致密气藏束缚与可动水研究

RESEARCH ON THE IRREDUCIBLE AND MOVABLE WATER OF TIGHT SANDSTONE GAS RESERVOIR

致密气藏一般束缚水含量高,但不少气井仍可产出水,这种现象很难解释。采用离心毛细管压力法、气驱水动态法及核磁共振法在实验室内研究测定气藏束缚水饱和度,用测井方法确定气藏原始含水饱和度,根据原始含水饱和度与束缚水饱和度相差值大小来确定可动水饱和度大小。采用中原油田桥口、白庙致密气藏及鄂尔多斯大牛地气田具有代表性的岩心开展渗流实验测试后发现:随着气流速度的增大,岩心含水饱和度逐渐减小,甚至会低于用常规方法测试的束缚水饱和度,相应气相渗透率也逐渐增大,说明气层束缚水是相对气相渗流速度的一个物理量,即具有速度敏感性,再不是一个定值,这就解释了致密气藏束缚水高、原始含水饱和度比束缚水低而地层水有可能流动的原因。研究结果对其他致密气藏的开发也有直接的指导意义。

Water produced in many gas wells despite usual high irreducible water content in tight gas reservoir is a phenomenon not easy to explain. In this regard, capillary pressure method, gas driven water dynamic method and nuclear magnetic resonance method are applied to measure gas reservoir irreducible water saturation in laboratory and gas reservoir in-situ water saturation is determined by well logging. Then movable water saturation is determined by the difference between reservoir in-situ water saturation and irreducible water saturation. Percolation test of typical coring from Qiaokou and Baimiao tight gas reservoir (Zhongyuan Oilfield) and Daniudi Gasfield (Ordos) reveal that coring water saturation decreases even to be lower than the saturation measured by conventional method with the increase of gas flow velocity and correspondingly gas phase permeability increases, which indicate gas formation irreducible water is a physical variable in relation to gas phase percolation velocity, sensitive to velocity and no longer a constant value. This explains why formation water may flow in tight gas reservoir where irreducible water content is high and in-situ water saturation is lower than irreducible water saturation. This discovery may be directly applied as guidance for the development of other tight gas reservoirs.