随钻脉冲中子测井识别天然气的数值模拟

由于天然气具有密度低、黏度小等特点,利用3种孔隙度资料可以定性识别天然气层,但定量评价存在困难。为此,在随钻过程中依据天然气与油、水的含氢指数不同,利用脉冲中子测井技术记录的近、远探测器热中子或俘获伽马计数率比的相对变化量来定量确定含气饱和度;在此基础上,利用蒙特卡罗方法建立计算模型,模拟不同井眼和地层条件下脉冲中子测井远近探测器记录的热中子或伽马计数,研究其比值与含气饱和度的测井响应。结果表明:油水层和气层的计数相对变化量能反映地层的含气饱和度,孔隙度越大,相对比值越大,对气层的定量评价越准确;岩性、泥质含量、地层水的矿化度、井眼流体和尺寸以及钻井液侵入等因素都会对天然气地层的脉冲中子测井响应产生影响。总之,利用脉冲中子测井技术可以定量评价天然气层,对提高天然气识别能力和气田高效勘探开发具有重要意义。

Numerical Simulation of gas detection through pulsed-neutron logging-while-drilling

Although qualitative identification of gas reservoirs is possible by using neutron porosity, density porosity and acoustic porosity data, it is very difficult to perform quantitative evaluation due to low density and low viscosity of natural gas. Therefore, while drilling, according to different hydrogen indexes of gas, oil and water, gas saturation can be calculated quantitatively by using a parameter D, i.e. the relative variation of the ratio between thermal neutron/capture gamma ray count rates of near detector and that of far detector. On basis of this, a calculation model is built by using the Monte Carlo method to simulate the thermal neutron or capture gamma ray counts recorded by the near and far detectors in different boreholes with different geologic conditions. The relationship between D and gas saturation is established. The simulation results show that the parameter D can reflect gas saturation of a reservoir. The higher the porosity is, the larger the D becomes, and the more accurate the quantitative evaluation of gas layers will be. Various factors, such as lithology, shale content, formation water salinity, borehole size, fluid type, and drilling mud invasion, can affect the pulsed neutron logging response of gas layers. In summary, this technology of gas detection through pulsed neutron logging while drilling can be used to quantitatively evaluate gas layers, and thus it is of great significance for gas detection and highly efficient exploration and development of gas fields.