Monte Carlo Simulation Of Logging-while-drilling Natural Gamma Ray Imaging

The natural gamma logging-while-drilling(LWD)logging technology can accurately reflect the boundary information of strata and is an effective method for stratigraphic division of strata.This technology can timely feedback reservoir-to-edge information,enabling the drill collar to adjust its direction along the reservoir and more efficiently ascertain the distribution of reservoir reserves.It is an essential component in the logging-while-drilling logging instrument system.The main problem solved in this paper is how to use Monte Carlo method to simulate the process and results of natural gamma LWD logging technology.The work of simulation was completed using the MCNPX program,and the results were presented in the form of forward modeling polychromatic maps.In order to assist in designing and improving the domestic natural gamma LWD logging system,the Monte Carlo simulation program was used to simulate the natural gamma LWD logging process and accumulate data for preparation.Monte Carlo design section introduces the simplification,design and improvement of the Monte Carlo model.The action of the drill collar spiraling in the strata is simplified as a superposition of stepping rotation and stepping advancement.The measurement results of each point are simulated by MCNPX program separately.The summary results after batch processing are equivalent to the entire LWD process.It takes four steps to finish the Monte Carlo design which are geometry construction,source definition,counting method and companion program.Necessary skills are used to complete the work and many adjustments and improvements have been undergone.The work to find an appropriate groove depth and a new progressive strata model are also described in this section.The imaging results section describes the variation of forward modeling polychromatic maps with different hole angles in a double layer strata model and the model lateral comparison results.When the detector is deep enough in the target strata,detectors in all angles show nearly the same number and can be transformed into API number.The maps are layered one above the other,and the boundary line is consistent with the cosine function.The larger the peak-to-peak value,the larger the difference between the counts of the upper and lower detectors and the larger the hole angle.There is an approximately proportional relationship between the peak-to-peak value and the tangent of the hole angle.Horizontal wells can be regarded as the limit of nonhorizontal wells.The distance between the drill collar and the interface of two strata can be judged according to the width of the stripe.Lateral contrast between similar Monte Carlo simulation results,laboratory measurement results,and actual natural gamma LWD logging results are consistent.Compared with other Monte Carlo simulation results,this model tends to be more theoretically calculated,and strives to solve practical problems such as structural design of drills and processing of forward imaging results.We will continue to improve the model,solve the problems encountered in the design of the instrument,and explore other theoretical possibilities that may be excavated from the forward imaging results.