南海神狐海域水合物三相混合层测井评价方法研究

Well Logging Evaluation for Three-Phase Zone with Gas Hydrate in the Shenhu Area,South China Sea

天然气水合物降压试采过程中,水合物、游离气和水的三相混合层中的游离气首先被采出,从而提高降压效率,促进水合物分解;因此利用岩心刻度测井的方法开展南海神狐海域水合物三相混合层测井评价方法研究,对水合物矿体储量计算以及产业化开采具有重要意义。三相混合层与水合物层相比,其密度和中子孔隙度值均减小,纵波速度明显下降;与气层相比,其剪切模量仍大于背景值,岩心孔隙水氯离子浓度降低。据此可以识别出三相混合层。水合物的密度和含氢指数与水接近,当岩性和物性相近时,密度和中子孔隙度测井仅受游离气的影响;发现三相混合层岩心分析含气饱和度与密度减少值、中子孔隙度减少值均具有较好的线性相关关系,因此建立密度-中子孔隙度双参数含气饱和度二元线性回归模型。在利用印度尼西亚方程计算地层含水饱和度以及利用双参数含气饱和度模型确定含气饱和度的基础上,即可得到三相混合层的水合物饱和度。计算结果表明,三相混合层的水合物饱和度与氯离子淡化水合物饱和度吻合度较高,说明本次研究的三相混合层测井评价方法是可行而可靠的。

During depressurization for the production test of natural gas hydrate,the free gas in the three-phase zone(gas hydrate,free gas,and water)was exploited first,which improves the depressurization efficiency and enhances the gas hydrate dissociation.Therefore,well-logging evaluation for three-phase zone with the core calibration logging method in Shenhu area(South China Sea)is important for the reserve calculation of gas hydrate bodies and subsequent industrial production.Compared with those in the gas hydrate layer,both the density and neutron porosity decrease,and the compressional wave speed drops dramatically in the three-phase zone.However,the shear modulus is still above the background and the chloridion concentration of core porewater falls below that in the free gas layer.Consequently,the three-phase zone can be identified.The density and hydrogen index of gas hydrate are close to those of water,thus the density and neutron porosity are only affected by the free gas,when the lithologies and physical properties are similar.We found that the reduction in density and neutron porosity are linearly correlated with core analysis gas saturation in the three-phase zone.Thus,for the gas saturation estimation,the dual-parameter binary linear regression model,which comprises the density and neutron porosity,was developed.Based on the water saturation and gas saturation achieved from the Indonesian formula and the dual-parameter free gas calculation model,respectively,the gas hydrate saturation was calcula-ted.The estimated gas hydrate saturation in the three-phase zone are in good agreement with the gas hydrate saturation derived from the chloridion content of core porewater,indicating that well-logging evaluation method for three-phase zone is reliable.