孔隙类型对页岩气赋存状态的影响——以川南长宁地区五峰组—龙马溪组页岩为例

Influence of pore type on the occurrence state of shale gas:Taking Wufeng-Longmaxi formation shale in Changning area of southern Sichuan as an example

本文选取四川盆地南部长宁地区两口页岩气井,利用不同流体充注下的核磁共振、场发射扫描电镜、有机地化、X-衍射、现场解析等实验测试,研究富有机质页岩中有机孔和无机孔的微观特征及其发育程度,重点探讨不同孔隙类型对吸附气、游离气赋存的影响,认为研究区内过成熟时期页岩中有机孔孔隙度增加不利于吸附气的赋存,游离气在有机孔中的储集受多种因素制约;而吸附气在无机孔中的赋存主要受黏土矿物影响,游离气含量随无机孔孔隙度增加而增加.研究结果表明:研究区五峰—龙马溪组页岩储层中有机孔孔隙度为0.55%~3.32%,平均为1.66%,无机孔孔隙度为1.12%~3.27%,平均为2.39%;游离气含量(平均为1.57 m~3/t)高于吸附气含量(平均为0.94 m~3/t);过成熟时期页岩有机孔孔隙度增加以孔径增大为主导,随着有机孔孔隙度从0.55%增加至3.32%,有机孔中吸附气含量则从1.25 m~3/t降至0.58 m~3/t;同时,在有机孔储集条件更优异的井中,游离气含量与有机孔孔隙度相关性较差(R~2=0.032),游离气在有机孔中的储集受到多个因素共同控制;黏土矿物多样化的结构丰富了页岩的比表面、影响无机孔孔隙度、控制页岩气在无机孔中的赋存,使得吸附气含量随着无机孔孔隙度的增加先增加后降低;类型丰富、分布全面的无机孔也是游离气的主要储集空间,游离气含量与无机孔孔隙度正相关.

In this paper, two shale gas wells in Changning area, southern Sichuan Basin were selected to study the microscopic characteristics and development degree of organic and inorganic pores in organic-rich shale by using nuclear magnetic resonance, field emission scanning electron microscope, organic geochemistry analysis, X-ray diffraction and field analytical tests under different fluid charging conditions. Focusing on the influence of different pore types on the occurrence of adsorbed gas and free gas, it is considered that the increase in the porosity of organic pores in the shale during the over-mature period in the study area is not conducive to the occurrence of adsorbed gas, and the storage of free gas in organic pores is affected. However, the occurrence of adsorbed gas in inorganic pores is mainly affected by clay minerals, and the content of free gas increases with the increase of porosity of inorganic pores.The results show that the porosity of organic pores in Wufeng-Longmaxi formation shale reservoir ranges from 0.55% to 3.32%(with an average of 1.66%), and that of inorganic pores ranges from 1.12% to 3.27%(with an average of 2.39%). Free gas content(with an average of 1.57 m~3/t) is higher than adsorbed gas content(with an average of 0.94 m~3/t). In the over-mature stage, the increase of organic pore porosity is dominated by the increase of pore size. With the increase of organic pore porosity from 0.55% to 3.32%, the adsorbed gas content in organic pore decreases from 1.25 m~3/t to 0.58 m~3/t. Meanwhile, in the wells with better organic pore storage conditions, the correlation between free gas content and organic pore porosity is poor(R~2=0.032), which means that the storage of free gas in organic pores is controlled by multiple factors. The diversity of clay minerals enriches the specific surface of shale, effecting the porosity of inorganic pores, and controlling the occurrence of shale gas in inorganic pores, making the adsorbed gas content increase first and then decrease with the increase of the porosity of inorganic pores. Inorganic pores with abundant types and comprehensive distribution are also the main storage space of free gas, and the content of free gas is positively correlated with the porosity of inorganic pores.