南海北部天然气水合物物性参数评价与分类体系构建

Evaluation of physical parameters and construction of a parameter classification system for natural gas hydrate in the northern South China Sea

由于目前海洋天然气水合物(以下简称水合物)钻采案例少、取样成本高、保温保压技术难度大,导致对其储层物性原位测试及资源等级定量分类评价认识不清、资源等级划分与开发模式响应关系不明。为此,根据南海北部实际水合物取样岩心资料,利用室内水合物声电力学测试装置,测定了不同组分、饱和度下含水合物沉积物的声波、电阻率、饱和度、渗透率、偏应力以及应变等物性参数,基于层次分析法,建立了水合物物性参数敏感度熵权综合评价模型,对水合物参数敏感性进行了评价;进而依据参数变化规律和敏感性分析结果,建立了海域水合物的物理参数评价方法与划分标准。研究结果表明:①南海北部水合物储层主要为胶结强度较差的砂泥质粉砂为主,粉砂、黏土含量较高,储层中位粒径主要介于9~30μm,水合物饱和度范围介于20%~60%,解析气主要为甲烷气体;②水合物参数敏感性从高到低顺序依次为:偏应力、应变、气量、纵波速度、电阻率、渗透率和横波速度;③南海北部水合物藏综合划分为非成岩Ⅰ类、非成岩Ⅱ类、非成岩Ⅲ类和成岩Ⅳ类、成岩Ⅴ类;④南海北部荔湾区域水合物储层属于泥质浅层沉积,无完整圈闭构造,结构较为松散,属于非成岩Ⅱ类水合物藏,水合物钻采面临工程风险、地质风险、设备风险3大安全风险。结论认为,该研究成果为后续海域水合物资源评价、分类以及与之对应的合理开发方式优选提供了理论指导和技术方法。

For lack of marine gas hydrate drilling and production scenarios and due to a high cost of sampling and a great difficulty in heat preservation and pressure preservation technologies,in-situ tests of hydrate reservoir physical properties and quantitative classification and evaluation of such resource grade are not so clear that the relationship between resource grade division and development mode response is not easy to understand.In this regard,based upon the actual hydrate sampling core data in the South China Sea,an indoor hydrate acoustoelectric testing device was adopted to measure the physical parameters of hydrate deposits with different composition and saturation such as acoustic wave,resistivity,saturation,permeability,partial stress and strain,etc.Then,an entropy weight comprehensive evaluation model for physical parameter sensitivity of gas hydrate was established on the basis of the analytic hierarchy process.Finally,according to the parameter variation and sensitivity analysis results,an evaluation method and a five-grade classification standard of physical parameters of marine hydrate were thus built up.The following results were achieved.(1)Hydrate reservoirs in the northern South China Sea are dominated by sandy argillaceous silt with poor cementation strength,and the content of silt and clay is high,the median grain size of reservoir hydrate saturation is 20-60%and the desorbed gas is mainly composed of methane gas.(2)The sensitivity order of hydrate parameters from high to low is stress,partial stress,analytical gas,longitudinal wave,resistivity,permeability,and shear wave.(3)Hydrate reservoirs in the northern South China Seas are classified into non-diagenetic gradesⅠ,Ⅱ,Ⅲand diagenetic gradesⅣandⅤ.(4)The Liwan hydrate reservoirs in the northern South China Sea are defined as the non-diagenetic gradeⅡ,which belongs to argillaceous shallow sedimentary hydrate deposits without complete trap structures,where hydrate drilling and exploitation will have to face engineering risk,geological risk and equipment risk.In conclusion,this study provides an important technical method and theoretical guidance for the evaluation and classification of marine gas hydrate resources and the optimization of corresponding reasonable development methods in the future.