冻土区天然气水合物各向异性储层的AMT响应特征

陆域天然气水合物通常发育于冻土层下方破碎带和岩层裂隙处，其储层会表现明显电性各向异性特征.音频大地电磁法（AMT）能有效探测陆域水合物储藏范围，且不易受高阻冻土层的压制和干扰，可用于陆域天然气水合物探测研究.本文采用AMT对角各向异性二维正演方法，对多种陆域天然气水合物各向异性储层模型及相关参数进行了模拟试算，分析其视电阻率和相位响应特征.结果表明AMT能清晰显示各向异性储层空间位置和分布情况，并对储层的水合物饱和度变化以及储层数量等特征都有所反映.天然气水合物电性各向异性模拟研究为这一新型潜力能源的勘查工作提供了新的思路.

Characteristics of AMT response of anisotropic natural gas hydrate reservoirs in permafrost regions

Natural gas hydrates (NGH) are ice-like crystalline solids formed with water and gas (CH₄ mostly) at low temperature and high pressure, which are usually present in ocean and seabed sediments or permafrost regions. NGH are a potential and clean energy resource with great amounts especially in ocean regions. Although the reserves of NGH in permafrost regions are smaller than ocean regions, their cover and trap of upper frozen soil layers, and shallow and certain storage depth, make NGH exploration in these regions easy with less threat of gas leakage.
In permafrost regions, NGH usually developed in rock fractures, cracks and lamination interlayers. As the hydrate is an electricity insulator, the considerable resistivity differences between the hydrate and strata make the NGH reservoirs show obvious electrical anisotropy. This paper presents a typical simplified reservoir model, in which the resistive NGH layers are interbeded with conductive strata. Hence the electrical anisotropy exists and can be expressed by horizontal resistivity and vertical resistivity jointly.
The audio-frequency magnetotelluric method (AMT) can be used to infer subsurface conductivity structure by measuring 2 horizontal components of the natural electric field and 2 horizontal and 1 vertical components of the natural magnetic field at earth's surface. With shallow penetration depth including hydrates layers' range, short measuring time and small cost, AMT is a useful tool for investigation of NGH reservoirs.
In this paper, a 2D AMT forward modeling method considering triaxial anisotropy is applied to examine the response characteristics of anisotropic NGH reservoirs in permafrost regions. Two synthetic models are presented, one is an 1D layer model and the other is a 2D block model, and both overly a relatively high resistivity layer representing a resistive frozen layer. In the 1D model, responses in xy mode differ from yx mode, which prove the existence of anisotropy. In the 2D model, while the width and depth become larger or smaller, the apparent resistivity and phase responses change correspondingly. The responses are also sensitive to the hydrate saturation and the block distribution in both models, showing significant differences between the original one and that without the frozen layer above. The overlying resistive layer becomes an electromagnetic disturbance to the underlying structure, and makes the electrical anomaly of the anisotropic block less obvious. Nonetheless, the main structure characteristics and the response of the block anomaly remain similar in spite of the existence of the overlying frozen layer. The results show the anisotropy phenomenon can change apparent resistivity and phase responses behavior with different model parameters. It is proved that AMT is an effective method to prospect NGH reservoirs in permafrost regions and the study about electrical anisotropy can provide some new insights into the NGH exploration both in land and marine areas.