根缘气(深盆气)成藏异常压力属性实验分析

Experiments on the abnormal pressure of source-contacting gas (basin-centered gas) accumulation

主实验采用内径7cm、高75cm的耐压玻璃筒,分15段填入粒径为0.05～4mm的白色等径石英砂,构成向上逐渐变粗的递变"砂柱",注入染成红色的水,石英砂柱分别与测压管相连。从玻璃筒底部向上注气,模拟注入的气体"活塞式"向上排驱孔隙水的过程,观察此过程中各砂粒段流体压力的变化过程。实验结果,注气速率较高时,粒径为0.3mm以下的砂粒段均可出现气水倒置并形成高异常流体压力,注气速率较低时只有粒径为0.15～0.2mm的砂粒段出现这些现象;不管注气速率高低,粒径大于0.3mm的砂粒段不出现气水倒置现象,保持常压。根据实验结果,认为典型根缘气(深盆气)成藏是高异常压力形成及其等压面推进的过程,因此原生地层压力应为高异常压力。盆地内部区域性高异常地层压力出现的顶界面可作为原生性根缘气藏的主要标志,可据该界面深度大致估计根缘气成藏时的注气压力。

A pressure-resistible glass tube with 7cm of inner diameter and 75 cm of height was adopted in the pressure experiments on source-contacting gas or basin-centered gas accumulation. After filled with red-dyed water, the glass tube was deposited with upward coursing sands of homogenized white quartz, whose grain diameters varied from 0.05 mm to 4 mm gradually in 15 segments of which the lower ones were connected respectively with piezometer tubes. In order to observe the variation process of fluid pressures in different sand segments, natural gas was injected into the glass tube from the bottom to simulate the piston-fashioned expelling of water by gas. The experiment result showed that the water over gas phenomenon occurred and abnormal pressures of fluids appeared in the segments in which sand grain diameters were less than 0.3 mm when the velocity of gas injection was relatively high while the sand grain diameters were less than 0.15-0.2 mm when the gas injection velocity was relatively lower. Regardless the gas injection velocities, there was nearly nothing to do with water over gas and overpressure when the sand grain diameters were greater than (0.3)mm. According to the experiment results, typical source-contacting gas is accumulated by the advancing of isobaric surface and characterized of overpressure.