用纵波首波幅度识别砂岩气层的方法

Identification of gas-bearing sand formation using the amplitude of head P wave

从纯砂岩、泥质砂岩(人工岩样)与其含气情况下的纵波首波幅度比与含气饱和度的实验发现,当砂岩内含气饱和度大于20%时,纵波首波幅度的衰减可达到最大值。而且,纵波首波幅度对砂岩内的天然气响应比纵波首波频率和速度敏感。基于这一实验结果,本文提出了用纵波首波幅度识别砂岩气层的方法。目的层岩石的纵波首波幅度可由声波波形分析图求取,而该目的层岩石的纵波首波幅度背景值,可用邻近含水层或含油层(轻质油层除外)岩石的纵波首波幅度代替。这样,当目的层岩石的纵波首波幅度小于该层的幅度背景值30%±10%时,则指示此目的层岩石泥浆侵入带内存在天然气。据此,可识别此处目的层是气层。另外,文中还给出了描述含气层岩石和含水层岩石纵波首波幅度的数学模型及应用纵波首波幅度识别砂岩气层的两个实例。

We made the experiment to find the relation between gas saturation and the ratio of head P wave amplitude in pure sandstone(or mudy sandstone)to head P wave amplitude in gas-saturated sandstone(mudy sandstone).The experiment result says that the attenuation of head P wave amplitude gets maximum when the gas saturation in sandstone is above 20%.Besides,the amplitude of head P wave is more sensitive to gas-bearing sandstone than the frequency and velocity.A method for indicating gas-bearing sand formation with the use of head P wave amplitude is put forward on the basis of the experiment result.The amplitude of head P wave from an objective formation can be obtained from sonic waveform analysis plot,and the background amplitude of head P wave from the objective formation can be substituted by the amplitude of head P wave from the neighbour water saturated or oil- bearing formation(except light oil formation).When the amplitude of head P wave from the objective formation is less than 30%(±10%)of the“substitute”background amplitude of this formation,we can say there is gas in mud-invading zone in the objective formation,and thus call this objective formation a gas reservoir. The paper shows the mathematic models which describe respectively the amplitudes of head P waves from gas-saturated and water-saturated sandstone formations.Two examples are given of identifying the gas reservoir by applying the amplitude of head P wave.