward modeling of azimuthal natural gamma ray logging, which is expected to be applied to geosteering and logging interpretation of high angle and horizontal wells.