Geosteering technology and application in horizontal wells for coal measure tight gas reservoirs in the areas of low exploration degree：Taking DJ-P37 bore field as an example

In the 2D seismic area, the reservoir with a low well control degree, frontal sedimentary of the meandering river's delta and other complex lithology reservoirs, the drilling ratio of reservoirs is low when horizontal wells are implemented. The application of the geosteering technology combining the iterative updating of 3D geologic models and geologic-engineering integration has been gradually extensive. Multiple tight sandstone reservoirs, such as the Beichagou sandstone in the Shan₂ ³ Submember of the Shanxi Formation, develop in the Neopaleozoic coal measure at the southeastern margin of the Ordos Basin. Among them, the DJ-P37 bore field of the Daning–Jixian Block has the thin sandstone bed, complex longitudinal lithology and fast horizontal change, with the micro-amplitude structure developing; in this bore field, there is generally no departure well, and the end well is in a long distance, resulting in difficult horizontal well steering. In order to enhance the drilling effects of the horizontal wells in the areas with a low degree of exploration, study is implemented by means of marker bed identification and determination, comparison between the coal bed and rock stratum, accurate analysis and completion of data, identification of reservoir top and bottom characteristics, etc. Firstly, the methods for quickly differentiating coal beds from carbonaceous mudstone in the study area were summarized, so as to solve the problem of difficult field lithology identification and build the stratigraphic framework in the study area. Secondly, on the basis of the meticulous stratigraphic correlation of multiple logging data, seismic data and geological data, one Class Ⅰ marker bed and three Class Ⅱ marker beds were determined; two Class Ⅱ marker beds were further determined in the Shan₂³ Submember. Finally, the technology of landing site control, the technology of improving the drilling ratio in horizontal sections and the method of determining whether a bit penetrates reservoirs were proposed, forming the geosteering technology for horizontal wells in delta’s frontal subfacies reservoirs. The technology was applied in the DJ-P37 bore field, and accumulatively 34 wells were completed, with 78.30% of the average drilling ratio in the sandstone, providing good drilling effects. The application can provide technological references for the coal measure horizontal well drilling in similar areas.