Study Of Fast Forward And Negative Inversion Method For Natural Gamma Logging While Drilling

Faced with the complicated international situation today,increasing the exploration and development of onshore deep,deep sea and unconventional oil and gas.Increasing oil and gas reserves and production is an important approach to ensure China’s energy security.As an important part of the logging-while-drilling(LWD)logging and geosteering technology,the fast forward and inversion technology of natural gamma logging can evaluate the reservoir properties in real time through the continuous iterative update of the geological model.This technology can guide drilling strategies and improve drilling encounter rates,is a key technology in oil and gas exploration and development efforts.Unfortunately,this technology has been monopolized by large foreign oil service companies for a long time.In recent years,Chinese researchers have made some research results in this area,but the following problems still exist: the geometric model of the orthorectified detector is over-simplified and the simulation accuracy needs to be improved;the existing method cannot perform high-precision multi-directional gamma imaging simulation calculation;there are difficulties in handling complex borehole trajectories and geological models;the impact of borehole size and intra-well fluids on simulation results is not considered;the algorithm is not applicable to The algorithm is not suitable for thin geological conditions and cannot simulate and analyze the response characteristics of thin mutual layers;there is a lack of non-human-machine interactive automated stratigraphic gamma value acquisition and stratigraphic interface delineation methods.Based on the above problems,the research is based on the time-independent Boltzmann equation to obtain a three-dimensional sensitivity function,which fully takes into account the relationship between the detector geometry and the location of the borehole.Finally,a set of azimuth gamma logging response rapid characterization methods were formed.In this study,geometric projection and double coordinate system conversion methods are used to obtain multi-directional spatial flux functions,and a set of drill-while-drilling azimuth gamma fast forward algorithms suitable for complex borehole trajectories and geological models is developed.The comparison and verification of the new algorithm and Monte Carlo numerical simulation results are completed,and the results show that the new method can handle the thin interlayer case.The calculation of the azimuth gamma curve is very consistent with the Monte Carlo calculation results,and the correlation coefficient is greater than 0.99.Based on the comparison and analysis of the proposed method and the measured gamma data,this paper provides guidance for the identification of on-site stratigraphic interface and the update of geological model.In addition,Monte Carlo numerical simulation method is used to study the characteristic changes of spatial sensitivity function under different logging conditions.Considering the influence of different formation densities and wellbore conditions on the logging response,the fast forward algorithm of LWD azimuth gamma was optimized.Based on the new algorithm,the gamma response of different formation thicknesses and inclination angles to the detector was studied,especially the gamma response characteristics of thin interlayer.It is found that the trend of gamma response curve is mainly dominated by the random thickness division of a single thin layer in the thin interlayer,and the gamma response value is about the product of the proportion of thin layers of mudstone and sandstone in the detection range and their corresponding radioactive values.Finally,combined with the LWD gamma fast forward algorithm,an automatic division inversion algorithm for formation interface and formation gamma value based on fitted Newton method(BFGS)is formed.The results of simulated data processing show that this method achieves good results in both straight wells and horizontal wells.The new method is expected to be attached to CIFLog,a professional logging interpretation software platform independently developed in China.The next step will be applied to the natural gamma logging site and post-interpretation evaluation.