水合物降压分解过程中沉积物孔隙结构动态演化规律  

Dynamic evolution of pore structures of hydrate-bearing sediments induced by step-wise depressurization

作　　者：王代刚[1,2] 魏伟[3] 孙静静 彭晖[3] 李承峰 贾春霞[5] 刘昌岭[4] 卢海龙[1] Daigang Wang;Wei Wei;Jingjing Sun;Hui Peng;Chengfeng Li;Chunxia Jia;Changling Liu;Hailong Lu(Beijing International Center for Gas Hydrate,Peking University,Beijing 100871,China;Key Laboratory of Unconventional Oil&Gas Development,Ministry of Education,China University of Petroleum(East China),Qingdao 266580,China;Research Institute of Petroleum Exploartion&Development,PetroChina,Beijing 100083,China;Key Laboratory of Gas Hydrate,Ministry of Natural Resources,Qingdao Institute of Marine Geology,China Geology Survey,Qingdao 266071,China;Mathematics&Science College,Shanghai Normal University,Shanghai 200234,China)

机构地区：[1]北京大学北京天然气水合物国际研究中心,北京100871 [2]中国石油大学(华东)非常规油气开发教育部重点实验室,青岛266580 [3]中国石油勘探开发研究院,北京100083 [4]中国地质调查局青岛海洋地质研究所,自然资源部天然气水合物重点实验室,青岛266071 [5]上海师范大学数理学院,上海200234

出　　处：《科学通报》2020年第21期2292-2302,共11页Chinese Science Bulletin

基　　金：国家自然科学基金(41806070,41876051);中国地质调查局项目(DD20190234);中国石油科技创新基金(2018D-5007-0201);中国博士后科学基金(2018M641069,2019T120022);非常规油气教育部重点实验室开放研究基金(19CX05005A-4)资助。

摘　　要：选取两种不同粒径分布的天然海砂样品,在相同实验温度及压力条件下合成甲烷水合物.开展水合物降压分解X-CT微观实验,获取不同时刻的沉积物内部构成图像.对X-CT扫描图像进行阈值分割、三维重建及拓扑等效等处理,建立不同粒径含水合物沉积介质不同分解阶段的三维孔隙网络模型,研究水合物降压分解过程中沉积物孔隙结构动态演化规律及其控制机理.研究表明,甲烷水合物微观分布非均质特征显著,分解过程开始于气-水合物接触的位置,分解初期水合物具有孔隙填充、颗粒胶结等多种赋存模式,而分解后期孔隙中主要赋存颗粒胶结型水合物.随着水合物饱和度的减小,不同粒径含水合物沉积物的平均孔喉半径、孔隙度、绝对渗透率以及两相共渗区宽度均不断增大,配位数、形状因子及束缚水饱和度逐渐减小.受水合物分解动力学行为影响,分解前期沉积物平均孔隙半径缓慢降低,水合物饱和度下降至某一临界值(小于0.1)后,平均孔隙半径和绝对渗透率急剧增大.粒径分布越窄,相同水合物饱和度下沉积物平均孔隙及孔喉半径、孔隙度和绝对渗透率越高,但不同粒径沉积介质配位数和形状因子的变化存在交叉点,表明含水合物沉积物微观孔隙结构特征受粒径分布、水合物微观赋存状态及空间分布的协同影响.Pore structures are of great importance to study gas production from natural gas hydrate reservoir.The dynamic evolution of pore structures over the course of methane hydrate dissociation and their association with particle size distribution of hydrate-bearing sediments remain poorly understood.In this study,a combination of X-ray computed tomography(CT)observation and pore network modeling was applied to exploring the dynamic evolution of pore structures with hydrate dissociation induced by depressurization in two initially brine-unsaturated sediments with different particle size distributions.Methane hydrates were synthesized in two silica sands,each of which is with different particle size distributions,at 2.0℃and 8.5 MPa.The reaction process continued nearly three weeks until the pore pressure and temperature inside the reaction vessel did not have significant changes for at least 7 d,to provide sufficient time for methane hydrate in sediment pores to reach steady spatial distribution.The X-ray micro-focus CT observation of methane hydrate dissociation,which was induced by step-wise depressurization,was further performed,to obtain a series of highresolution CT images of hydrate-bearing sediments at different degrees of hydrate saturation.The corresponding pore structures of the two sediments with different hydrate saturation are studied by the digital image processing methods,including image filter,2-D threshold segmentation of binarized CT images,3-D digital core reconstruction,and topological extraction of actual pore structure.To overcome the obstacle caused by the similar X-ray attenuation coefficients of methane hydrate and deionized water,the phase-contrast between methane hydrate and pore water was enhanced by the addition of NaCl into pore water.To have abetter configuration about the distributions of materials in sediments,the bulky patchy-like hydrate adhering to the inner wall of the reaction vessel or occupying macroscopic pores was used to calibrate the CT values of sand,water,gas,and methane h

关 键 词：甲烷水合物 降压分解 X-CT扫描 孔隙结构 颗粒粒径 网络模型 

分 类 号：P744.4[天文地球—海洋科学]