沉积物中天然气水合物生成过程的二维电阻层析成像观测

基于室内模拟实验模拟沉积物中天然气水合物(以下简称水合物)的生成过程及其物性演化规律,可以为现场实际水合物储层动态响应规律分析提供重要的理论支撑。为此,基于ITS电阻层析成像仪,研制了适用于沉积物中水合物原位合成—分解动态探测的电阻层析成像模拟装置,并以天然海滩砂为沉积介质,模拟了含盐水沉积物体系中水合物的生成过程,根据电阻层析成像测试结果分析了水合物在沉积物截面上的生成位置和非均质性。研究结果表明:①水合物合成过程中,受排盐效应和水合物生成速率两个因素的影响,沉积物体系平均电阻率呈波动上升趋势;②较之于水合物合成,沉积物内部排盐效应对沉积物体系平均电阻率的影响存在着明显的滞后效应,水合物合成结束后沉积物体系内部仍存在盐离子浓度差异控制的传质过程;③水合物合成过程中不同区域的电导率偏移量变化幅度表现出明显的非均质性,指示水合物在沉积物内部的生成过程具有明显的非均质性。结论认为,起始条件下沉积物中气—水非均质分布特征是导致沉积物中最终水合物饱和度非均质分布的决定因素,而水合物合成过程中排盐效应导致的沉积物内部盐离子浓度的空间分布差异则是水合物合成位置变化的控制因素。

2-D electrical resistivity tomography assessment of hydrate formation in sandy sediments

Laboratory hydrate formation process simulation and its physical property evolution monitoring within the sediment provides a theoretical support for determining the dynamic revolution behaviors of actual hydrate reservoirs. In view of this, based on the ITS electrical resistance tomography (ERT) module, a dedicated apparatus was developed to simulate the in-situ core-scale distribution of natural gas hydrate directly during the process of hydrate formation and dissociation in artificial sediments. Beach sand was used as porous media to simulate the formation of methane hydrate and assess the 2-D ERT evolution characteristics simultaneously. The findings were obtained. (1) The apparent average resistance values of hydrate-bearing sediment system are determined by the combination of hydrate forming rate and salt-removing effect, showing fluctuation rise behaviors during the process of hydrate formation. (2) When the hydrate forming rate is high enough, hysteresis influence of salt-removing effect on apparent average resistance value is observed. After the hydrate formation, there still exists a mass transfer process controlled by the difference of salt ion concentration. (3) Difference between original and real-time ERT distribution indicates a heterogeneous formation process of hydrate within the sediments. In conclusion, initial gas-water contact relationship and distribution is the virtual factors for hydrate heterogeneous distribution behaviors, while that of spatial difference of ion concentration caused by the salt-removing effect plays a key role in promoting local hydrate formation position change.