Mineral compositions and microstructural characteristics of the tight sandstone reservoir in the Sulige area and their potential influence on hydraulic fracturing
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摘要: 致密砂岩气是重要的非常规天然气资源,致密砂岩储层的矿物组成特征与孔隙微观结构不仅会影响致密储层中天然气的赋存状态,同时也对水力压裂改造效果具有重要影响。然而,有关致密砂岩矿物成分及其微观赋存形态对气井水力压裂潜在影响的研究尚不充分,针对这一问题,文章以鄂尔多斯盆地苏里格地区主开采层——二叠系下石盒子组盒8下段致密砂岩为研究对象,钻取5口气井岩心样品,利用XRF、XRD、铸体薄片、SEM及EDS能谱等技术系统分析了该段致密砂岩矿物组成、微观形貌、孔隙分布等储层特征。基于分析结果,探讨了上述储层特征因素对水力压裂的潜在影响。研究发现,苏里格地区盒8下段致密砂岩主要为岩屑砂岩和岩屑石英砂岩,密度介于2.44~2.56 g/cm3之间,孔隙度为7.7%~12.6%,渗透率为0.16~1.42 mD,属于典型的低渗透气藏。矿物成分主要为石英和黏土矿物,而长石矿物和碳酸盐矿物含量极低。其中黏土矿物含量占比为16.5%~47.4%,以高岭石、伊利石和绿泥石为主。高岭石在该区致密砂岩中广泛发育,呈书页状和蠕虫状填充于粒间孔隙及粒表,形成大量高岭石晶间孔。此外,致密砂岩中粒间孔隙、粒内溶孔、粒间裂隙及粒内缝均有发育,为致密砂岩气的赋存提供了良好的储集空间。矿物分析以及压裂液致密砂岩水岩实验结果显示黏土矿物稳定性对于苏里格地区致密砂岩气水力压裂效果至关重要,尤其是易分散运移的高岭石和伊利石矿物。水力压裂前需要对储层矿物成分及地层水成分展开详细研究,选择合适的黏土稳定剂优化水力压裂。Abstract: Tight sandstone gas is an important unconventional natural gas resource. Mineral characteristics of the tight sandstone not only affect the occurrence of gas, but also have a significant impact on the hydraulic fracturing effectiveness. The potential influence of tight sandstone mineral compositions and microscopic morphology on gas reservoir during hydraulic fracturing was seldom examined. In this study, five tight sandstone gas wells in the Sulige gasfield located in the Ordos Basin were drilled. The core samples of the 8th formation of the lower member of the Permian Shihezi Group (He 8) were collected. The mineral characteristics, microstructure and pore feature of the tight sandstone are systematically analyzed by using XRF, XRD, casting thin section, SEM and EDS technologies and the potential effects of the above factors on hydraulic fracturing are discussed. The results show that the tight sandstone is mainly composed of lithic sandstone and lithic quartz sandstone. The density of the tight sandstone ranges from 2.44 to 2.56 g/cm3, the porosity from 7.7% to 12.6%, and the permeability from 0.16 to 1.42 mD. The mineral compositions are mainly quartz and clay minerals (16.5%−47.4%), and feldspar and carbonate are absent. Kaolinite, illite and chlorite are the main clay minerals. Kaolinite is widely developed, filling the intergranular pores and surface in the forms of “booklets” and worm-like. The intergranular pores, intragranular solution voids, intergranular and intragranular fractures occur in the tight sandstone, which provide basic reservoir space for natural gas. The results of mineral analysis and fracturing fluid-tight sandstone interactions reveal that the stability of clay minerals, especially kaolinite and illite, are crucial for hydraulic fracturing in the Sulige gasfield. It is necessary to understand the mineral compositions and formation water of the tight reservoir and select the appropriate clay stabilizer to optimize hydraulic fracturing.
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Key words:
- tight sandstone /
- microstructure /
- clay minerals /
- hydraulic fracturing /
- Sulige gasfield
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表 1 储层岩心主要元素含量
Table 1. Main elements of the reservoir core
/(mg·kg−1) 样品编号 Zr Sr Rb Zn Fe Mn Cr V Ti Ca K 桃10 183.6 71.95 51.43 44.48 24028 216.1 < LOD 50.81 2384 3261 15949 召42 426.0 112.30 97.93 42.34 31273 350.0 33.76 114.6 7667 2316 32037 统98 218.9 84.50 58.42 33.52 19717 397.2 < LOD < LOD 3607 14964 21756 召60 131.8 82.16 29.92 39.58 16930 1429.0 < LOD 36.85 1812 50787 9202 召62 279.8 128.40 103.00 71.20 29066 272.1 < LOD 104.7 4542 5527 39314 注:<LOD表示低于检出限。 表 2 储层岩心矿物组成
Table 2. Mineral compositions of the reservoir core
/(wt%) 样品编号 石英 斜长石 钾长石 方解石 伊利石 绿泥石+高岭石 赤铁矿 方沸石 桃10 74.6 − − 0.4 4.2 19.8 1.0 − 召42 65.6 0.2 0.2 0.4 9.8 22.9 0.9 − 统98 81.6 − − 1.0 5.6 10.9 0.9 − 召60 45.9 − − 18.0 3.9 31.8 − 0.4 召62 51.5 − − 0.4 27.3 20.1 0.7 − -
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