含天然气水合物沉积物的介电常数是水合物资源勘探开发过程中的重要参数。祁连山冻土区钻探表明水合物储集层多属致密型岩石类型,因此无法精确定位水合物储层及估算资源量,探地雷达及介电测井成为重要的备选方案。本文设计并建立含水合物岩心介电常数测试系统,对祁连山水合物钻探区DK-8井岩心样品进行实验。结果表明,含水合物砂岩样品的介电常数与外加电场的频率相关,即介电常数总趋势是随频率增高而减小,而在低频端出现高值,介电常数与频率呈现指数关系,含水合物砂岩的实验结果符合岩石介电常数规律。同一岩石样品,含水合物岩石的介电常数介于饱水岩石和干岩石介电常数之间。在气饱和条件下,同一岩石孔隙中较高介电常数的极化水转化为较低介电常数水合物导致含水合物岩石介电常数低于饱水岩石;但是水合物的介电常数高于空气的介电常数使含水合物岩石的介电常数高于同一块干岩石的介电常数,说明岩石中水合物增大了岩石的介电常数。含水合物样品在10 ~ 40 MHz频段存在频散特性;频率高于50 MHz,频散特性变弱,介电常数变化很小。
The dielectric constant of hydrate-bearing-sediments (HBS) is a significant parameter in the exploration and development of hydrate resources. From the drilling results, the HBS of the permafrost region in Qilian Mountain mainly belongs to consolidated rock type. So it is difficult to locate the hydrate reservoir and estimate resources accurately, Ground Penetrating Radar and dielectric well logging become an important option. In this paper, the dielectric constant test system of HBS was designed and established to test core samples from DK-8 well in the hydrate drilling area of Qilian Mountain. The results showed that the dielectric constant of sandstone samples is associated with the frequency of the external electric field, which means a general trend of dielectric constant decreases with frequency increasing. However, high dielectric constants showed in the low frequency, while dielectric constants showed the exponential relationship with the frequency. The experiment results of HBS conform to the law of dielectric constant in rocks. In the same rock sample, the dielectric constant of HBS was between that of water-saturated rock and dry rock. In the gas saturated circumstances, the polarised water with high dielectric constant was transformed into hydrate with low dielectric constant in the pores of the same rock, which led to the dielectric constant of HBS lower than that of water-saturated rock. However, the dielectric constant of hydrate was higher than that of air, so that the dielectric constant of HBS was higher than that of the same dry rock, which indicated that hydrate in rock increases the dielectric constant of rock. The HBS samples showed the dispersion characteristics in 10 ~ 40 MHz frequency. However, the dispersion characteristic weakens and the dielectric constant changes little when the frequency was above 50 MHz.
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