Log Subtle Interpretation Method of Changqing Gas ― bearing low Porosity and Permeability Sandstones

摘要

The reservoir is mostly close sand in Upperpaleozoic of Changqing gas field and it's welllogging evaluation is strongly affected by low Signal Noise Ratio, formation damage and anisotropy. The study on the "four properties" relation indicates that the constituent of rock definitively controls the micro ― pore ―structure, physical properties and gas ― bearing saturation. Usually, quartzitic sandstone develops the residual intergranular pore and kaolinite intercrystal pore, which has high permeability and production, the log put up short sonic interval transit time and high resistivity; but litharenite and lithic quartzitic sandstone mainly develops vugular pore and intercrystal pore, which has low permeability and production,the log shows long sonic interval transit time and low resistivity. According to the characteristic of tight gas sand,the paper introduced or put forward some new gas-bearing bed identification method, such as the three porosity overiap, gas sand well logging porosity lower limit, apparent elastic modulus minus ratio, time difference of sonic transit time and other statistical method based on the gas test data, this makes the precision of gas bed identification beyond 90%. Through "core scaling log", the paper computes the volume content of quartz, lithoclast and day, it also gets the high precision regressive formula of porosity, permeability and water saturation. The paper provides a log method for reserves calculation and reservoir commercial evaluation.